Model Output Discussion - Winter arrives

[damianslaw]

1 minute ago, phil nw. said:

Yes noticeable that the blocking signal further east doesn't  go away and does look like returning in 10 days or so.Probably about the right timing taking into account the move east of the MJO into phase 7 about now and the lag effect.

Two Decembers that brought a Euro high that stretched into UK were 2006 and 2007, lots of inversion cold and fog. However, the difference this time is the continent looks cooler and if heights ridge further north into Scandi a flow off the near continent will be cold. In mid December high pressure overhead is rarely mild, unless it has built in from the azores. The azores high looks like it may merge with it to become a major block, all eyes then on how the jet interacts, possibility we could end up with a scandi trough slicing through it, and it then advecting further north and west, but not sure on this given the signal is for heights to remain strong over Urals/NW Russia. Lots to resolve..

[Mike Poole]

7 minutes ago, Cold Winter Night said:

Not much new on the clusters compared to this morning.
Days 5-10 are NAO+ dominant, with a minor cluster with NE blocking.

We still see in the extended, 264h-360h, that the majority (33 members) moves towards a BLO+ regime, 23 of them in a cluster that has heights over most of Europe, I can imagine inversion cold with that, and 10 members in a cluster with some modest Scandi heights.
The other 18 go NAO+/Euro High.

Yes, I think pretty much all of the last 7 GFS runs, going back to pub run on 30th Nov, have ended up here 12z T384:

Which has the positives that we’re not getting a rampaging Atlantic raging at us, puts pressure on the strat (more on that in a minute), settled and feeling cold, but we are not really in any kind of cold air, T850s:

Not that that matters as there won’t be much precipitation around.  By the way I’m talking at this timescale about wider than this specific run, it is about what the ensembles are also saying - one of those weird situations that I’m more confident about the state of play as it affects the UK in 15 days than I am in 5 with the uncertainty over THAT low!!  

Well it certainty wouldn’t be a washout.  Or a snow out!  It may grow colder but I cannot see the obvious route to snow from here.  Blessed that there is no prolonged attack from the Atlantic, sure.  

So has the trop and strat coupled?  NAM index (0z GFS) suggests yes:

And same model shows a period of vortex intensification:

It seems that despite this, the more amplified pattern in the trop that has been here for months now, still needs to have a say.  And the massive UK-Ukraine block that seems to be result of most modelling at the outer reaches, will put pressure on the strat vortex, leading to either jam or marmite in due course.  

 

[sebastiaan1973]

26 minutes ago, MATTWOLVES said:

Ok folks I'm out of retirement...I've let off my steam  

I'm liking the 46 anomalies...some strong hints of extensive Hights across the UK in towards scandy big time.

Seems to me they are from 29-11.

[sebastiaan1973]

12 minutes ago, damianslaw said:

Two Decembers that brought a Euro high that stretched into UK were 2006 and 2007, lots of inversion cold and fog. However, the difference this time is the continent looks cooler and if heights ridge further north into Scandi a flow off the near continent will be cold. In mid December high pressure overhead is rarely mild, unless it has built in from the azores. The azores high looks like it may merge with it to become a major block, all eyes then on how the jet interacts, possibility we could end up with a scandi trough slicing through it, and it then advecting further north and west, but not sure on this given the signal is for heights to remain strong over Urals/NW Russia. Lots to resolve..

December 2007 showed a MJO phase 8 and 1-2-3-4.

[knocker]

The medium term anomalies are all indicating the tpv in charge with the other key player the Urals tidge.  precisely how this aligns with the tpv lobe/trough ext in the eastern Atlantic will determine the detail through the period as the strong westerly upper flow diverges . Likely to be unsettled with the possibilty of some strong winds at times.

[Battleground Snow]

Just need a little shift north in those anomalies from the ECM 46.

Edit: added in the extended mjo forecast which looks good, a little bit more amplitude than gefs

Edited December 2, 2021 by Battleground Snow

[MATTWOLVES 3]

Apologies folks...I've completely lost the plot today...the anomalies I posted were from Mondays update...

More to follow as they update.

In all fairness we are heading in a similar direction anyway...quite a blockade of Heights there. So if we do go a tad zonal for a while,it's not going on this be lasting any real length of time.

Edited December 2, 2021 by MATTWOLVES

[damianslaw]

13 minutes ago, Battleground Snow said:

Just need a little shift north in those anomalies from the ECM 46.

Those heights are ridging north in time, with low heights over the med, could be a slow burner evolution to a colder pattern as we move through second half of the month.

[bluearmy]

Ec 46 will surely be surface cold at the very least once the high takes control …..as the run progresses, it shows the high drifting further north which raises the chances of some CAA. 
 

the spv forecast is also less strong which may reflect wave 1 forcing from the blocking 

[Mike Poole]

GFS 18z T

 

Well, where from here?  The op runs seem keen on a UK-Ukraine block.  The ensembles less so, but it is there, ECM clusters T192-T240 and T264+

Particularly at the end of the ECM clusters. 

Think we’re just going to have to see how it pans out now, not that hopeful for wintry weather in the next week or two after Monday maybe, looks like we’ll likely be pinning our hopes on a SSW after all, with all the trials and tribulations that entails…hey ho!   Goodbye until something new and interesting crops up!

Regards

Mike

Oops, that went too early, I haven’t put a GFS chart in!  Think I’ll leave it though, wouldn’t change the message!

Edited December 2, 2021 by Mike Poole

[Bricriu]

I am more confident now of a settled run up to Christmas. The pv will likely hold sway for a week or two before a block to the north east sends it into retreat around Mid December.

 

I, like most people here, would much prefer snow, but i'll settle for cold and frosty mornings. To some it maybe faux cold/inversion due to the likely position of the high, to me it will feel like real cold if i need to wear gloves and a coat on a morning stroll:)

Edited December 2, 2021 by Bricriu

[Eagle Eye]

Here's to thinking that combining La Nina, the QBO, this year's SSW and current models in one post (later) is a good idea

Alright so I'll be explaining what I've read about the QBO here and it's effect on models, La Nina will be when I have the time

Also feel free to correct any mistakes, this is just interferences from what I've read and also quoted from what I've read a lot of it

Most of the explanations will be direct quotes from the texts before the two conclusions one of which is mine here goes :
All models and their references for the first part




Part 1

Introduction

At the time that the QBO (Quasi Biennial zonal wind Oscillation) was discovered, there were no observations of tropical atmospheric waves and there were absolutely no theories predicting their existence. “The search for an explanation for the QBO initially involved a variety of causes: some internal feedback mechanism, a natural period of atmospheric oscillation, an external process, or some combination of these mechanisms.” None of these attempts explained features such as the downward propagation and maintenance of the amplitude of the QBO (hence the increase in energy density) as it descends. Forcing by zonally asymmetric waves is needed to explain the equatorial westerly wind maximum. “Wallace and Holton tried to drive the QBO in a numerical model through heat sources or through extratropical planetary-scale waves propagating toward the equator. They showed conclusively that lateral momentum transfer by planetary waves could not explain the downward propagation of the QBO without loss of amplitude. Booker and Bretherton's [1967] paper on the absorption of gravity waves at a critical level sparked that, that would lead to an understanding of how the QBO is driven. It was Lindzen's leap of insight to realize that vertically propagating gravity waves could supply the necessary wave forcing for which of the QBO.  "

I'll be honest I didn't know anything about zonal wind before this so it's best to just directly quote the whole paragraph
 

Zonal Wind  

“A composite of the QBO in equatorial zonal winds (Figure 1) [Pawson et al., 1993] shows faster and more regular downward propagation of the westerly phase and the stronger intensity and longer duration of the easterly phase. The mean period of the QBO for data during 1953-1995 is 28.2 months (about 2 and a half years), slightly longer than the 27.7 months (about 2 and a half years) obtained from the shorter record of Naujokat [1986]. The standard deviation about the composite QBO is also included in Figure 1, showing maxima in variability close to the descending easterly and westerly shear zones (larger for the westerly phase). This reflects deviations in the duration of each phase. Dunkerton [1990] showed that the QBO may be synchronized to the annual cycle, showing that the onset of the easterly regime at 50 hPa tends to occur during NH late spring or summer. His analysis is updated in Figure 2, which shows the onset of each wind regime at 50 hPa. The easterly and westerly transitions both show a strong preference to occur during April- June. The latitudinal structure of the QBO in zonal wind is shown in Figure 3, derived from long time series of wind observations at many tropical ß stations [Dunkerton and Delisi, 1985]. The amplitude of the QBO is latitudinally symmetric, and the maximum is centered over the equator, with a meridional half width of approximately 12 ø. Similar QBO structure is derived from assimilated meteorological analyses, but the amplitude is often underestimated in comparison with rawinsonde measurements [Pawson and Fiorino, 1998; Randel et al., 1999]. Plate 2 supplies an overview of the QBO, its sources, and its global dynamical effects, as well as a foundation for the discussion of the details of the QBO in the following sections. The diagram spans the troposphere, stratosphere, and mesosphere from pole to pole and shows schematically the differences in zonal wind between the 40-hPa easterly and westerly phases of the OBO. Convection in the tropical troposphere, ranging from the scale of mesoscale convective complexes (spanning more than 100 km) to planetary-scale phenomena, produces a broad spectrum of waves (orange wavy arrows), including gravity, inertia-gravity, Kelvin, and Rossby gravity waves (see section 3). These waves, with a variety of vertical and horizontal wavelengths and phase speeds, propagate into the stratosphere, transporting easterly and westerly zonal momentum. Most of this zonal momentum is deposited at stratospheric levels, driving the zonal wind anomalies of the QBO. For each wave, the vertical profile of the zonal wind decides the critical level at or below which the momentum is deposited. The critical levels for these waves depend, in part, on the shear zones of the QBO. Some gravity waves propagate through the entire stratosphere and produce a QBO near the mesopause known as the mesospheric QBO, or MQBO (section 6). In the tropical lower stratosphere the time-averaged wind speeds are small, so the easterly minus westerly composite in Plate 2 is similar in appearance to the actual winds during the easterly phase of the QBO. At high latitudes, there is a pronounced annual cycle, with strong westerly winds during the winter season. To the north of the equator in the lower stratosphere, tropical winds alter the effective waveguide for upward and equatorward propagating planetary-scale waves (curved purple arrows). The effect of the zonal wind structure in the easterly phase of the QBO is to focus more wave activity toward the pole, where the waves converge and slow the zonal-mean flow. Thus the polar vortex north of --•45øN shows weaker westerly winds (or easterly anomaly, shown in light blue). The high-latitude wind anomalies penetrate the troposphere and provide a mechanism for the QBO to have a small influence on tropospheric weather patterns (section 6).” 
Part 2

References

Temperature and Meridional Circulation

The QBO exhibits a clear signature in temperature, with strong signals in both the tropics and extra-tropics. The tropical temperature QBO is in a thermal wind balance with the vertical shear of the zonal winds, which is expressed for the equatorial [•-plane as
""

The equatorial temperature anomalies with the QBO in the lower stratosphere are of the order of plus/minus 4 Kelvin, maximizing around 30-50 hPa. Figure 4 compares time series (after subtraction of the seasonal cycle) of 30-hPa temperature measurements at Singapore corresponding  with the zonal wind upwards shear in the 30 to 50-hPa level, showing correlation (see Plate 1).  Smaller anomalies expand down, with QBO variations of the order being plus/minus 0.5 Kelvin near the tropopause [Angell and Korshover, 1964]. These QBO temperature anomalies extend into the middle and upper stratosphere, whereby they are different with the lower stratosphere anomalies. Figure 5 in an example of temperature anomalies that are associated with an easterly phase of the QBO during the NH winter in 1994. Although these data quite possible underestimate the magnitude of the temperature QBO, the out-of-phase vertical structure is a robust feature also observed in long time records of satellite radiance measurements [Randel et al., 1999]. A big aspect of extratropical temperature anomalies is that they are seasonally synchronized and occur mainly in Winter and into spring in each hemisphere. Column ozone measurements are nearly identical whilst also being seasonally synchronised,  one key aspect of the global QBO is extratropical variability. Low-frequency temperature anomalies are closely linked with variations in the meridional circulation, global circulation patterns associated with the QBO are also highly unbalanced at solstice. Furthermore, temperature patterns show signals in both polar regions and maximize in spring in each hemisphere. "The modulation by the QBO of zonal-mean wind (Plate 1) is coupled to modulation of the zonally averaged mean meridional circulation. The climatological circulation is characterized by large-scale ascent in the tropics, broad poleward transport in the stratosphere, and compensating sinking through the extratropical tropopause [Holton et al., 1995]. The transport of chemical trace species into, within, and out of the stratosphere is the result of both large-scale circulations and mixing processes associated with waves. Chemical processes, such as those resulting in ozone depletion, not only depend on the concentrations of trace species, but may also depend critically on temperature. Since the QBO modulates the global stratospheric circulation, including the polar regions, an understanding of the effects of the QBO not only on dynamics and temperature but also on the distribution of trace species is essential in order to understand global climate variability and change. Many long-lived trace species, such as N20 and CH4, originate in the troposphere and are transported into the stratosphere through the tropical tropopause. "


Part 3

References



QBO Mechanism

"Since the QBO is approximately longitudinally symmetric [Belmont and Dartt, 1968], it is natural to try to explain it within a model that considers the dynamics of a longitudinally symmetric atmosphere. In a rotating atmosphere the temperature and wind fields are closely coupled, and correspondingly, both heating or mechanical forcing (i.e., forcing in the momentum equations) can give rise to a velocity response." As noted in part 1 the current view is that mechanical forcing, provided by wave momentum fluxes, is essential for the QBO, the coupling between temperature and wind fields is and must be  taken into account when you are trying to explain many aspects of the structure. The mechanism for the oscillation in essence can be demonstrated in just simply the representation of interaction of upward propagating gravity waves with a background flow that is itself a function of height [Plumb, 1977]. "Consider two discrete upward propagating internal gravity waves, forced at a lower boundary with identical amplitudes and equal but opposite zonal phase speeds. The waves are assumed to be quasi-linear (interacting with the mean flow, but not with each other), steady, hydrostatic, unaffected by rotation, and subject to linear damping. The superposition of these waves corresponds exactly to a single "standing" wave. As each wave component propagates vertically, its amplitude is diminished by damping, generating a force on the mean flow due to convergence of the vertical flux of zonal momentum. This force locally accelerates the mean flow in the direction of the dominant wave's zonal phase propagation. The momentum flux convergence depends on the rate of upward propagation and hence on the vertical structure of zonal-mean wind. With waves of equal amplitude but opposite phase speed, zero mean flow is a possible equilibrium, but unless vertical diffusion is strong, it is an unstable equilibrium; any small deviation from zero will inevitably grow with time. Plumb [1977] showed that the zonal-mean wind anomalies descend in time. Each wave propagates vertically until its group velocity is slowed, and the wave is damped as it encounters a shear zone. When the easterly jet decays, the westward wave moves to upper levels of the atmosphere and a new easterly shear zone forms aloft it. The  eventual time period of the oscillation is determined although within others, by the eastward and westward momentum flux at the lower boundary and by the amount of atmospheric mass affected by the waves. "In Plumb's [1977] Boussinesq formulation the QBO period is inversely proportional to momentum flux. The same is true in a quasi-compressible atmosphere, but the decrease in atmospheric density with height results in a substantially shorter period. Simple representation such as Plumb's capture the essential wave mean-flow interaction mechanism leading the QBO. However, they cannot explain why the QBO is an equatorial phenomenon (notwithstanding its important links to the extratropics). One reason the QBO is equatorially confined may be that it is driven by equatorially trapped waves. However, it is also possible that the QBO is driven by additional waves and is confined near the equator for another, more fundamental, reason. Some simple insights on this point come from considering the equations for the evolution of a longitudinally symmetric atmosphere subject to mechanical forcing. A suitable set of model equations for such a longitudinally symmetric atmosphere is as follows:

"
Waves in the Tropical Lower Stratosphere
There are many different types of waves in the tropics which contribute to the Quasi-Bilateral Oscillation. There is a combination of evidence to believe that a combination of Kelvin, Rossby-gravity, inertia-gravity, and smaller-scale gravity waves provide most of the flux of momentum needed to help drive the QBO .These waves propagate vertically and interact with the QBO having originated in the tropical troposphere. Tropical waves are significantly generated by convection. Modes are formed through lateral propagation, refraction, and reflection within an equatorial waveguide, the extent on a plane of which, which depends on the properties of the different waves. "Equatorward propagating waves originating outside the tropics, such as planetary Rossby waves from the winter hemisphere, may have some influence in upper levels of the QBO [Ortland, 1997]. The lower region of the QBO (---20-23 km) near the equator is relatively well shielded from the intrusion of extratropical planetary waves [O'Sullivan, 1997]. Vertically propagating waves relevant to the QBO are either those with slow vertical group propagation undergoing absorption (due to radiative or mechanical damping) at such a rate that their momentum is deposited at QBO altitudes, or those with fast vertical group propagation up to a critical level lying within the range of QBO wind speeds [Dunkerton, 1997]." Vertically at which momentum is mostly left depends on the vertical group velocity.  Waves with very slow group propagation are confined within a few kilometres of the tropopause, whereas the other hand, waves with fast vertical group velocity and with phase speeds lying outside the range of QBO wind speeds propagate more or less transparently through the QBO." Long-period waves tend to dominate spectra of horizontal wind and temperature. However, higher-frequency waves contribute more to momentum fluxes than might be expected from consideration of temperature alone. We can organize the waves relevant to the QBO into three categories: (1) Kelvin and Rossby-gravity waves, which are equatorially trapped; periods of •>3 days; wave numbers 1-4 (zonal wavelengths •>10,000 km); (2) inertia-gravity waves, which may or may not be equatorially trapped; periods of ---1-3 days; wave numbers ---4-40 (zonal wavelengths --• 1000-10,000 km); and (3) gravity waves; periods of 40 (zonal wavelengths --•10-1000 km) propagating rapidly in the vertical. (Waves with very short horizontal wavelengths <•10 km tend to be trapped vertically at tropospheric levels near the altitude where they are forced and are not believed to play a significant role in middle atmosphere dynamics.)" When reviewing the information, intermediate and high-frequency waves can help to drive the QBO. With the wave momentum flux spectrum there is still caution to be had, with regard to actual values of flux and the relative contribution from various parts of the spectrum. Momentum flux in mesoscale waves is locally very large, although it is necessary to know the spatial and temporal distribution of these wave to determine their position in terms to role within the QBO." Available observations are insufficient for this purpose. For intermediate-scale waves, it is unclear what fraction of the waves is important to the QBO without a more precise estimate of their phase speeds, modal structure, and absorption characteristics. Twice-daily rawinsondes provide an accurate picture of vertical structure but have poor horizontal and temporal coverage. Their description of horizontal structure is inadequate, and temporal aliasing may occur, obscuring the true frequency of the waves. The QBO, in principle, depends on wave driving from the entire tropical belt, but the observing network can only sample a small fraction of horizontal area and time. Thus it is uncertain how to translate the information from local observations of intermediate and small-scale waves into a useful estimate of QBO wave driving on a global scale. Ultimately, satellite observations will provide the needed coverage in space and time. These observations have already proven useful for planetary scale equatorial waves and small-scale extratropical gravity waves with deep vertical wavelength. Significant improvement in the vertical resolution of satellite instruments and their ability to measure or infer horizontal wind components will be necessary, however, before such observations are quantitatively useful for estimates of momentum flux due to intermediate and small-scale waves in the QBO region."
Kelvin and Rossby-gravity waves

The identification of equatorial modes is pretty easy in regions with good area coverage  so that the actual propagation may be observed. "Long records of rawinsonde data from high-quality stations have been used to derive seasonal and QBO related variations of Kelvin and Rossby-gravity wave activity near the equator [Maruyama, 1991; Dunkerton, 199 lb, 1993; Shiotani and Horinouchi, 1993; Sato et al., 1994; Wikle et al., 1997]. The QBO variation of Kelvin wave activity observed in fluctuations of zonal wind and temperature is consistent with the expected amplification of these waves in descending westerly shear zones." Yearly variation of Rossby-gravity wave activity is observed in the lowermost equatorial stratosphere and may help to explain the observed seasonal variation of QBO onsets near 50 hPa [Dunkerton, 1990]. Equatorially trapped waves have been observed in temperature and trace constituent data obtained from various satellite instruments. Most of these studies dealt with waves in the upper stratosphere relevant to the stratopause semiannual oscillation (SAO); a few, however, also observed waves in the equatorial lower stratosphere relevant to the QBO [e.g., Salby et al., 1984 Randel, 1990; Ziemke and Stanford, 1994; Canziani et al., 1995; Kawamoto et al., 1997; Shiotani et al., 1997; Mote et al., 1998; Canziani and Holton, 1998]. It is difficult to detect the weak, shallow temperature signals associated with vertically propagating equatorial waves, and satellite sampling usually recovers only the lowest zonal wave numbers (e.g., waves 1-6). Nevertheless, satellite observations are valuable for their global view, complementing the irregular sampling of the rawinsonde network. Two-dimensional modeling studies [Gray and Pyle, 1989; Dunkerton, 1991a, 1997] showed that Kelvin and Rossby-gravity waves are insufficient to account for the required vertical flux of momentum to drive the QBO. The required momentum flux is much larger than was previously assumed because the tropical stratospheric air moves upward with the Brewer-Dobson circulation. When realistic equatorial upwelling is included in models, the required total wave flux for a realistic QBO is 2-4 times as large as that of the observed large-scale, long-period Kelvin and Rossby-gravity waves. Threedimensional simulations [e.g., Takahashi and Boville, 1992; Hayashi and Golder, 1994; Takahashi, 1996] described in section 3.3.2 confirm the need for additional wave fluxes. Therefore it is necessary to understand better from observations the morphology of smallerscale inertia-gravity and gravity waves and their possible role in the QBO.

inertia-gravity waves

"Eastward propagating equatorial inertia-gravity waves are seen in westerly shear phases of the QBO, while westward propagating waves are seen in easterly shear phases. Observational campaigns using rawinsondes have provided data with high temporal and vertical resolution, so that analysis is possible both for temporal and vertical phase variations. Cadet and Teitelbaum [1979] conducted a pioneering study on inertia-gravity waves in the equatorial region, analysing 3-hourly rawinsonde data at 8.5øN, 23.5øW during the Global Atmospheric Research Project Atlantic Tropical Experiment (GATE). The QBO was in an easterly shear phase. They detected a short vertical wavelength (•1.5 km) inertia-gravity wave-like structure having a period of 30-40 hours. The zonal phase velocity was estimated to be westward. Tsuda et al. [1994a, 1994b] conducted an observational campaign focusing on waves in the lower stratosphere at Watukosek, Indonesia (7.6øS, 112.7øE), for 24 days in February-March 1990 when the QBO was in a westerly shear phase. Wind and temperature data were obtained with a temporal interval of 6 hours and vertical resolution of 150 m. Figure 8 shows a time-height section of temperature fluctuations with periods shorter than 4 days. Clear downward phase propagation is observed in the lower stratosphere (above about 16 km altitude). The vertical wavelength is about 3 km, and the wave period is about 2 days. Similar wave structure was seen also for zonal (u) and meridional wind (v) fluctuations. The amplitudes of horizontal wind and temperature fluctuations were about 3 rn s -• and 2 K, respectively. On the basis of hodographic analysis, assuming that these fluctuations are due to plane inertia-gravity waves, Tsuda et al. [1994b] showed that most wave activity propagated eastward and upward in the lower stratosphere. Similar characteristics were observed in their second campaign, in Bandung, Indonesia (107.6øE, 6.9øS), during another westerly shear phase of the QBO (November 1992 to April 1993) [Shimizu and Tsuda, 1997]. Statistical studies of equatorial inertia-gravity waves have been made using operational rawinsonde data at Singapore (1.4øN, 104.0øE). Maruyama [1994] and Sato et al. [1994] analyzed the year-to-year variation of 1- to 3-day wave activity in the lower stratosphere using data from Singapore spanning 10 years. Extraction of waves by their periods is useful since the ground-based wave frequency is invariant during the wave propagation in a steady background field. The QBO can be considered sufficiently steady for these purposes for inertia-gravity waves having periods shorter than several days"

Effects in the extratropical stratosphere

Connection between the QBO and the extratropical atmosphere must be viewed in the context of the seasonal cycle and variability of the extratropical stratosphere. Compared with the troposphere, the circulation in the extratropical stratosphere seasonal reversal is much stronger as an actual reversal of winds from winter to summer. During the winter season the high-latitude stratosphere cools (naturally), forming a deep, strong westerly vortex typically. Therefore strong westerlies are replaced by easterlies whereby increasing solar heating in the spring and summer." In both hemispheres the smoothly varying seasonal cycle described above is modified by the effects of planetary Rossby waves (here in after referred to simply as planetary waves) which are forced in part by land-sea contrasts and surface topography. These waves propagate vertically and meridionally into the winter stratosphere (Plate 2), but are evanescent in the mean easterly winds of the summer hemisphere [Charney and Drazin, 1961; Andrews et al., 1987]. The NH has much greater land-sea contrast and larger mountain ranges than the SH, resulting in larger amplitude tropospheric planetary waves. Consequently, the northern winter stratosphere tends to be much more disturbed by planetary waves than the southern winter stratosphere. Large-amplitude waves can rapidly disrupt the northern polar vortex, even in midwinter, replacing westerly winds with easterly winds in high latitudes and causing the polar stratosphere to dramatically warm. Such events are called major stratospheric warmings. The transition from westerlies to easterlies in the springtime usually occurs in conjunction with a planetary wave event and is called the final warming. In the NH the timing of the final warming is highly variable and tends to occur during March or April. In the SH the final warming occurs in November and December, with less interannual variability [Waugh and Randel, 1999]. In the NH the planetary wave amplitudes are just large enough for midwinter sudden warmings to occur during some years but not others. Thus the northern stratosphere is sensitive to the effects of vertically propagating planetary waves, resulting in large interannual variability in the strength of the polar vortex. It appears that this sensitivity to the upward and equatorward propagation of planetary waves allows the equatorial QBO to influence the polar stratosphere by modulating the flux of wave activity or Eliassen-Palm flux [e.g., Dunkerton and Baldwin, 1991]. The definitive identification of an extratropical QBO signal has been difficult due to the brevity and limited height range of data sets. In the NH, data up to 10 hPa appear to be reliable beginning in the 1950s. Above the 10-hPa level, and in the SH lower stratosphere, the lack of rawinsonde coverage has limited the production of reliable gridded data to the period beginning in the late 1970s, when satellite temperature retrievals began. Most of the literature on the extratropical influence of the QBO has focused on the NH simply because the data record is longer and more reliable. Part of the difficulty in identifying a NH QBO signal is that the QBO accounts for only a fraction of the variance. In addition to the variability of tropospheric forcing, other signals, such as the l 1-year solar cycle, volcanic eruptions, and sea surface temperature anomalies, appear to influence the variability of the extratropical stratosphere. Holton and Tan [1980, 1982] presented strong evidence that the QBO influences the extratropical northern stratøsphere by using gridded data for 16 NH winters (1962-1977) to form easterly and westerly phase composites of 50-hPa geopotential. They showed that geopotential height at high latitudes is significantly lower during the westerly phase of the QBO. They also found a statistically significant modulation of the springtime zonal wind in the SH. In the NH, Labitzke [1987] and Labitzke and van Loon [1988] found a strong relation to the l 1-year solar cycle during January and February, suggesting that solar influence modifies the signal during late winter. Naito and Hirota [1997] confirmed their findings and also showed that a robust OBO signal is present during November and December."

Interaction of the QBO With Other Low-Frequency Signals

"The extratropical QBO signal may be identified statistically in a long data record, but it is only part of the large interannual variability of the northern winter stratosphere. Several other signals contribute to variability or interact with the QBO signal to produce other frequencies of variability in the observed data record. BaMwin and Tung [1994] showed that the QBO modulates the extratropical annual cycle signal so that the signature of the QBO in angular momentum, rather than having only a single spectral frequency peak at •28 months, includes two additional spectral peaks at the annual frequency plus or minus the QBO frequency. These studies demonstrated that the "three-peak QBO" spectrum [Tung and Yang, 1994a] can be expected from the Holton-Tan effect acting to modulate the annual cycle. Tung and Yang considered a harmonic with the period of the QBO that acts to modulate a signal consisting of an annual mean plus a sinusoid with an annual period. The combined signal of the QBO in the extratropics together with the annual cycle can be represented mathematically as 

Studies using time series of stratospheric temperatures [e.g., Salby et al., 1997] suggest that low-frequency variability of the middle and upper stratosphere includes a biennial mode with a period of exactly 24 months. Such a purely biennial signal cannot be the result of quasibiennial forcing. Salby et al. as well as BaMwin and Dunkerton [1998a] speculated that a biennial mode might propagate into the stratosphere from the upper troposphere. It is unclear why a biennial mode, which may be found in the troposphere, would be amplified to become important in the polar stratosphere. Baldwin and Dunkerton could find no explanation for such a biennial mode and noted that the statistical significance of the biennial spectral peak is not high; the mode may simply be an artifact of using a short (32-year) data record that happened to have biennial variability. They noted that the Holton-Tan mechanism would tend to make polar anomalies in potential vorticity (PV) change sign from year to year. This tendency, together with random chance, could account for the observed biennial mode. If this interpretation is correct, then the biennial mode, in all likelihood, will not continue. Several researchers have considered that remote effects from E1 Nifio-Southern Oscillation (ENSO) could influence the extratropical stratosphere. Such an influence could masquerade as a QBO signal, or at least be difficult to separate from a QBO signal. Wallace and Chang [1982] were unable to separate the effects of ENSO and the QBO on the tropical stratosphere in 21 winters of NH 30-hPa geopotential. Van Loon and Labitzke [1987] also found that the phases of the QBO and ENSO tended to coincide. By removing cold and warm ENSO years (keeping years only with weak ENSO anomalies), they displayed results similar to those of Holton and Tan. Subsequent observational studies [e.g., Hamilton, 1993; Baldwin and O'Sullivan, 1995] and modeling [Hamilton, 1995] show a consistent picture in which the influence of ENSO on the zonal-mean structure of the vortex is largely confined to the troposphere. In the lower stratosphere, ENSO appears to modulate the amplitudes of large-scale stationary waves. Decadal variability, possibly related to the l 1-year solar cycle, clearly exists in data records which began in the 1950s. Labitzke [1987] and Labitzke and van Loon [1988] studied the observed late-winter NH circulation classified by both the level of solar activity and the QBO phase. They found a strong relation to the solar cycle during late winter. Naito and Hirota [1997] confirmed this relationship and found that early winter is dominated by a robust QBO signal. Figure 19 summarizes the solar-QBO results as scatterplots of mean 30-hPa geopotential heights during January and February above the North Pole versus 10.7-cm solar radio flux (a proxy for the l 1-year cycle in solar activity). The data set can be grouped into four categories based on the QBO phase and solar activity level. In years with low solar activity the polar winter vortex tends to be disturbed and weak when the QBO is easterly, but deeper and undisturbed when the QBO is westerly. In years with strong solar activity, however, westerly phases of the QBO are associated with disturbed winters, whereas easterly phases of the QBO are accompanied by deep and undisturbed polar vortices. Hence the QBO acts as predicted by Holton and Tan [1980] in years with low solar activity but appears to reverse its behavior during years with high solar activIty. Only two cases do not fit this scheme: 1989 and 1997. It is the subject of active debate whether or not decadal variability is caused by the l 1-year solar cycle, but there is increasing evidence through modelling that the solar cycle has a significant influence on winds and temperatures in the upper stratosphere."

Figure 14



30


31


Effect of the QBO on the Extratropical Troposphere

"The QBO, by modulating the wave guide for vertically propagating planetary waves, affects the circulation of the extratropical winter stratosphere. This modulation is more easily seen in the NH, where wave amplitudes are larger and the stratospheric circulation is disrupted by major warmings. Figure 14 showed that modulation of the zonal wind by the QBO in the NH in January extends below the tropopause. Angell and Korshover [1975] showed a strong correlation between Balboa 50-hPa zonal wind and the displacement of the northern vortex at 300 hPa, near the tropopause. The surface signature of the QBO was first examined by Holton and Tan [1980], who showed the difference between 1000-hPa geopotential for the two phases of the QBO. An update of Holton and Tan's calculation, for 1964-1996 data, is shown in Figure 31. The pattern is characterized by modulation of the strength of the polar vortex and anomalies of opposite sign at low to middle latitudes. The pattern in Figure 31 is similar to that shown by Holton and Tan. Hamilton [1998b], in a 48-year GCM simulation with an imposed QBO, found that the QBO composite difference in the strength of the upper tropospheric polar vortex, while small, was statistically significant. There is increasing evidence from observations, numerical models, and conceptual models that stratospheric anomalies do indeed influence the troposphere. (It is not necessary to limit our discussion to the influence of the QBO, but to think of any circulation anomaly in the stratosphere: for example, due to solar influence, the QBO, a volcanic eruption, etc.) Boville [1984] showed, using a GCM, that a change in the high-latitude zonal wind structure in the stratosphere introduced changes in the zonal-mean flow down to the Earth's surface, as well as in planetary wave structures. He concluded that the degree of trapping of planetary waves in the troposphere is determined by the strength and structure of the stratospheric zonal-mean wind, resulting in sensitivity of the troposphere to the stratospheric zonal wind structure. Boville [1986] explained further that when high-latitude winds in the lower stratosphere are strong, they tend to inhibit vertical propagation of wave activity into the polar stratosphere. If winds are weak, on the other hand, wave activity can propagate more effectively into the polar stratosphere. The process was found to be tightly coupled to the tropospheric generation of vertically propagating planetary waves. Kodera et al. [1990] used both observations and a GCM to show that anomalies in the midlatitude upper stratosphere (1 hPa) in December tend to move poleward and downward, reaching the troposphere approximately 2 months later. In general, these effects can be understood in terms of the modification of the zonalmean zonal wind, which acts as a waveguide for planetary wave propagation. Stratospheric anomalies tend to induce changes in wave propagation at lower levels, which affect the convergence of the waves, which further modifies the zonal-mean flow. Over time, the net effect appears as a downward and poleward movement of anomalies. A complementary approach to understanding the downward link to the troposphere has been to examine "modes of variability." Such modes may be thought of as patterns which tend to recur and which account for a large fraction of variance; patterns should be robust and be found through different analysis schemes. For example, the NH winter zonal wind tends to vary in a dipole pattern (Figure 14). This coupled mode of variability between the northern winter stratosphere and troposphere was discussed by Nigam [1990], who examined rotated EOFs of zonal-mean zonal wind. Nigam's result showed that the dominant mode of variability in zonalmean wind appears as a deep north-south dipole with a node near 40ø-45øN (similar to Figure 14). The poleward part of the dipole represents fluctuations in the strength of the polar vortex. Coupling between the stratosphere and troposphere was further explored by Baldwin et al. [1994], who examined geopotential patterns in the middle troposphere linked to the stratosphere. Using singular value decomposition (also called maximum covariance analysis) between 500-hPa geopotential and zonal-mean wind, they showed that the leading mode had a strong dipole signature in zonal-mean wind, extending from the surface to above 10 hPa. The north-south dipole mode accounts for a large fraction of the variance in zonal wind and is found by a variety of techniques. The leading mode of variability of the northern extratropical troposphere/stratosphere is characterized by a deep, zonally symmetric or "annular" structure [Thompson and Wallace, 2000]. This dipole mode in zonal-mean zonal wind is coupled to a horizontal wave structure of geopotential anomalies in the troposphere. The surface pattern resembles the North Atlantic Oscillation, but is more symmetric in longitude. Thompson and Wallace [1998] showed that the surface pattern corresponds to the leading EOF of wintertime monthlymean sea level pressure. The mode, at any level, is known as the Northern Annular Mode (NAM). The surface NAM pattern is also known as the Arctic Oscillation [Thompson and Wallace, 1998] and is broadly similar to the QBO signature shown in Figure 31, suggesting that the QBO may act to modulate the NAM. It is now becoming clear that all studies of modes of NH variability produce patterns that are, in essence, slight variants of the NAM. The NAM represents a dominant, robust, naturally occurring mode of variability, and if the QBO can affect the NAM in the stratosphere, it can be expected that there would also be a surface signature of the QBO. The NAM is closely linked to stratospheric sudden warmings [Baldwin and Dunkerton, 1999], and every major warming shows a clear signature in the magnitude of the stratospheric NAM. This relationship can be expected because both phenomena, in the stratosphere, relate to the strength of the polar vortex. As the strength of the stratospheric polar vortex changes, the surface NAM signature tends to vary. Baldwin and Dunkerton examined this relationship and demonstrated that large, sustained variations in the strength of the stratospheric polar vortex tend to propagate downward to the Earth's surface. The time to propagate from 10 hPa to the surface was found to vary, averaging about 3 weeks. They also examined the relationship between the QBO and the NAM, which was found to be strongest during December in the middle stratosphere and weaker as winter progressed. The QBO is one of several factors that influences the NAM, modulating the strength of the polar vortex from the lower mesosphere to the Earth's surface."

Conclusion
"In a paper summarizing work on the then recently discovered QBO, Reed [1967, p. 393] stated Perhaps a simple explanation will soon be found, and what now seems an intriguing mystery will be relegated to the category of a meteorological freak. Or perhaps the phenomenon will prove to have a greater significance than we now might envisage, either because of some intrinsic property it possesses or because of its effect on other related areas of research. With the benefit of more than 3 decades of QBO related research, we can now say assuredly that the QBO is more than a meteorological "Outlier" Indeed, as demonstrated in this review, the QBO has a much larger role beyond that envisaged in the 1960s, both for its inherent fluid dynamical characteristics and its relevance to issues of global atmospheric chemistry and climate. The QBO is a spectacular demonstration of the role of wave, mean-flow interactions in the fluid dynamics of a rotating stratified atmosphere. As elegantly argued by Mcintyre [1993], what makes the dynamics of a rotating stratified atmosphere special is the ubiquitous occurrence of wave motions and the fact that wave propagation and refraction are generally accompanied by a transport of momentum. The QBO would not exist were it not for momentum transfer by wave propagation and refraction. The dependence of wave refraction on the mean flow provides the mechanism whereby wave-induced momentum fluxes in the equatorial stratosphere can produce a feedback onto the mean flow. In the QBO, not only do the oscillating waves interact with the mean flow to produce a flow rectification, but also the rectified flow itself oscillates on a period completely different from that of the driving waves. Plate 1 shows that the QBO (which in the 1960s was regarded by some as most likely a transient phenomenon) is a persistent feature of the circulation of the equatorial stratosphere. We have observed directly 20 full cycles of the oscillation, and there is indirect evidence covering a much longer time. By study of longterm variations in the solar semidiurnal tidal signal in surface pressure at equatorial stations (which is sensitive to the zonal winds in the stratosphere), Hamilton [1983] and Teitelbaum et al. [1995] argued that the QBO must have existed for at least the past 120 years. This robust nature of the QBO suggests that similar phenomena should be present on other planets with rotating stratified atmospheres and equatorial convection zones. Indeed, an analogous oscillation, the quasiquadrennial oscillation (QQO), has been documented in the equatorial atmosphere of Jupiter [Leovy et al., 1991; Friedson, 1999]. The observed meridional scale of the QQO on Jupiter (---7 ø) is about half that of the terrestrial QBO. For the parameters given by Friedson, this is consistent with the transition scale discussed in section 3.1, provided that the vertical scale of forcing is set at 12 km, rather than the 4-km value appropriate for the terrestrial QBO. Mcintyre [1994] suggested that a similar oscillation may occur in the solar interior. The possibility of broader implications of the QBO for other areas of research, as suggested by Reed [1967] in the above quotation, has certainly proved to be true. As discussed in section 6.2, the influence of the QBO on interannual climate variations in the extratropical troposphere and stratosphere is a major subject of current interest. Attempts to better understand and predict trends and variability of atmospheric ozone require careful consideration of direct and indirect effects of the equatorial QBO on the ozone layer (see section 5). Thus models of interannual climate variability and of global stratospheric chemistry both should include the effects of the QBO either explicitly or through some parameterization. Unfortunately, simulation of the QBO remains a great challenge for general circulation models. Such models are currently being used for prediction of climate trends and variability associated with human-induced changes in the concentrations of various greenhouse gases. Yet, as discussed in section 3.3.2, many models are unable to spontaneously generate a realistic QBO. The atmosphere, however, has no such difficulties. A sceptic might argue that the absence of such a robust global scale dynamical phenomenon demonstrates that models are still far from reality. It would be more accurate to say that the QBO places stringent requirements on a numerical model, requiring an accurate computational method, fine spatial resolution, and low diffusion. The need for accurate parameterization of subgrid-scale momentum fluxes is also clear. The arguments presented above suggest that gravity waves generated by equatorial convection are essential to forcing of the QBO. This implies that among other things, better modelling of the dynamics of mesoscale convective systems, and the synoptic-scale tropical waves in which these systems are embedded, is required if GCMs are to routinely reproduce the fascinating wave, mean-flow feedback interactions that result in the equatorial OBO."
My conclusion

 
Simply put, the QBO is the variations of wind around the equator and it fluctuates on average every 28 months (about 2 and a half years) as the standard deviation between easterly and westerly winds. The last few easterly QBO were in the years 2016-2018, 2012-2013 and 2009-10. Other significant easterly QBO’s were 1986-87 along with many of the 80s and 90s years, although this years easterly doesn’t look much at all like those Winter’s which their QBO’s easterly phase began much earlier and so the QBO phases were much stronger for their Winter’s with the natural Polar Vortex changes across the different seasons. This year seems to be the most like 2012/13 but also has links with the 2010 easterly and the 2018 QBO. Each phase happens at different times, typically the beginning of the phases a high zonal originates and for easterlies then there seems to be a transition phase before the end is a sliding easterly up and down which is what brings the actual ‘easterly’ people talk about. These easterly QBO's typically originate a year or two before the actual event per se, such as the 2010 'event' easterly winds began in 2009 and can be traced back further. When looking at zonal winds to understand it easier you must first understand the nature between these that there is a significantly stronger downward propagation in the westerly phase whereas in the easterly phase there is a longer prolonging of this phase along with. You can see here pronounced signals within the different QBO signals and phases, the tropical area of the QBO is in thermal wind balance with the shear of the zonal winds.

To understand Rossby waves first we must see how it is formed. Rossby waves, formed when tropical air is moving poleward and polar air moves towards the equator. Therefore because of the difference in temperature between the Equator and the poles due to the difference in the amount of solar heating received, heat tends to flow from low to high latitudes. This is attained in part, by these air movements. Rossby waves are also the principal component of the Ferrel circulation. Heat is transferred to the poleward by the tropical waves and polar air absorbs heat as it moves toward the equator. The existence of the Rossby waves also happen to explain cyclones and anticyclones. Atmospheric Rossby waves  result from the conservation of the potential vorticity and are affected by the Coriolis force and the pressure gradient.  Rotation causes fluid to move towards the right as they move in the  Northern hemisphere and to the left as they move in the Southern hemisphere. For example, a fluid moving from the equator toward the north pole will deviate towards the east whereas the fluid movie moving from the north towards the equations will deviate towards the west. These types of deviations are generated by Coriolis force and conservation of potential vorticity which gives rise to change in relative vorticity. This is similar to the conservation of angular momentum.  In the Planetary atmosphere, including Earth, the Rossby waves are caused due to changes in the Coriolis effect with latitude. The Jet stream is a current of fast-moving air that is generally many thousand miles ago and is relatively thin. The jet streams are found in the upper layer of the atmosphere at the tropopause- the boundary between the stratosphere and the troposphere. The changing of the jet stream is known as these Rossby waves.  The Rossby waves were first observed by Carl- Gustaf Rossby hence the name Rossby waves.

The different phases of the wave is simply a change of pattern within Oscillation, so phase velocity simply refers to the movement of these patterns. Most of the time Oscillations occur as a result of overlapping waves with a band of different wave numbers, one idea is that "as a stone falls into a tank of water, a circular ring-wave is generated in the water, which expands in the radial direction and spreads outward" much like Oscillation within weather and describes the QBO pretty well.

"Momentum flux is the vertical flux of horizontal momentum, equal to the force per unit area, or stress. The Reynolds stress can be determined from the covariance of the fluctuations of the horizontal  and verticalwind components, eddy-correlation techniques, or an indirect determination can be made using Monin–Obukhov similarity flux-profile relationships (
also called universal functions). The relation between the velocity scale (friction velocity u*) and the momentum flux is Momentum flux can be associated with either mean velocity components, internal gravity waves, or with turbulent velocity fluctuations. For turbulence, the momentum flux is also called the Reynolds stress. For waves, it is related to mountain wave drag
So then finally to link this all together
The dependence of waves on the mean flow provides the mechanism whereby momentum fluxes in the equatorial stratosphere can produce a feed onto the flow.  Within the QBO, oscillating waves interact with the mean flow to produce a flow but also the flow itself oscillates in periods completely different from that of the driving waves."

I'm not sure whether I should put this in the Stratosphere thread but seeing as this deals with so many levels of the atmosphere and a lot to do with understanding the models right now I've left it here for now, hopefully this helps if you don't understand the QBO fully here
I know that most of this is quotation just so you can get my references and you've got the most important parts of the original though and so a lot the scientific bits have been left there because there's not a lot you can change in them. 
I can't believe how long it took me to write this but I got there eventually
Most of my Sources

Quasi-Biennial Oscillation (QBO)

WWW.METOFFICE.GOV.UK

---

The Quasi-Biennial Oscillation (QBO) is a regular variation of the winds that blow high above the equator.

https://www2.physics.ox.ac.uk/sites/default/files/2011-10-15/how_does_the_quasi_biennial_oscillation_affect_the_41176.pdf
My head hurts
Xander

 

[Snowman.]

Such a nasty feature.

Would hate to be a fish in that.

[mountsbaysnow]

1 minute ago, Snowman. said:

Such a nasty feature.

Would hate to be a fish in that.

A Michael Fish.

 

[weirpig]

2 minutes ago, Snowman. said:

Such a nasty feature.

Would hate to be a fish in that.

Ha ha

[Mike Poole]

Just one more and I’m out of here for a bit, but check out the wolf on GFS T132:

But the nascent Atlantic ridge and the heights in N Scandi are what starts the big UK-Ural high.  And at the moment the two eyes of the wolf, i.e. the trop vortex, are the same size.  

[Stu_London]

Got to hope the GFS is over-egging this storm for next week.

Would be horrendous for rush hour on Wednesday morning with the winds on the southern flank.

Probably not quite October 87 - but would be a big headline maker, all the same.

 

[Don]

48 minutes ago, Mike Poole said:

 

Goodbye until something new and interesting crops up!

Regards

Mike

Oops, that went too early, I haven’t put a GFS chart in!  Think I’ll leave it though, wouldn’t change the message!

Hmm, not sure you will be away that long Mike?!

[MATTWOLVES 3]

33 minutes ago, Eagle Eye said:

Here's to thinking that combining La Nina, the QBO, this year's SSW and current models in one post (later) is a good idea

Alright so I'll be explaining what I've read about the QBO here and it's effect on models, La Nina will be when I have the time

Also feel free to correct any mistakes, this is just interferences from what I've read and also quoted from what I've read a lot of it

Most of the explanations will be direct quotes from the texts before the two conclusions one of which is mine here goes :
All models and their references for the first part




Part 1

Introduction

At the time that the QBO (Quasi Biennial zonal wind Oscillation) was discovered, there were no observations of tropical atmospheric waves and there were absolutely no theories predicting their existence. “The search for an explanation for the QBO initially involved a variety of causes: some internal feedback mechanism, a natural period of atmospheric oscillation, an external process, or some combination of these mechanisms.” None of these attempts explained features such as the downward propagation and maintenance of the amplitude of the QBO (hence the increase in energy density) as it descends. Forcing by zonally asymmetric waves is needed to explain the equatorial westerly wind maximum. “Wallace and Holton tried to drive the QBO in a numerical model through heat sources or through extratropical planetary-scale waves propagating toward the equator. They showed conclusively that lateral momentum transfer by planetary waves could not explain the downward propagation of the QBO without loss of amplitude. Booker and Bretherton's [1967] paper on the absorption of gravity waves at a critical level sparked that, that would lead to an understanding of how the QBO is driven. It was Lindzen's leap of insight to realize that vertically propagating gravity waves could supply the necessary wave forcing for which of the QBO.  "

I'll be honest I didn't know anything about zonal wind before this so it's best to just directly quote the whole paragraph
 

Zonal Wind  

“A composite of the QBO in equatorial zonal winds (Figure 1) [Pawson et al., 1993] shows faster and more regular downward propagation of the westerly phase and the stronger intensity and longer duration of the easterly phase. The mean period of the QBO for data during 1953-1995 is 28.2 months (about 2 and a half years), slightly longer than the 27.7 months (about 2 and a half years) obtained from the shorter record of Naujokat [1986]. The standard deviation about the composite QBO is also included in Figure 1, showing maxima in variability close to the descending easterly and westerly shear zones (larger for the westerly phase). This reflects deviations in the duration of each phase. Dunkerton [1990] showed that the QBO may be synchronized to the annual cycle, showing that the onset of the easterly regime at 50 hPa tends to occur during NH late spring or summer. His analysis is updated in Figure 2, which shows the onset of each wind regime at 50 hPa. The easterly and westerly transitions both show a strong preference to occur during April- June. The latitudinal structure of the QBO in zonal wind is shown in Figure 3, derived from long time series of wind observations at many tropical ß stations [Dunkerton and Delisi, 1985]. The amplitude of the QBO is latitudinally symmetric, and the maximum is centered over the equator, with a meridional half width of approximately 12 ø. Similar QBO structure is derived from assimilated meteorological analyses, but the amplitude is often underestimated in comparison with rawinsonde measurements [Pawson and Fiorino, 1998; Randel et al., 1999]. Plate 2 supplies an overview of the QBO, its sources, and its global dynamical effects, as well as a foundation for the discussion of the details of the QBO in the following sections. The diagram spans the troposphere, stratosphere, and mesosphere from pole to pole and shows schematically the differences in zonal wind between the 40-hPa easterly and westerly phases of the OBO. Convection in the tropical troposphere, ranging from the scale of mesoscale convective complexes (spanning more than 100 km) to planetary-scale phenomena, produces a broad spectrum of waves (orange wavy arrows), including gravity, inertia-gravity, Kelvin, and Rossby gravity waves (see section 3). These waves, with a variety of vertical and horizontal wavelengths and phase speeds, propagate into the stratosphere, transporting easterly and westerly zonal momentum. Most of this zonal momentum is deposited at stratospheric levels, driving the zonal wind anomalies of the QBO. For each wave, the vertical profile of the zonal wind decides the critical level at or below which the momentum is deposited. The critical levels for these waves depend, in part, on the shear zones of the QBO. Some gravity waves propagate through the entire stratosphere and produce a QBO near the mesopause known as the mesospheric QBO, or MQBO (section 6). In the tropical lower stratosphere the time-averaged wind speeds are small, so the easterly minus westerly composite in Plate 2 is similar in appearance to the actual winds during the easterly phase of the QBO. At high latitudes, there is a pronounced annual cycle, with strong westerly winds during the winter season. To the north of the equator in the lower stratosphere, tropical winds alter the effective waveguide for upward and equatorward propagating planetary-scale waves (curved purple arrows). The effect of the zonal wind structure in the easterly phase of the QBO is to focus more wave activity toward the pole, where the waves converge and slow the zonal-mean flow. Thus the polar vortex north of --•45øN shows weaker westerly winds (or easterly anomaly, shown in light blue). The high-latitude wind anomalies penetrate the troposphere and provide a mechanism for the QBO to have a small influence on tropospheric weather patterns (section 6).” 
Part 2

References

Temperature and Meridional Circulation

The QBO exhibits a clear signature in temperature, with strong signals in both the tropics and extra-tropics. The tropical temperature QBO is in a thermal wind balance with the vertical shear of the zonal winds, which is expressed for the equatorial [•-plane as
""

The equatorial temperature anomalies with the QBO in the lower stratosphere are of the order of plus/minus 4 Kelvin, maximizing around 30-50 hPa. Figure 4 compares time series (after subtraction of the seasonal cycle) of 30-hPa temperature measurements at Singapore corresponding  with the zonal wind upwards shear in the 30 to 50-hPa level, showing correlation (see Plate 1).  Smaller anomalies expand down, with QBO variations of the order being plus/minus 0.5 Kelvin near the tropopause [Angell and Korshover, 1964]. These QBO temperature anomalies extend into the middle and upper stratosphere, whereby they are different with the lower stratosphere anomalies. Figure 5 in an example of temperature anomalies that are associated with an easterly phase of the QBO during the NH winter in 1994. Although these data quite possible underestimate the magnitude of the temperature QBO, the out-of-phase vertical structure is a robust feature also observed in long time records of satellite radiance measurements [Randel et al., 1999]. A big aspect of extratropical temperature anomalies is that they are seasonally synchronized and occur mainly in Winter and into spring in each hemisphere. Column ozone measurements are nearly identical whilst also being seasonally synchronised,  one key aspect of the global QBO is extratropical variability. Low-frequency temperature anomalies are closely linked with variations in the meridional circulation, global circulation patterns associated with the QBO are also highly unbalanced at solstice. Furthermore, temperature patterns show signals in both polar regions and maximize in spring in each hemisphere. "The modulation by the QBO of zonal-mean wind (Plate 1) is coupled to modulation of the zonally averaged mean meridional circulation. The climatological circulation is characterized by large-scale ascent in the tropics, broad poleward transport in the stratosphere, and compensating sinking through the extratropical tropopause [Holton et al., 1995]. The transport of chemical trace species into, within, and out of the stratosphere is the result of both large-scale circulations and mixing processes associated with waves. Chemical processes, such as those resulting in ozone depletion, not only depend on the concentrations of trace species, but may also depend critically on temperature. Since the QBO modulates the global stratospheric circulation, including the polar regions, an understanding of the effects of the QBO not only on dynamics and temperature but also on the distribution of trace species is essential in order to understand global climate variability and change. Many long-lived trace species, such as N20 and CH4, originate in the troposphere and are transported into the stratosphere through the tropical tropopause. "


Part 3

References



QBO Mechanism

"Since the QBO is approximately longitudinally symmetric [Belmont and Dartt, 1968], it is natural to try to explain it within a model that considers the dynamics of a longitudinally symmetric atmosphere. In a rotating atmosphere the temperature and wind fields are closely coupled, and correspondingly, both heating or mechanical forcing (i.e., forcing in the momentum equations) can give rise to a velocity response." As noted in part 1 the current view is that mechanical forcing, provided by wave momentum fluxes, is essential for the QBO, the coupling between temperature and wind fields is and must be  taken into account when you are trying to explain many aspects of the structure. The mechanism for the oscillation in essence can be demonstrated in just simply the representation of interaction of upward propagating gravity waves with a background flow that is itself a function of height [Plumb, 1977]. "Consider two discrete upward propagating internal gravity waves, forced at a lower boundary with identical amplitudes and equal but opposite zonal phase speeds. The waves are assumed to be quasi-linear (interacting with the mean flow, but not with each other), steady, hydrostatic, unaffected by rotation, and subject to linear damping. The superposition of these waves corresponds exactly to a single "standing" wave. As each wave component propagates vertically, its amplitude is diminished by damping, generating a force on the mean flow due to convergence of the vertical flux of zonal momentum. This force locally accelerates the mean flow in the direction of the dominant wave's zonal phase propagation. The momentum flux convergence depends on the rate of upward propagation and hence on the vertical structure of zonal-mean wind. With waves of equal amplitude but opposite phase speed, zero mean flow is a possible equilibrium, but unless vertical diffusion is strong, it is an unstable equilibrium; any small deviation from zero will inevitably grow with time. Plumb [1977] showed that the zonal-mean wind anomalies descend in time. Each wave propagates vertically until its group velocity is slowed, and the wave is damped as it encounters a shear zone. When the easterly jet decays, the westward wave moves to upper levels of the atmosphere and a new easterly shear zone forms aloft it. The  eventual time period of the oscillation is determined although within others, by the eastward and westward momentum flux at the lower boundary and by the amount of atmospheric mass affected by the waves. "In Plumb's [1977] Boussinesq formulation the QBO period is inversely proportional to momentum flux. The same is true in a quasi-compressible atmosphere, but the decrease in atmospheric density with height results in a substantially shorter period. Simple representation such as Plumb's capture the essential wave mean-flow interaction mechanism leading the QBO. However, they cannot explain why the QBO is an equatorial phenomenon (notwithstanding its important links to the extratropics). One reason the QBO is equatorially confined may be that it is driven by equatorially trapped waves. However, it is also possible that the QBO is driven by additional waves and is confined near the equator for another, more fundamental, reason. Some simple insights on this point come from considering the equations for the evolution of a longitudinally symmetric atmosphere subject to mechanical forcing. A suitable set of model equations for such a longitudinally symmetric atmosphere is as follows:

"
Waves in the Tropical Lower Stratosphere
There are many different types of waves in the tropics which contribute to the Quasi-Bilateral Oscillation. There is a combination of evidence to believe that a combination of Kelvin, Rossby-gravity, inertia-gravity, and smaller-scale gravity waves provide most of the flux of momentum needed to help drive the QBO .These waves propagate vertically and interact with the QBO having originated in the tropical troposphere. Tropical waves are significantly generated by convection. Modes are formed through lateral propagation, refraction, and reflection within an equatorial waveguide, the extent on a plane of which, which depends on the properties of the different waves. "Equatorward propagating waves originating outside the tropics, such as planetary Rossby waves from the winter hemisphere, may have some influence in upper levels of the QBO [Ortland, 1997]. The lower region of the QBO (---20-23 km) near the equator is relatively well shielded from the intrusion of extratropical planetary waves [O'Sullivan, 1997]. Vertically propagating waves relevant to the QBO are either those with slow vertical group propagation undergoing absorption (due to radiative or mechanical damping) at such a rate that their momentum is deposited at QBO altitudes, or those with fast vertical group propagation up to a critical level lying within the range of QBO wind speeds [Dunkerton, 1997]." Vertically at which momentum is mostly left depends on the vertical group velocity.  Waves with very slow group propagation are confined within a few kilometres of the tropopause, whereas the other hand, waves with fast vertical group velocity and with phase speeds lying outside the range of QBO wind speeds propagate more or less transparently through the QBO." Long-period waves tend to dominate spectra of horizontal wind and temperature. However, higher-frequency waves contribute more to momentum fluxes than might be expected from consideration of temperature alone. We can organize the waves relevant to the QBO into three categories: (1) Kelvin and Rossby-gravity waves, which are equatorially trapped; periods of •>3 days; wave numbers 1-4 (zonal wavelengths •>10,000 km); (2) inertia-gravity waves, which may or may not be equatorially trapped; periods of ---1-3 days; wave numbers ---4-40 (zonal wavelengths --• 1000-10,000 km); and (3) gravity waves; periods of 40 (zonal wavelengths --•10-1000 km) propagating rapidly in the vertical. (Waves with very short horizontal wavelengths <•10 km tend to be trapped vertically at tropospheric levels near the altitude where they are forced and are not believed to play a significant role in middle atmosphere dynamics.)" When reviewing the information, intermediate and high-frequency waves can help to drive the QBO. With the wave momentum flux spectrum there is still caution to be had, with regard to actual values of flux and the relative contribution from various parts of the spectrum. Momentum flux in mesoscale waves is locally very large, although it is necessary to know the spatial and temporal distribution of these wave to determine their position in terms to role within the QBO." Available observations are insufficient for this purpose. For intermediate-scale waves, it is unclear what fraction of the waves is important to the QBO without a more precise estimate of their phase speeds, modal structure, and absorption characteristics. Twice-daily rawinsondes provide an accurate picture of vertical structure but have poor horizontal and temporal coverage. Their description of horizontal structure is inadequate, and temporal aliasing may occur, obscuring the true frequency of the waves. The QBO, in principle, depends on wave driving from the entire tropical belt, but the observing network can only sample a small fraction of horizontal area and time. Thus it is uncertain how to translate the information from local observations of intermediate and small-scale waves into a useful estimate of QBO wave driving on a global scale. Ultimately, satellite observations will provide the needed coverage in space and time. These observations have already proven useful for planetary scale equatorial waves and small-scale extratropical gravity waves with deep vertical wavelength. Significant improvement in the vertical resolution of satellite instruments and their ability to measure or infer horizontal wind components will be necessary, however, before such observations are quantitatively useful for estimates of momentum flux due to intermediate and small-scale waves in the QBO region."
Kelvin and Rossby-gravity waves

The identification of equatorial modes is pretty easy in regions with good area coverage  so that the actual propagation may be observed. "Long records of rawinsonde data from high-quality stations have been used to derive seasonal and QBO related variations of Kelvin and Rossby-gravity wave activity near the equator [Maruyama, 1991; Dunkerton, 199 lb, 1993; Shiotani and Horinouchi, 1993; Sato et al., 1994; Wikle et al., 1997]. The QBO variation of Kelvin wave activity observed in fluctuations of zonal wind and temperature is consistent with the expected amplification of these waves in descending westerly shear zones." Yearly variation of Rossby-gravity wave activity is observed in the lowermost equatorial stratosphere and may help to explain the observed seasonal variation of QBO onsets near 50 hPa [Dunkerton, 1990]. Equatorially trapped waves have been observed in temperature and trace constituent data obtained from various satellite instruments. Most of these studies dealt with waves in the upper stratosphere relevant to the stratopause semiannual oscillation (SAO); a few, however, also observed waves in the equatorial lower stratosphere relevant to the QBO [e.g., Salby et al., 1984 Randel, 1990; Ziemke and Stanford, 1994; Canziani et al., 1995; Kawamoto et al., 1997; Shiotani et al., 1997; Mote et al., 1998; Canziani and Holton, 1998]. It is difficult to detect the weak, shallow temperature signals associated with vertically propagating equatorial waves, and satellite sampling usually recovers only the lowest zonal wave numbers (e.g., waves 1-6). Nevertheless, satellite observations are valuable for their global view, complementing the irregular sampling of the rawinsonde network. Two-dimensional modeling studies [Gray and Pyle, 1989; Dunkerton, 1991a, 1997] showed that Kelvin and Rossby-gravity waves are insufficient to account for the required vertical flux of momentum to drive the QBO. The required momentum flux is much larger than was previously assumed because the tropical stratospheric air moves upward with the Brewer-Dobson circulation. When realistic equatorial upwelling is included in models, the required total wave flux for a realistic QBO is 2-4 times as large as that of the observed large-scale, long-period Kelvin and Rossby-gravity waves. Threedimensional simulations [e.g., Takahashi and Boville, 1992; Hayashi and Golder, 1994; Takahashi, 1996] described in section 3.3.2 confirm the need for additional wave fluxes. Therefore it is necessary to understand better from observations the morphology of smallerscale inertia-gravity and gravity waves and their possible role in the QBO.

inertia-gravity waves

"Eastward propagating equatorial inertia-gravity waves are seen in westerly shear phases of the QBO, while westward propagating waves are seen in easterly shear phases. Observational campaigns using rawinsondes have provided data with high temporal and vertical resolution, so that analysis is possible both for temporal and vertical phase variations. Cadet and Teitelbaum [1979] conducted a pioneering study on inertia-gravity waves in the equatorial region, analysing 3-hourly rawinsonde data at 8.5øN, 23.5øW during the Global Atmospheric Research Project Atlantic Tropical Experiment (GATE). The QBO was in an easterly shear phase. They detected a short vertical wavelength (•1.5 km) inertia-gravity wave-like structure having a period of 30-40 hours. The zonal phase velocity was estimated to be westward. Tsuda et al. [1994a, 1994b] conducted an observational campaign focusing on waves in the lower stratosphere at Watukosek, Indonesia (7.6øS, 112.7øE), for 24 days in February-March 1990 when the QBO was in a westerly shear phase. Wind and temperature data were obtained with a temporal interval of 6 hours and vertical resolution of 150 m. Figure 8 shows a time-height section of temperature fluctuations with periods shorter than 4 days. Clear downward phase propagation is observed in the lower stratosphere (above about 16 km altitude). The vertical wavelength is about 3 km, and the wave period is about 2 days. Similar wave structure was seen also for zonal (u) and meridional wind (v) fluctuations. The amplitudes of horizontal wind and temperature fluctuations were about 3 rn s -• and 2 K, respectively. On the basis of hodographic analysis, assuming that these fluctuations are due to plane inertia-gravity waves, Tsuda et al. [1994b] showed that most wave activity propagated eastward and upward in the lower stratosphere. Similar characteristics were observed in their second campaign, in Bandung, Indonesia (107.6øE, 6.9øS), during another westerly shear phase of the QBO (November 1992 to April 1993) [Shimizu and Tsuda, 1997]. Statistical studies of equatorial inertia-gravity waves have been made using operational rawinsonde data at Singapore (1.4øN, 104.0øE). Maruyama [1994] and Sato et al. [1994] analyzed the year-to-year variation of 1- to 3-day wave activity in the lower stratosphere using data from Singapore spanning 10 years. Extraction of waves by their periods is useful since the ground-based wave frequency is invariant during the wave propagation in a steady background field. The QBO can be considered sufficiently steady for these purposes for inertia-gravity waves having periods shorter than several days"

Effects in the extratropical stratosphere

Connection between the QBO and the extratropical atmosphere must be viewed in the context of the seasonal cycle and variability of the extratropical stratosphere. Compared with the troposphere, the circulation in the extratropical stratosphere seasonal reversal is much stronger as an actual reversal of winds from winter to summer. During the winter season the high-latitude stratosphere cools (naturally), forming a deep, strong westerly vortex typically. Therefore strong westerlies are replaced by easterlies whereby increasing solar heating in the spring and summer." In both hemispheres the smoothly varying seasonal cycle described above is modified by the effects of planetary Rossby waves (here in after referred to simply as planetary waves) which are forced in part by land-sea contrasts and surface topography. These waves propagate vertically and meridionally into the winter stratosphere (Plate 2), but are evanescent in the mean easterly winds of the summer hemisphere [Charney and Drazin, 1961; Andrews et al., 1987]. The NH has much greater land-sea contrast and larger mountain ranges than the SH, resulting in larger amplitude tropospheric planetary waves. Consequently, the northern winter stratosphere tends to be much more disturbed by planetary waves than the southern winter stratosphere. Large-amplitude waves can rapidly disrupt the northern polar vortex, even in midwinter, replacing westerly winds with easterly winds in high latitudes and causing the polar stratosphere to dramatically warm. Such events are called major stratospheric warmings. The transition from westerlies to easterlies in the springtime usually occurs in conjunction with a planetary wave event and is called the final warming. In the NH the timing of the final warming is highly variable and tends to occur during March or April. In the SH the final warming occurs in November and December, with less interannual variability [Waugh and Randel, 1999]. In the NH the planetary wave amplitudes are just large enough for midwinter sudden warmings to occur during some years but not others. Thus the northern stratosphere is sensitive to the effects of vertically propagating planetary waves, resulting in large interannual variability in the strength of the polar vortex. It appears that this sensitivity to the upward and equatorward propagation of planetary waves allows the equatorial QBO to influence the polar stratosphere by modulating the flux of wave activity or Eliassen-Palm flux [e.g., Dunkerton and Baldwin, 1991]. The definitive identification of an extratropical QBO signal has been difficult due to the brevity and limited height range of data sets. In the NH, data up to 10 hPa appear to be reliable beginning in the 1950s. Above the 10-hPa level, and in the SH lower stratosphere, the lack of rawinsonde coverage has limited the production of reliable gridded data to the period beginning in the late 1970s, when satellite temperature retrievals began. Most of the literature on the extratropical influence of the QBO has focused on the NH simply because the data record is longer and more reliable. Part of the difficulty in identifying a NH QBO signal is that the QBO accounts for only a fraction of the variance. In addition to the variability of tropospheric forcing, other signals, such as the l 1-year solar cycle, volcanic eruptions, and sea surface temperature anomalies, appear to influence the variability of the extratropical stratosphere. Holton and Tan [1980, 1982] presented strong evidence that the QBO influences the extratropical northern stratøsphere by using gridded data for 16 NH winters (1962-1977) to form easterly and westerly phase composites of 50-hPa geopotential. They showed that geopotential height at high latitudes is significantly lower during the westerly phase of the QBO. They also found a statistically significant modulation of the springtime zonal wind in the SH. In the NH, Labitzke [1987] and Labitzke and van Loon [1988] found a strong relation to the l 1-year solar cycle during January and February, suggesting that solar influence modifies the signal during late winter. Naito and Hirota [1997] confirmed their findings and also showed that a robust OBO signal is present during November and December."

Interaction of the QBO With Other Low-Frequency Signals

"The extratropical QBO signal may be identified statistically in a long data record, but it is only part of the large interannual variability of the northern winter stratosphere. Several other signals contribute to variability or interact with the QBO signal to produce other frequencies of variability in the observed data record. BaMwin and Tung [1994] showed that the QBO modulates the extratropical annual cycle signal so that the signature of the QBO in angular momentum, rather than having only a single spectral frequency peak at •28 months, includes two additional spectral peaks at the annual frequency plus or minus the QBO frequency. These studies demonstrated that the "three-peak QBO" spectrum [Tung and Yang, 1994a] can be expected from the Holton-Tan effect acting to modulate the annual cycle. Tung and Yang considered a harmonic with the period of the QBO that acts to modulate a signal consisting of an annual mean plus a sinusoid with an annual period. The combined signal of the QBO in the extratropics together with the annual cycle can be represented mathematically as 

Studies using time series of stratospheric temperatures [e.g., Salby et al., 1997] suggest that low-frequency variability of the middle and upper stratosphere includes a biennial mode with a period of exactly 24 months. Such a purely biennial signal cannot be the result of quasibiennial forcing. Salby et al. as well as BaMwin and Dunkerton [1998a] speculated that a biennial mode might propagate into the stratosphere from the upper troposphere. It is unclear why a biennial mode, which may be found in the troposphere, would be amplified to become important in the polar stratosphere. Baldwin and Dunkerton could find no explanation for such a biennial mode and noted that the statistical significance of the biennial spectral peak is not high; the mode may simply be an artifact of using a short (32-year) data record that happened to have biennial variability. They noted that the Holton-Tan mechanism would tend to make polar anomalies in potential vorticity (PV) change sign from year to year. This tendency, together with random chance, could account for the observed biennial mode. If this interpretation is correct, then the biennial mode, in all likelihood, will not continue. Several researchers have considered that remote effects from E1 Nifio-Southern Oscillation (ENSO) could influence the extratropical stratosphere. Such an influence could masquerade as a QBO signal, or at least be difficult to separate from a QBO signal. Wallace and Chang [1982] were unable to separate the effects of ENSO and the QBO on the tropical stratosphere in 21 winters of NH 30-hPa geopotential. Van Loon and Labitzke [1987] also found that the phases of the QBO and ENSO tended to coincide. By removing cold and warm ENSO years (keeping years only with weak ENSO anomalies), they displayed results similar to those of Holton and Tan. Subsequent observational studies [e.g., Hamilton, 1993; Baldwin and O'Sullivan, 1995] and modeling [Hamilton, 1995] show a consistent picture in which the influence of ENSO on the zonal-mean structure of the vortex is largely confined to the troposphere. In the lower stratosphere, ENSO appears to modulate the amplitudes of large-scale stationary waves. Decadal variability, possibly related to the l 1-year solar cycle, clearly exists in data records which began in the 1950s. Labitzke [1987] and Labitzke and van Loon [1988] studied the observed late-winter NH circulation classified by both the level of solar activity and the QBO phase. They found a strong relation to the solar cycle during late winter. Naito and Hirota [1997] confirmed this relationship and found that early winter is dominated by a robust QBO signal. Figure 19 summarizes the solar-QBO results as scatterplots of mean 30-hPa geopotential heights during January and February above the North Pole versus 10.7-cm solar radio flux (a proxy for the l 1-year cycle in solar activity). The data set can be grouped into four categories based on the QBO phase and solar activity level. In years with low solar activity the polar winter vortex tends to be disturbed and weak when the QBO is easterly, but deeper and undisturbed when the QBO is westerly. In years with strong solar activity, however, westerly phases of the QBO are associated with disturbed winters, whereas easterly phases of the QBO are accompanied by deep and undisturbed polar vortices. Hence the QBO acts as predicted by Holton and Tan [1980] in years with low solar activity but appears to reverse its behavior during years with high solar activIty. Only two cases do not fit this scheme: 1989 and 1997. It is the subject of active debate whether or not decadal variability is caused by the l 1-year solar cycle, but there is increasing evidence through modelling that the solar cycle has a significant influence on winds and temperatures in the upper stratosphere."

Figure 14



30


31


Effect of the QBO on the Extratropical Troposphere

"The QBO, by modulating the wave guide for vertically propagating planetary waves, affects the circulation of the extratropical winter stratosphere. This modulation is more easily seen in the NH, where wave amplitudes are larger and the stratospheric circulation is disrupted by major warmings. Figure 14 showed that modulation of the zonal wind by the QBO in the NH in January extends below the tropopause. Angell and Korshover [1975] showed a strong correlation between Balboa 50-hPa zonal wind and the displacement of the northern vortex at 300 hPa, near the tropopause. The surface signature of the QBO was first examined by Holton and Tan [1980], who showed the difference between 1000-hPa geopotential for the two phases of the QBO. An update of Holton and Tan's calculation, for 1964-1996 data, is shown in Figure 31. The pattern is characterized by modulation of the strength of the polar vortex and anomalies of opposite sign at low to middle latitudes. The pattern in Figure 31 is similar to that shown by Holton and Tan. Hamilton [1998b], in a 48-year GCM simulation with an imposed QBO, found that the QBO composite difference in the strength of the upper tropospheric polar vortex, while small, was statistically significant. There is increasing evidence from observations, numerical models, and conceptual models that stratospheric anomalies do indeed influence the troposphere. (It is not necessary to limit our discussion to the influence of the QBO, but to think of any circulation anomaly in the stratosphere: for example, due to solar influence, the QBO, a volcanic eruption, etc.) Boville [1984] showed, using a GCM, that a change in the high-latitude zonal wind structure in the stratosphere introduced changes in the zonal-mean flow down to the Earth's surface, as well as in planetary wave structures. He concluded that the degree of trapping of planetary waves in the troposphere is determined by the strength and structure of the stratospheric zonal-mean wind, resulting in sensitivity of the troposphere to the stratospheric zonal wind structure. Boville [1986] explained further that when high-latitude winds in the lower stratosphere are strong, they tend to inhibit vertical propagation of wave activity into the polar stratosphere. If winds are weak, on the other hand, wave activity can propagate more effectively into the polar stratosphere. The process was found to be tightly coupled to the tropospheric generation of vertically propagating planetary waves. Kodera et al. [1990] used both observations and a GCM to show that anomalies in the midlatitude upper stratosphere (1 hPa) in December tend to move poleward and downward, reaching the troposphere approximately 2 months later. In general, these effects can be understood in terms of the modification of the zonalmean zonal wind, which acts as a waveguide for planetary wave propagation. Stratospheric anomalies tend to induce changes in wave propagation at lower levels, which affect the convergence of the waves, which further modifies the zonal-mean flow. Over time, the net effect appears as a downward and poleward movement of anomalies. A complementary approach to understanding the downward link to the troposphere has been to examine "modes of variability." Such modes may be thought of as patterns which tend to recur and which account for a large fraction of variance; patterns should be robust and be found through different analysis schemes. For example, the NH winter zonal wind tends to vary in a dipole pattern (Figure 14). This coupled mode of variability between the northern winter stratosphere and troposphere was discussed by Nigam [1990], who examined rotated EOFs of zonal-mean zonal wind. Nigam's result showed that the dominant mode of variability in zonalmean wind appears as a deep north-south dipole with a node near 40ø-45øN (similar to Figure 14). The poleward part of the dipole represents fluctuations in the strength of the polar vortex. Coupling between the stratosphere and troposphere was further explored by Baldwin et al. [1994], who examined geopotential patterns in the middle troposphere linked to the stratosphere. Using singular value decomposition (also called maximum covariance analysis) between 500-hPa geopotential and zonal-mean wind, they showed that the leading mode had a strong dipole signature in zonal-mean wind, extending from the surface to above 10 hPa. The north-south dipole mode accounts for a large fraction of the variance in zonal wind and is found by a variety of techniques. The leading mode of variability of the northern extratropical troposphere/stratosphere is characterized by a deep, zonally symmetric or "annular" structure [Thompson and Wallace, 2000]. This dipole mode in zonal-mean zonal wind is coupled to a horizontal wave structure of geopotential anomalies in the troposphere. The surface pattern resembles the North Atlantic Oscillation, but is more symmetric in longitude. Thompson and Wallace [1998] showed that the surface pattern corresponds to the leading EOF of wintertime monthlymean sea level pressure. The mode, at any level, is known as the Northern Annular Mode (NAM). The surface NAM pattern is also known as the Arctic Oscillation [Thompson and Wallace, 1998] and is broadly similar to the QBO signature shown in Figure 31, suggesting that the QBO may act to modulate the NAM. It is now becoming clear that all studies of modes of NH variability produce patterns that are, in essence, slight variants of the NAM. The NAM represents a dominant, robust, naturally occurring mode of variability, and if the QBO can affect the NAM in the stratosphere, it can be expected that there would also be a surface signature of the QBO. The NAM is closely linked to stratospheric sudden warmings [Baldwin and Dunkerton, 1999], and every major warming shows a clear signature in the magnitude of the stratospheric NAM. This relationship can be expected because both phenomena, in the stratosphere, relate to the strength of the polar vortex. As the strength of the stratospheric polar vortex changes, the surface NAM signature tends to vary. Baldwin and Dunkerton examined this relationship and demonstrated that large, sustained variations in the strength of the stratospheric polar vortex tend to propagate downward to the Earth's surface. The time to propagate from 10 hPa to the surface was found to vary, averaging about 3 weeks. They also examined the relationship between the QBO and the NAM, which was found to be strongest during December in the middle stratosphere and weaker as winter progressed. The QBO is one of several factors that influences the NAM, modulating the strength of the polar vortex from the lower mesosphere to the Earth's surface."

Conclusion
"In a paper summarizing work on the then recently discovered QBO, Reed [1967, p. 393] stated Perhaps a simple explanation will soon be found, and what now seems an intriguing mystery will be relegated to the category of a meteorological freak. Or perhaps the phenomenon will prove to have a greater significance than we now might envisage, either because of some intrinsic property it possesses or because of its effect on other related areas of research. With the benefit of more than 3 decades of QBO related research, we can now say assuredly that the QBO is more than a meteorological "Outlier" Indeed, as demonstrated in this review, the QBO has a much larger role beyond that envisaged in the 1960s, both for its inherent fluid dynamical characteristics and its relevance to issues of global atmospheric chemistry and climate. The QBO is a spectacular demonstration of the role of wave, mean-flow interactions in the fluid dynamics of a rotating stratified atmosphere. As elegantly argued by Mcintyre [1993], what makes the dynamics of a rotating stratified atmosphere special is the ubiquitous occurrence of wave motions and the fact that wave propagation and refraction are generally accompanied by a transport of momentum. The QBO would not exist were it not for momentum transfer by wave propagation and refraction. The dependence of wave refraction on the mean flow provides the mechanism whereby wave-induced momentum fluxes in the equatorial stratosphere can produce a feedback onto the mean flow. In the QBO, not only do the oscillating waves interact with the mean flow to produce a flow rectification, but also the rectified flow itself oscillates on a period completely different from that of the driving waves. Plate 1 shows that the QBO (which in the 1960s was regarded by some as most likely a transient phenomenon) is a persistent feature of the circulation of the equatorial stratosphere. We have observed directly 20 full cycles of the oscillation, and there is indirect evidence covering a much longer time. By study of longterm variations in the solar semidiurnal tidal signal in surface pressure at equatorial stations (which is sensitive to the zonal winds in the stratosphere), Hamilton [1983] and Teitelbaum et al. [1995] argued that the QBO must have existed for at least the past 120 years. This robust nature of the QBO suggests that similar phenomena should be present on other planets with rotating stratified atmospheres and equatorial convection zones. Indeed, an analogous oscillation, the quasiquadrennial oscillation (QQO), has been documented in the equatorial atmosphere of Jupiter [Leovy et al., 1991; Friedson, 1999]. The observed meridional scale of the QQO on Jupiter (---7 ø) is about half that of the terrestrial QBO. For the parameters given by Friedson, this is consistent with the transition scale discussed in section 3.1, provided that the vertical scale of forcing is set at 12 km, rather than the 4-km value appropriate for the terrestrial QBO. Mcintyre [1994] suggested that a similar oscillation may occur in the solar interior. The possibility of broader implications of the QBO for other areas of research, as suggested by Reed [1967] in the above quotation, has certainly proved to be true. As discussed in section 6.2, the influence of the QBO on interannual climate variations in the extratropical troposphere and stratosphere is a major subject of current interest. Attempts to better understand and predict trends and variability of atmospheric ozone require careful consideration of direct and indirect effects of the equatorial QBO on the ozone layer (see section 5). Thus models of interannual climate variability and of global stratospheric chemistry both should include the effects of the QBO either explicitly or through some parameterization. Unfortunately, simulation of the QBO remains a great challenge for general circulation models. Such models are currently being used for prediction of climate trends and variability associated with human-induced changes in the concentrations of various greenhouse gases. Yet, as discussed in section 3.3.2, many models are unable to spontaneously generate a realistic QBO. The atmosphere, however, has no such difficulties. A sceptic might argue that the absence of such a robust global scale dynamical phenomenon demonstrates that models are still far from reality. It would be more accurate to say that the QBO places stringent requirements on a numerical model, requiring an accurate computational method, fine spatial resolution, and low diffusion. The need for accurate parameterization of subgrid-scale momentum fluxes is also clear. The arguments presented above suggest that gravity waves generated by equatorial convection are essential to forcing of the QBO. This implies that among other things, better modelling of the dynamics of mesoscale convective systems, and the synoptic-scale tropical waves in which these systems are embedded, is required if GCMs are to routinely reproduce the fascinating wave, mean-flow feedback interactions that result in the equatorial OBO."
My conclusion

 
Simply put, the QBO is the variations of wind around the equator and it fluctuates on average every 28 months (about 2 and a half years) as the standard deviation between easterly and westerly winds. The last few easterly QBO were in the years 2016-2018, 2012-2013 and 2009-10. Other significant easterly QBO’s were 1986-87 along with many of the 80s and 90s years, although this years easterly doesn’t look much at all like those Winter’s which their QBO’s easterly phase began much earlier and so the QBO phases were much stronger for their Winter’s with the natural Polar Vortex changes across the different seasons. This year seems to be the most like 2012/13 but also has links with the 2010 easterly and the 2018 QBO. Each phase happens at different times, typically the beginning of the phases a high zonal originates and for easterlies then there seems to be a transition phase before the end is a sliding easterly up and down which is what brings the actual ‘easterly’ people talk about. These easterly QBO's typically originate a year or two before the actual event per se, such as the 2010 'event' easterly winds began in 2009 and can be traced back further. When looking at zonal winds to understand it easier you must first understand the nature between these that there is a significantly stronger downward propagation in the westerly phase whereas in the easterly phase there is a longer prolonging of this phase along with. You can see here pronounced signals within the different QBO signals and phases, the tropical area of the QBO is in thermal wind balance with the shear of the zonal winds.

To understand Rossby waves first we must see how it is formed. Rossby waves, formed when tropical air is moving poleward and polar air moves towards the equator. Therefore because of the difference in temperature between the Equator and the poles due to the difference in the amount of solar heating received, heat tends to flow from low to high latitudes. This is attained in part, by these air movements. Rossby waves are also the principal component of the Ferrel circulation. Heat is transferred to the poleward by the tropical waves and polar air absorbs heat as it moves toward the equator. The existence of the Rossby waves also happen to explain cyclones and anticyclones. Atmospheric Rossby waves  result from the conservation of the potential vorticity and are affected by the Coriolis force and the pressure gradient.  Rotation causes fluid to move towards the right as they move in the  Northern hemisphere and to the left as they move in the Southern hemisphere. For example, a fluid moving from the equator toward the north pole will deviate towards the east whereas the fluid movie moving from the north towards the equations will deviate towards the west. These types of deviations are generated by Coriolis force and conservation of potential vorticity which gives rise to change in relative vorticity. This is similar to the conservation of angular momentum.  In the Planetary atmosphere, including Earth, the Rossby waves are caused due to changes in the Coriolis effect with latitude. The Jet stream is a current of fast-moving air that is generally many thousand miles ago and is relatively thin. The jet streams are found in the upper layer of the atmosphere at the tropopause- the boundary between the stratosphere and the troposphere. The changing of the jet stream is known as these Rossby waves.  The Rossby waves were first observed by Carl- Gustaf Rossby hence the name Rossby waves.

The different phases of the wave is simply a change of pattern within Oscillation, so phase velocity simply refers to the movement of these patterns. Most of the time Oscillations occur as a result of overlapping waves with a band of different wave numbers, one idea is that "as a stone falls into a tank of water, a circular ring-wave is generated in the water, which expands in the radial direction and spreads outward" much like Oscillation within weather and describes the QBO pretty well.

"Momentum flux is the vertical flux of horizontal momentum, equal to the force per unit area, or stress. The Reynolds stress can be determined from the covariance of the fluctuations of the horizontal  and verticalwind components, eddy-correlation techniques, or an indirect determination can be made using Monin–Obukhov similarity flux-profile relationships (
also called universal functions). The relation between the velocity scale (friction velocity u*) and the momentum flux is Momentum flux can be associated with either mean velocity components, internal gravity waves, or with turbulent velocity fluctuations. For turbulence, the momentum flux is also called the Reynolds stress. For waves, it is related to mountain wave drag
So then finally to link this all together
The dependence of waves on the mean flow provides the mechanism whereby momentum fluxes in the equatorial stratosphere can produce a feed onto the flow.  Within the QBO, oscillating waves interact with the mean flow to produce a flow but also the flow itself oscillates in periods completely different from that of the driving waves."

I'm not sure whether I should put this in the Stratosphere thread but seeing as this deals with so many levels of the atmosphere and a lot to do with understanding the models right now I've left it here for now, hopefully this helps if you don't understand the QBO fully here
I know that most of this is quotation just so you can get my references and you've got the most important parts of the original though and so a lot the scientific bits have been left there because there's not a lot you can change in them. 
I can't believe how long it took me to write this but I got there eventually
Most of my Sources

Quasi-Biennial Oscillation (QBO)

WWW.METOFFICE.GOV.UK

---

The Quasi-Biennial Oscillation (QBO) is a regular variation of the winds that blow high above the equator.

https://www2.physics.ox.ac.uk/sites/default/files/2011-10-15/how_does_the_quasi_biennial_oscillation_affect_the_41176.pdf
My head hurts
Xander

 

When I've managed to read this I will be asking you a few questions...should be sometime next Christmas.

Excellent stuff Xander...

[Mike Poole]

Just now, Don said:

Hmm, not sure you will be away that long Mike?!

No, correct, I wasn’t

[swfc]

On 02/12/2021 at 22:00, Eagle Eye said:

Here's to thinking that combining La Nina, the QBO, this year's SSW and current models in one post (later) is a good idea

Alright so I'll be explaining what I've read about the QBO here and it's effect on models, La Nina will be when I have the time

Also feel free to correct any mistakes, this is just interferences from what I've read and also quoted from what I've read a lot of it

Most of the explanations will be direct quotes from the texts before the two conclusions one of which is mine here goes :
All models and their references for the first part




Part 1

Introduction

At the time that the QBO (Quasi Biennial zonal wind Oscillation) was discovered, there were no observations of tropical atmospheric waves and there were absolutely no theories predicting their existence. “The search for an explanation for the QBO initially involved a variety of causes: some internal feedback mechanism, a natural period of atmospheric oscillation, an external process, or some combination of these mechanisms.” None of these attempts explained features such as the downward propagation and maintenance of the amplitude of the QBO (hence the increase in energy density) as it descends. Forcing by zonally asymmetric waves is needed to explain the equatorial westerly wind maximum. “Wallace and Holton tried to drive the QBO in a numerical model through heat sources or through extratropical planetary-scale waves propagating toward the equator. They showed conclusively that lateral momentum transfer by planetary waves could not explain the downward propagation of the QBO without loss of amplitude. Booker and Bretherton's [1967] paper on the absorption of gravity waves at a critical level sparked that, that would lead to an understanding of how the QBO is driven. It was Lindzen's leap of insight to realize that vertically propagating gravity waves could supply the necessary wave forcing for which of the QBO.  "

I'll be honest I didn't know anything about zonal wind before this so it's best to just directly quote the whole paragraph
 

Zonal Wind  

“A composite of the QBO in equatorial zonal winds (Figure 1) [Pawson et al., 1993] shows faster and more regular downward propagation of the westerly phase and the stronger intensity and longer duration of the easterly phase. The mean period of the QBO for data during 1953-1995 is 28.2 months (about 2 and a half years), slightly longer than the 27.7 months (about 2 and a half years) obtained from the shorter record of Naujokat [1986]. The standard deviation about the composite QBO is also included in Figure 1, showing maxima in variability close to the descending easterly and westerly shear zones (larger for the westerly phase). This reflects deviations in the duration of each phase. Dunkerton [1990] showed that the QBO may be synchronized to the annual cycle, showing that the onset of the easterly regime at 50 hPa tends to occur during NH late spring or summer. His analysis is updated in Figure 2, which shows the onset of each wind regime at 50 hPa. The easterly and westerly transitions both show a strong preference to occur during April- June. The latitudinal structure of the QBO in zonal wind is shown in Figure 3, derived from long time series of wind observations at many tropical ß stations [Dunkerton and Delisi, 1985]. The amplitude of the QBO is latitudinally symmetric, and the maximum is centered over the equator, with a meridional half width of approximately 12 ø. Similar QBO structure is derived from assimilated meteorological analyses, but the amplitude is often underestimated in comparison with rawinsonde measurements [Pawson and Fiorino, 1998; Randel et al., 1999]. Plate 2 supplies an overview of the QBO, its sources, and its global dynamical effects, as well as a foundation for the discussion of the details of the QBO in the following sections. The diagram spans the troposphere, stratosphere, and mesosphere from pole to pole and shows schematically the differences in zonal wind between the 40-hPa easterly and westerly phases of the OBO. Convection in the tropical troposphere, ranging from the scale of mesoscale convective complexes (spanning more than 100 km) to planetary-scale phenomena, produces a broad spectrum of waves (orange wavy arrows), including gravity, inertia-gravity, Kelvin, and Rossby gravity waves (see section 3). These waves, with a variety of vertical and horizontal wavelengths and phase speeds, propagate into the stratosphere, transporting easterly and westerly zonal momentum. Most of this zonal momentum is deposited at stratospheric levels, driving the zonal wind anomalies of the QBO. For each wave, the vertical profile of the zonal wind decides the critical level at or below which the momentum is deposited. The critical levels for these waves depend, in part, on the shear zones of the QBO. Some gravity waves propagate through the entire stratosphere and produce a QBO near the mesopause known as the mesospheric QBO, or MQBO (section 6). In the tropical lower stratosphere the time-averaged wind speeds are small, so the easterly minus westerly composite in Plate 2 is similar in appearance to the actual winds during the easterly phase of the QBO. At high latitudes, there is a pronounced annual cycle, with strong westerly winds during the winter season. To the north of the equator in the lower stratosphere, tropical winds alter the effective waveguide for upward and equatorward propagating planetary-scale waves (curved purple arrows). The effect of the zonal wind structure in the easterly phase of the QBO is to focus more wave activity toward the pole, where the waves converge and slow the zonal-mean flow. Thus the polar vortex north of --•45øN shows weaker westerly winds (or easterly anomaly, shown in light blue). The high-latitude wind anomalies penetrate the troposphere and provide a mechanism for the QBO to have a small influence on tropospheric weather patterns (section 6).” 
Part 2

References

Temperature and Meridional Circulation

The QBO exhibits a clear signature in temperature, with strong signals in both the tropics and extra-tropics. The tropical temperature QBO is in a thermal wind balance with the vertical shear of the zonal winds, which is expressed for the equatorial [•-plane as
""

The equatorial temperature anomalies with the QBO in the lower stratosphere are of the order of plus/minus 4 Kelvin, maximizing around 30-50 hPa. Figure 4 compares time series (after subtraction of the seasonal cycle) of 30-hPa temperature measurements at Singapore corresponding  with the zonal wind upwards shear in the 30 to 50-hPa level, showing correlation (see Plate 1).  Smaller anomalies expand down, with QBO variations of the order being plus/minus 0.5 Kelvin near the tropopause [Angell and Korshover, 1964]. These QBO temperature anomalies extend into the middle and upper stratosphere, whereby they are different with the lower stratosphere anomalies. Figure 5 in an example of temperature anomalies that are associated with an easterly phase of the QBO during the NH winter in 1994. Although these data quite possible underestimate the magnitude of the temperature QBO, the out-of-phase vertical structure is a robust feature also observed in long time records of satellite radiance measurements [Randel et al., 1999]. A big aspect of extratropical temperature anomalies is that they are seasonally synchronized and occur mainly in Winter and into spring in each hemisphere. Column ozone measurements are nearly identical whilst also being seasonally synchronised,  one key aspect of the global QBO is extratropical variability. Low-frequency temperature anomalies are closely linked with variations in the meridional circulation, global circulation patterns associated with the QBO are also highly unbalanced at solstice. Furthermore, temperature patterns show signals in both polar regions and maximize in spring in each hemisphere. "The modulation by the QBO of zonal-mean wind (Plate 1) is coupled to modulation of the zonally averaged mean meridional circulation. The climatological circulation is characterized by large-scale ascent in the tropics, broad poleward transport in the stratosphere, and compensating sinking through the extratropical tropopause [Holton et al., 1995]. The transport of chemical trace species into, within, and out of the stratosphere is the result of both large-scale circulations and mixing processes associated with waves. Chemical processes, such as those resulting in ozone depletion, not only depend on the concentrations of trace species, but may also depend critically on temperature. Since the QBO modulates the global stratospheric circulation, including the polar regions, an understanding of the effects of the QBO not only on dynamics and temperature but also on the distribution of trace species is essential in order to understand global climate variability and change. Many long-lived trace species, such as N20 and CH4, originate in the troposphere and are transported into the stratosphere through the tropical tropopause. "


Part 3

References



QBO Mechanism

"Since the QBO is approximately longitudinally symmetric [Belmont and Dartt, 1968], it is natural to try to explain it within a model that considers the dynamics of a longitudinally symmetric atmosphere. In a rotating atmosphere the temperature and wind fields are closely coupled, and correspondingly, both heating or mechanical forcing (i.e., forcing in the momentum equations) can give rise to a velocity response." As noted in part 1 the current view is that mechanical forcing, provided by wave momentum fluxes, is essential for the QBO, the coupling between temperature and wind fields is and must be  taken into account when you are trying to explain many aspects of the structure. The mechanism for the oscillation in essence can be demonstrated in just simply the representation of interaction of upward propagating gravity waves with a background flow that is itself a function of height [Plumb, 1977]. "Consider two discrete upward propagating internal gravity waves, forced at a lower boundary with identical amplitudes and equal but opposite zonal phase speeds. The waves are assumed to be quasi-linear (interacting with the mean flow, but not with each other), steady, hydrostatic, unaffected by rotation, and subject to linear damping. The superposition of these waves corresponds exactly to a single "standing" wave. As each wave component propagates vertically, its amplitude is diminished by damping, generating a force on the mean flow due to convergence of the vertical flux of zonal momentum. This force locally accelerates the mean flow in the direction of the dominant wave's zonal phase propagation. The momentum flux convergence depends on the rate of upward propagation and hence on the vertical structure of zonal-mean wind. With waves of equal amplitude but opposite phase speed, zero mean flow is a possible equilibrium, but unless vertical diffusion is strong, it is an unstable equilibrium; any small deviation from zero will inevitably grow with time. Plumb [1977] showed that the zonal-mean wind anomalies descend in time. Each wave propagates vertically until its group velocity is slowed, and the wave is damped as it encounters a shear zone. When the easterly jet decays, the westward wave moves to upper levels of the atmosphere and a new easterly shear zone forms aloft it. The  eventual time period of the oscillation is determined although within others, by the eastward and westward momentum flux at the lower boundary and by the amount of atmospheric mass affected by the waves. "In Plumb's [1977] Boussinesq formulation the QBO period is inversely proportional to momentum flux. The same is true in a quasi-compressible atmosphere, but the decrease in atmospheric density with height results in a substantially shorter period. Simple representation such as Plumb's capture the essential wave mean-flow interaction mechanism leading the QBO. However, they cannot explain why the QBO is an equatorial phenomenon (notwithstanding its important links to the extratropics). One reason the QBO is equatorially confined may be that it is driven by equatorially trapped waves. However, it is also possible that the QBO is driven by additional waves and is confined near the equator for another, more fundamental, reason. Some simple insights on this point come from considering the equations for the evolution of a longitudinally symmetric atmosphere subject to mechanical forcing. A suitable set of model equations for such a longitudinally symmetric atmosphere is as follows:

"
Waves in the Tropical Lower Stratosphere
There are many different types of waves in the tropics which contribute to the Quasi-Bilateral Oscillation. There is a combination of evidence to believe that a combination of Kelvin, Rossby-gravity, inertia-gravity, and smaller-scale gravity waves provide most of the flux of momentum needed to help drive the QBO .These waves propagate vertically and interact with the QBO having originated in the tropical troposphere. Tropical waves are significantly generated by convection. Modes are formed through lateral propagation, refraction, and reflection within an equatorial waveguide, the extent on a plane of which, which depends on the properties of the different waves. "Equatorward propagating waves originating outside the tropics, such as planetary Rossby waves from the winter hemisphere, may have some influence in upper levels of the QBO [Ortland, 1997]. The lower region of the QBO (---20-23 km) near the equator is relatively well shielded from the intrusion of extratropical planetary waves [O'Sullivan, 1997]. Vertically propagating waves relevant to the QBO are either those with slow vertical group propagation undergoing absorption (due to radiative or mechanical damping) at such a rate that their momentum is deposited at QBO altitudes, or those with fast vertical group propagation up to a critical level lying within the range of QBO wind speeds [Dunkerton, 1997]." Vertically at which momentum is mostly left depends on the vertical group velocity.  Waves with very slow group propagation are confined within a few kilometres of the tropopause, whereas the other hand, waves with fast vertical group velocity and with phase speeds lying outside the range of QBO wind speeds propagate more or less transparently through the QBO." Long-period waves tend to dominate spectra of horizontal wind and temperature. However, higher-frequency waves contribute more to momentum fluxes than might be expected from consideration of temperature alone. We can organize the waves relevant to the QBO into three categories: (1) Kelvin and Rossby-gravity waves, which are equatorially trapped; periods of •>3 days; wave numbers 1-4 (zonal wavelengths •>10,000 km); (2) inertia-gravity waves, which may or may not be equatorially trapped; periods of ---1-3 days; wave numbers ---4-40 (zonal wavelengths --• 1000-10,000 km); and (3) gravity waves; periods of 40 (zonal wavelengths --•10-1000 km) propagating rapidly in the vertical. (Waves with very short horizontal wavelengths <•10 km tend to be trapped vertically at tropospheric levels near the altitude where they are forced and are not believed to play a significant role in middle atmosphere dynamics.)" When reviewing the information, intermediate and high-frequency waves can help to drive the QBO. With the wave momentum flux spectrum there is still caution to be had, with regard to actual values of flux and the relative contribution from various parts of the spectrum. Momentum flux in mesoscale waves is locally very large, although it is necessary to know the spatial and temporal distribution of these wave to determine their position in terms to role within the QBO." Available observations are insufficient for this purpose. For intermediate-scale waves, it is unclear what fraction of the waves is important to the QBO without a more precise estimate of their phase speeds, modal structure, and absorption characteristics. Twice-daily rawinsondes provide an accurate picture of vertical structure but have poor horizontal and temporal coverage. Their description of horizontal structure is inadequate, and temporal aliasing may occur, obscuring the true frequency of the waves. The QBO, in principle, depends on wave driving from the entire tropical belt, but the observing network can only sample a small fraction of horizontal area and time. Thus it is uncertain how to translate the information from local observations of intermediate and small-scale waves into a useful estimate of QBO wave driving on a global scale. Ultimately, satellite observations will provide the needed coverage in space and time. These observations have already proven useful for planetary scale equatorial waves and small-scale extratropical gravity waves with deep vertical wavelength. Significant improvement in the vertical resolution of satellite instruments and their ability to measure or infer horizontal wind components will be necessary, however, before such observations are quantitatively useful for estimates of momentum flux due to intermediate and small-scale waves in the QBO region."
Kelvin and Rossby-gravity waves

The identification of equatorial modes is pretty easy in regions with good area coverage  so that the actual propagation may be observed. "Long records of rawinsonde data from high-quality stations have been used to derive seasonal and QBO related variations of Kelvin and Rossby-gravity wave activity near the equator [Maruyama, 1991; Dunkerton, 199 lb, 1993; Shiotani and Horinouchi, 1993; Sato et al., 1994; Wikle et al., 1997]. The QBO variation of Kelvin wave activity observed in fluctuations of zonal wind and temperature is consistent with the expected amplification of these waves in descending westerly shear zones." Yearly variation of Rossby-gravity wave activity is observed in the lowermost equatorial stratosphere and may help to explain the observed seasonal variation of QBO onsets near 50 hPa [Dunkerton, 1990]. Equatorially trapped waves have been observed in temperature and trace constituent data obtained from various satellite instruments. Most of these studies dealt with waves in the upper stratosphere relevant to the stratopause semiannual oscillation (SAO); a few, however, also observed waves in the equatorial lower stratosphere relevant to the QBO [e.g., Salby et al., 1984 Randel, 1990; Ziemke and Stanford, 1994; Canziani et al., 1995; Kawamoto et al., 1997; Shiotani et al., 1997; Mote et al., 1998; Canziani and Holton, 1998]. It is difficult to detect the weak, shallow temperature signals associated with vertically propagating equatorial waves, and satellite sampling usually recovers only the lowest zonal wave numbers (e.g., waves 1-6). Nevertheless, satellite observations are valuable for their global view, complementing the irregular sampling of the rawinsonde network. Two-dimensional modeling studies [Gray and Pyle, 1989; Dunkerton, 1991a, 1997] showed that Kelvin and Rossby-gravity waves are insufficient to account for the required vertical flux of momentum to drive the QBO. The required momentum flux is much larger than was previously assumed because the tropical stratospheric air moves upward with the Brewer-Dobson circulation. When realistic equatorial upwelling is included in models, the required total wave flux for a realistic QBO is 2-4 times as large as that of the observed large-scale, long-period Kelvin and Rossby-gravity waves. Threedimensional simulations [e.g., Takahashi and Boville, 1992; Hayashi and Golder, 1994; Takahashi, 1996] described in section 3.3.2 confirm the need for additional wave fluxes. Therefore it is necessary to understand better from observations the morphology of smallerscale inertia-gravity and gravity waves and their possible role in the QBO.

inertia-gravity waves

"Eastward propagating equatorial inertia-gravity waves are seen in westerly shear phases of the QBO, while westward propagating waves are seen in easterly shear phases. Observational campaigns using rawinsondes have provided data with high temporal and vertical resolution, so that analysis is possible both for temporal and vertical phase variations. Cadet and Teitelbaum [1979] conducted a pioneering study on inertia-gravity waves in the equatorial region, analysing 3-hourly rawinsonde data at 8.5øN, 23.5øW during the Global Atmospheric Research Project Atlantic Tropical Experiment (GATE). The QBO was in an easterly shear phase. They detected a short vertical wavelength (•1.5 km) inertia-gravity wave-like structure having a period of 30-40 hours. The zonal phase velocity was estimated to be westward. Tsuda et al. [1994a, 1994b] conducted an observational campaign focusing on waves in the lower stratosphere at Watukosek, Indonesia (7.6øS, 112.7øE), for 24 days in February-March 1990 when the QBO was in a westerly shear phase. Wind and temperature data were obtained with a temporal interval of 6 hours and vertical resolution of 150 m. Figure 8 shows a time-height section of temperature fluctuations with periods shorter than 4 days. Clear downward phase propagation is observed in the lower stratosphere (above about 16 km altitude). The vertical wavelength is about 3 km, and the wave period is about 2 days. Similar wave structure was seen also for zonal (u) and meridional wind (v) fluctuations. The amplitudes of horizontal wind and temperature fluctuations were about 3 rn s -• and 2 K, respectively. On the basis of hodographic analysis, assuming that these fluctuations are due to plane inertia-gravity waves, Tsuda et al. [1994b] showed that most wave activity propagated eastward and upward in the lower stratosphere. Similar characteristics were observed in their second campaign, in Bandung, Indonesia (107.6øE, 6.9øS), during another westerly shear phase of the QBO (November 1992 to April 1993) [Shimizu and Tsuda, 1997]. Statistical studies of equatorial inertia-gravity waves have been made using operational rawinsonde data at Singapore (1.4øN, 104.0øE). Maruyama [1994] and Sato et al. [1994] analyzed the year-to-year variation of 1- to 3-day wave activity in the lower stratosphere using data from Singapore spanning 10 years. Extraction of waves by their periods is useful since the ground-based wave frequency is invariant during the wave propagation in a steady background field. The QBO can be considered sufficiently steady for these purposes for inertia-gravity waves having periods shorter than several days"

Effects in the extratropical stratosphere

Connection between the QBO and the extratropical atmosphere must be viewed in the context of the seasonal cycle and variability of the extratropical stratosphere. Compared with the troposphere, the circulation in the extratropical stratosphere seasonal reversal is much stronger as an actual reversal of winds from winter to summer. During the winter season the high-latitude stratosphere cools (naturally), forming a deep, strong westerly vortex typically. Therefore strong westerlies are replaced by easterlies whereby increasing solar heating in the spring and summer." In both hemispheres the smoothly varying seasonal cycle described above is modified by the effects of planetary Rossby waves (here in after referred to simply as planetary waves) which are forced in part by land-sea contrasts and surface topography. These waves propagate vertically and meridionally into the winter stratosphere (Plate 2), but are evanescent in the mean easterly winds of the summer hemisphere [Charney and Drazin, 1961; Andrews et al., 1987]. The NH has much greater land-sea contrast and larger mountain ranges than the SH, resulting in larger amplitude tropospheric planetary waves. Consequently, the northern winter stratosphere tends to be much more disturbed by planetary waves than the southern winter stratosphere. Large-amplitude waves can rapidly disrupt the northern polar vortex, even in midwinter, replacing westerly winds with easterly winds in high latitudes and causing the polar stratosphere to dramatically warm. Such events are called major stratospheric warmings. The transition from westerlies to easterlies in the springtime usually occurs in conjunction with a planetary wave event and is called the final warming. In the NH the timing of the final warming is highly variable and tends to occur during March or April. In the SH the final warming occurs in November and December, with less interannual variability [Waugh and Randel, 1999]. In the NH the planetary wave amplitudes are just large enough for midwinter sudden warmings to occur during some years but not others. Thus the northern stratosphere is sensitive to the effects of vertically propagating planetary waves, resulting in large interannual variability in the strength of the polar vortex. It appears that this sensitivity to the upward and equatorward propagation of planetary waves allows the equatorial QBO to influence the polar stratosphere by modulating the flux of wave activity or Eliassen-Palm flux [e.g., Dunkerton and Baldwin, 1991]. The definitive identification of an extratropical QBO signal has been difficult due to the brevity and limited height range of data sets. In the NH, data up to 10 hPa appear to be reliable beginning in the 1950s. Above the 10-hPa level, and in the SH lower stratosphere, the lack of rawinsonde coverage has limited the production of reliable gridded data to the period beginning in the late 1970s, when satellite temperature retrievals began. Most of the literature on the extratropical influence of the QBO has focused on the NH simply because the data record is longer and more reliable. Part of the difficulty in identifying a NH QBO signal is that the QBO accounts for only a fraction of the variance. In addition to the variability of tropospheric forcing, other signals, such as the l 1-year solar cycle, volcanic eruptions, and sea surface temperature anomalies, appear to influence the variability of the extratropical stratosphere. Holton and Tan [1980, 1982] presented strong evidence that the QBO influences the extratropical northern stratøsphere by using gridded data for 16 NH winters (1962-1977) to form easterly and westerly phase composites of 50-hPa geopotential. They showed that geopotential height at high latitudes is significantly lower during the westerly phase of the QBO. They also found a statistically significant modulation of the springtime zonal wind in the SH. In the NH, Labitzke [1987] and Labitzke and van Loon [1988] found a strong relation to the l 1-year solar cycle during January and February, suggesting that solar influence modifies the signal during late winter. Naito and Hirota [1997] confirmed their findings and also showed that a robust OBO signal is present during November and December."

Interaction of the QBO With Other Low-Frequency Signals

"The extratropical QBO signal may be identified statistically in a long data record, but it is only part of the large interannual variability of the northern winter stratosphere. Several other signals contribute to variability or interact with the QBO signal to produce other frequencies of variability in the observed data record. BaMwin and Tung [1994] showed that the QBO modulates the extratropical annual cycle signal so that the signature of the QBO in angular momentum, rather than having only a single spectral frequency peak at •28 months, includes two additional spectral peaks at the annual frequency plus or minus the QBO frequency. These studies demonstrated that the "three-peak QBO" spectrum [Tung and Yang, 1994a] can be expected from the Holton-Tan effect acting to modulate the annual cycle. Tung and Yang considered a harmonic with the period of the QBO that acts to modulate a signal consisting of an annual mean plus a sinusoid with an annual period. The combined signal of the QBO in the extratropics together with the annual cycle can be represented mathematically as 

Studies using time series of stratospheric temperatures [e.g., Salby et al., 1997] suggest that low-frequency variability of the middle and upper stratosphere includes a biennial mode with a period of exactly 24 months. Such a purely biennial signal cannot be the result of quasibiennial forcing. Salby et al. as well as BaMwin and Dunkerton [1998a] speculated that a biennial mode might propagate into the stratosphere from the upper troposphere. It is unclear why a biennial mode, which may be found in the troposphere, would be amplified to become important in the polar stratosphere. Baldwin and Dunkerton could find no explanation for such a biennial mode and noted that the statistical significance of the biennial spectral peak is not high; the mode may simply be an artifact of using a short (32-year) data record that happened to have biennial variability. They noted that the Holton-Tan mechanism would tend to make polar anomalies in potential vorticity (PV) change sign from year to year. This tendency, together with random chance, could account for the observed biennial mode. If this interpretation is correct, then the biennial mode, in all likelihood, will not continue. Several researchers have considered that remote effects from E1 Nifio-Southern Oscillation (ENSO) could influence the extratropical stratosphere. Such an influence could masquerade as a QBO signal, or at least be difficult to separate from a QBO signal. Wallace and Chang [1982] were unable to separate the effects of ENSO and the QBO on the tropical stratosphere in 21 winters of NH 30-hPa geopotential. Van Loon and Labitzke [1987] also found that the phases of the QBO and ENSO tended to coincide. By removing cold and warm ENSO years (keeping years only with weak ENSO anomalies), they displayed results similar to those of Holton and Tan. Subsequent observational studies [e.g., Hamilton, 1993; Baldwin and O'Sullivan, 1995] and modeling [Hamilton, 1995] show a consistent picture in which the influence of ENSO on the zonal-mean structure of the vortex is largely confined to the troposphere. In the lower stratosphere, ENSO appears to modulate the amplitudes of large-scale stationary waves. Decadal variability, possibly related to the l 1-year solar cycle, clearly exists in data records which began in the 1950s. Labitzke [1987] and Labitzke and van Loon [1988] studied the observed late-winter NH circulation classified by both the level of solar activity and the QBO phase. They found a strong relation to the solar cycle during late winter. Naito and Hirota [1997] confirmed this relationship and found that early winter is dominated by a robust QBO signal. Figure 19 summarizes the solar-QBO results as scatterplots of mean 30-hPa geopotential heights during January and February above the North Pole versus 10.7-cm solar radio flux (a proxy for the l 1-year cycle in solar activity). The data set can be grouped into four categories based on the QBO phase and solar activity level. In years with low solar activity the polar winter vortex tends to be disturbed and weak when the QBO is easterly, but deeper and undisturbed when the QBO is westerly. In years with strong solar activity, however, westerly phases of the QBO are associated with disturbed winters, whereas easterly phases of the QBO are accompanied by deep and undisturbed polar vortices. Hence the QBO acts as predicted by Holton and Tan [1980] in years with low solar activity but appears to reverse its behavior during years with high solar activIty. Only two cases do not fit this scheme: 1989 and 1997. It is the subject of active debate whether or not decadal variability is caused by the l 1-year solar cycle, but there is increasing evidence through modelling that the solar cycle has a significant influence on winds and temperatures in the upper stratosphere."

Figure 14



30


31


Effect of the QBO on the Extratropical Troposphere

"The QBO, by modulating the wave guide for vertically propagating planetary waves, affects the circulation of the extratropical winter stratosphere. This modulation is more easily seen in the NH, where wave amplitudes are larger and the stratospheric circulation is disrupted by major warmings. Figure 14 showed that modulation of the zonal wind by the QBO in the NH in January extends below the tropopause. Angell and Korshover [1975] showed a strong correlation between Balboa 50-hPa zonal wind and the displacement of the northern vortex at 300 hPa, near the tropopause. The surface signature of the QBO was first examined by Holton and Tan [1980], who showed the difference between 1000-hPa geopotential for the two phases of the QBO. An update of Holton and Tan's calculation, for 1964-1996 data, is shown in Figure 31. The pattern is characterized by modulation of the strength of the polar vortex and anomalies of opposite sign at low to middle latitudes. The pattern in Figure 31 is similar to that shown by Holton and Tan. Hamilton [1998b], in a 48-year GCM simulation with an imposed QBO, found that the QBO composite difference in the strength of the upper tropospheric polar vortex, while small, was statistically significant. There is increasing evidence from observations, numerical models, and conceptual models that stratospheric anomalies do indeed influence the troposphere. (It is not necessary to limit our discussion to the influence of the QBO, but to think of any circulation anomaly in the stratosphere: for example, due to solar influence, the QBO, a volcanic eruption, etc.) Boville [1984] showed, using a GCM, that a change in the high-latitude zonal wind structure in the stratosphere introduced changes in the zonal-mean flow down to the Earth's surface, as well as in planetary wave structures. He concluded that the degree of trapping of planetary waves in the troposphere is determined by the strength and structure of the stratospheric zonal-mean wind, resulting in sensitivity of the troposphere to the stratospheric zonal wind structure. Boville [1986] explained further that when high-latitude winds in the lower stratosphere are strong, they tend to inhibit vertical propagation of wave activity into the polar stratosphere. If winds are weak, on the other hand, wave activity can propagate more effectively into the polar stratosphere. The process was found to be tightly coupled to the tropospheric generation of vertically propagating planetary waves. Kodera et al. [1990] used both observations and a GCM to show that anomalies in the midlatitude upper stratosphere (1 hPa) in December tend to move poleward and downward, reaching the troposphere approximately 2 months later. In general, these effects can be understood in terms of the modification of the zonalmean zonal wind, which acts as a waveguide for planetary wave propagation. Stratospheric anomalies tend to induce changes in wave propagation at lower levels, which affect the convergence of the waves, which further modifies the zonal-mean flow. Over time, the net effect appears as a downward and poleward movement of anomalies. A complementary approach to understanding the downward link to the troposphere has been to examine "modes of variability." Such modes may be thought of as patterns which tend to recur and which account for a large fraction of variance; patterns should be robust and be found through different analysis schemes. For example, the NH winter zonal wind tends to vary in a dipole pattern (Figure 14). This coupled mode of variability between the northern winter stratosphere and troposphere was discussed by Nigam [1990], who examined rotated EOFs of zonal-mean zonal wind. Nigam's result showed that the dominant mode of variability in zonalmean wind appears as a deep north-south dipole with a node near 40ø-45øN (similar to Figure 14). The poleward part of the dipole represents fluctuations in the strength of the polar vortex. Coupling between the stratosphere and troposphere was further explored by Baldwin et al. [1994], who examined geopotential patterns in the middle troposphere linked to the stratosphere. Using singular value decomposition (also called maximum covariance analysis) between 500-hPa geopotential and zonal-mean wind, they showed that the leading mode had a strong dipole signature in zonal-mean wind, extending from the surface to above 10 hPa. The north-south dipole mode accounts for a large fraction of the variance in zonal wind and is found by a variety of techniques. The leading mode of variability of the northern extratropical troposphere/stratosphere is characterized by a deep, zonally symmetric or "annular" structure [Thompson and Wallace, 2000]. This dipole mode in zonal-mean zonal wind is coupled to a horizontal wave structure of geopotential anomalies in the troposphere. The surface pattern resembles the North Atlantic Oscillation, but is more symmetric in longitude. Thompson and Wallace [1998] showed that the surface pattern corresponds to the leading EOF of wintertime monthlymean sea level pressure. The mode, at any level, is known as the Northern Annular Mode (NAM). The surface NAM pattern is also known as the Arctic Oscillation [Thompson and Wallace, 1998] and is broadly similar to the QBO signature shown in Figure 31, suggesting that the QBO may act to modulate the NAM. It is now becoming clear that all studies of modes of NH variability produce patterns that are, in essence, slight variants of the NAM. The NAM represents a dominant, robust, naturally occurring mode of variability, and if the QBO can affect the NAM in the stratosphere, it can be expected that there would also be a surface signature of the QBO. The NAM is closely linked to stratospheric sudden warmings [Baldwin and Dunkerton, 1999], and every major warming shows a clear signature in the magnitude of the stratospheric NAM. This relationship can be expected because both phenomena, in the stratosphere, relate to the strength of the polar vortex. As the strength of the stratospheric polar vortex changes, the surface NAM signature tends to vary. Baldwin and Dunkerton examined this relationship and demonstrated that large, sustained variations in the strength of the stratospheric polar vortex tend to propagate downward to the Earth's surface. The time to propagate from 10 hPa to the surface was found to vary, averaging about 3 weeks. They also examined the relationship between the QBO and the NAM, which was found to be strongest during December in the middle stratosphere and weaker as winter progressed. The QBO is one of several factors that influences the NAM, modulating the strength of the polar vortex from the lower mesosphere to the Earth's surface."

Conclusion
"In a paper summarizing work on the then recently discovered QBO, Reed [1967, p. 393] stated Perhaps a simple explanation will soon be found, and what now seems an intriguing mystery will be relegated to the category of a meteorological freak. Or perhaps the phenomenon will prove to have a greater significance than we now might envisage, either because of some intrinsic property it possesses or because of its effect on other related areas of research. With the benefit of more than 3 decades of QBO related research, we can now say assuredly that the QBO is more than a meteorological "Outlier" Indeed, as demonstrated in this review, the QBO has a much larger role beyond that envisaged in the 1960s, both for its inherent fluid dynamical characteristics and its relevance to issues of global atmospheric chemistry and climate. The QBO is a spectacular demonstration of the role of wave, mean-flow interactions in the fluid dynamics of a rotating stratified atmosphere. As elegantly argued by Mcintyre [1993], what makes the dynamics of a rotating stratified atmosphere special is the ubiquitous occurrence of wave motions and the fact that wave propagation and refraction are generally accompanied by a transport of momentum. The QBO would not exist were it not for momentum transfer by wave propagation and refraction. The dependence of wave refraction on the mean flow provides the mechanism whereby wave-induced momentum fluxes in the equatorial stratosphere can produce a feedback onto the mean flow. In the QBO, not only do the oscillating waves interact with the mean flow to produce a flow rectification, but also the rectified flow itself oscillates on a period completely different from that of the driving waves. Plate 1 shows that the QBO (which in the 1960s was regarded by some as most likely a transient phenomenon) is a persistent feature of the circulation of the equatorial stratosphere. We have observed directly 20 full cycles of the oscillation, and there is indirect evidence covering a much longer time. By study of longterm variations in the solar semidiurnal tidal signal in surface pressure at equatorial stations (which is sensitive to the zonal winds in the stratosphere), Hamilton [1983] and Teitelbaum et al. [1995] argued that the QBO must have existed for at least the past 120 years. This robust nature of the QBO suggests that similar phenomena should be present on other planets with rotating stratified atmospheres and equatorial convection zones. Indeed, an analogous oscillation, the quasiquadrennial oscillation (QQO), has been documented in the equatorial atmosphere of Jupiter [Leovy et al., 1991; Friedson, 1999]. The observed meridional scale of the QQO on Jupiter (---7 ø) is about half that of the terrestrial QBO. For the parameters given by Friedson, this is consistent with the transition scale discussed in section 3.1, provided that the vertical scale of forcing is set at 12 km, rather than the 4-km value appropriate for the terrestrial QBO. Mcintyre [1994] suggested that a similar oscillation may occur in the solar interior. The possibility of broader implications of the QBO for other areas of research, as suggested by Reed [1967] in the above quotation, has certainly proved to be true. As discussed in section 6.2, the influence of the QBO on interannual climate variations in the extratropical troposphere and stratosphere is a major subject of current interest. Attempts to better understand and predict trends and variability of atmospheric ozone require careful consideration of direct and indirect effects of the equatorial QBO on the ozone layer (see section 5). Thus models of interannual climate variability and of global stratospheric chemistry both should include the effects of the QBO either explicitly or through some parameterization. Unfortunately, simulation of the QBO remains a great challenge for general circulation models. Such models are currently being used for prediction of climate trends and variability associated with human-induced changes in the concentrations of various greenhouse gases. Yet, as discussed in section 3.3.2, many models are unable to spontaneously generate a realistic QBO. The atmosphere, however, has no such difficulties. A sceptic might argue that the absence of such a robust global scale dynamical phenomenon demonstrates that models are still far from reality. It would be more accurate to say that the QBO places stringent requirements on a numerical model, requiring an accurate computational method, fine spatial resolution, and low diffusion. The need for accurate parameterization of subgrid-scale momentum fluxes is also clear. The arguments presented above suggest that gravity waves generated by equatorial convection are essential to forcing of the QBO. This implies that among other things, better modelling of the dynamics of mesoscale convective systems, and the synoptic-scale tropical waves in which these systems are embedded, is required if GCMs are to routinely reproduce the fascinating wave, mean-flow feedback interactions that result in the equatorial OBO."
My conclusion

 
Simply put, the QBO is the variations of wind around the equator and it fluctuates on average every 28 months (about 2 and a half years) as the standard deviation between easterly and westerly winds. The last few easterly QBO were in the years 2016-2018, 2012-2013 and 2009-10. Other significant easterly QBO’s were 1986-87 along with many of the 80s and 90s years, although this years easterly doesn’t look much at all like those Winter’s which their QBO’s easterly phase began much earlier and so the QBO phases were much stronger for their Winter’s with the natural Polar Vortex changes across the different seasons. This year seems to be the most like 2012/13 but also has links with the 2010 easterly and the 2018 QBO. Each phase happens at different times, typically the beginning of the phases a high zonal originates and for easterlies then there seems to be a transition phase before the end is a sliding easterly up and down which is what brings the actual ‘easterly’ people talk about. These easterly QBO's typically originate a year or two before the actual event per se, such as the 2010 'event' easterly winds began in 2009 and can be traced back further. When looking at zonal winds to understand it easier you must first understand the nature between these that there is a significantly stronger downward propagation in the westerly phase whereas in the easterly phase there is a longer prolonging of this phase along with. You can see here pronounced signals within the different QBO signals and phases, the tropical area of the QBO is in thermal wind balance with the shear of the zonal winds.

To understand Rossby waves first we must see how it is formed. Rossby waves, formed when tropical air is moving poleward and polar air moves towards the equator. Therefore because of the difference in temperature between the Equator and the poles due to the difference in the amount of solar heating received, heat tends to flow from low to high latitudes. This is attained in part, by these air movements. Rossby waves are also the principal component of the Ferrel circulation. Heat is transferred to the poleward by the tropical waves and polar air absorbs heat as it moves toward the equator. The existence of the Rossby waves also happen to explain cyclones and anticyclones. Atmospheric Rossby waves  result from the conservation of the potential vorticity and are affected by the Coriolis force and the pressure gradient.  Rotation causes fluid to move towards the right as they move in the  Northern hemisphere and to the left as they move in the Southern hemisphere. For example, a fluid moving from the equator toward the north pole will deviate towards the east whereas the fluid movie moving from the north towards the equations will deviate towards the west. These types of deviations are generated by Coriolis force and conservation of potential vorticity which gives rise to change in relative vorticity. This is similar to the conservation of angular momentum.  In the Planetary atmosphere, including Earth, the Rossby waves are caused due to changes in the Coriolis effect with latitude. The Jet stream is a current of fast-moving air that is generally many thousand miles ago and is relatively thin. The jet streams are found in the upper layer of the atmosphere at the tropopause- the boundary between the stratosphere and the troposphere. The changing of the jet stream is known as these Rossby waves.  The Rossby waves were first observed by Carl- Gustaf Rossby hence the name Rossby waves.

The different phases of the wave is simply a change of pattern within Oscillation, so phase velocity simply refers to the movement of these patterns. Most of the time Oscillations occur as a result of overlapping waves with a band of different wave numbers, one idea is that "as a stone falls into a tank of water, a circular ring-wave is generated in the water, which expands in the radial direction and spreads outward" much like Oscillation within weather and describes the QBO pretty well.

"Momentum flux is the vertical flux of horizontal momentum, equal to the force per unit area, or stress. The Reynolds stress can be determined from the covariance of the fluctuations of the horizontal  and verticalwind components, eddy-correlation techniques, or an indirect determination can be made using Monin–Obukhov similarity flux-profile relationships (
also called universal functions). The relation between the velocity scale (friction velocity u*) and the momentum flux is Momentum flux can be associated with either mean velocity components, internal gravity waves, or with turbulent velocity fluctuations. For turbulence, the momentum flux is also called the Reynolds stress. For waves, it is related to mountain wave drag
So then finally to link this all together
The dependence of waves on the mean flow provides the mechanism whereby momentum fluxes in the equatorial stratosphere can produce a feed onto the flow.  Within the QBO, oscillating waves interact with the mean flow to produce a flow but also the flow itself oscillates in periods completely different from that of the driving waves."

I'm not sure whether I should put this in the Stratosphere thread but seeing as this deals with so many levels of the atmosphere and a lot to do with understanding the models right now I've left it here for now, hopefully this helps if you don't understand the QBO fully here
I know that most of this is quotation just so you can get my references and you've got the most important parts of the original though and so a lot the scientific bits have been left there because there's not a lot you can change in them. 
I can't believe how long it took me to write this but I got there eventually
Most of my Sources

Quasi-Biennial Oscillation (QBO)

WWW.METOFFICE.GOV.UK

---

The Quasi-Biennial Oscillation (QBO) is a regular variation of the winds that blow high above the equator.

https://www2.physics.ox.ac.uk/sites/default/files/2011-10-15/how_does_the_quasi_biennial_oscillation_affect_the_41176.pdf
My head hurts
Xander

 

Christ what does that mean

[damianslaw]

No comments on GFS 18z this eve... the lone ranger run.. what is it doing tonight more bizarreness.. first a low that barrels around like a washing machine and fills in situ.. then a pattern that doesn't change at all for about 7 days with a long fetch Szw flow.. odd.. then signal for major amplification with warm air advection the next run would show northern blocking.. out of nowhere... all odd. Next please..

[Purga]

GFS 18z showing some good developments at the tail end of FI.

Hopefully its reputation for picking up long distance trends proves valid!

Edited December 3, 2021 by Purga

[SLEETY]

On 02/12/2021 at 22:40, Don said:

Just another 16 or so weeks and we can relax and look forward to spring without winter chaos/dramas lol!

Seen to get most our snow and cold weather in the spring  now anyway. 

Look how cold this spring was for example, little sign now of any particularly cold weather the next two weeks mainly very unsettled weather with snow mainly on Scottish mountains is how the next couple of weeks are looking like now. 

No easterly happening now, close but no cigar again! 

[MATTWOLVES 3]

There are some cracking ens tonight...I actually feel confident of quite a shake up come mid month...if I'm wrong,so be it..The vortex looks stressed on quite a few ens...some want to build heights further North into the Arctic...WAA...The urals block pushing further stress on the strat...theres potentially alot going on,with plenty of water to go under the bridge just yet...but I like what I'm seeing.

Just to finish...3rd of December today,and guess what? Its snowing and settling...3rd time already...absolutely know need to panic....I think we could be getting many suprises this time around.

A very good night...

Edited December 3, 2021 by MATTWOLVES