Gwo And Global Angular Momentum

thanks for this thread Brick, a good learning area for quite a few of us; no doubt you will get a lot of questions, I'll certainly have some once I have time to read and absorb your post

Thanks for that good read BF. I think that you have just made global angular momentum as easy to understand as I have seen but that does stir me into wanting to know more!

Is the angular momentum measured at sea level or cumulatively throughout the whole troposphere?

Also, do we get greater levels of angular momentum in flat jet or greater amplified jet scenarios, or is this more dependent on the track of the jet and subsequent surface troughs, across land masses and mountain ranges?

Regards and a Happy New Year.

c

That slightly tricky to answer. Lets try to answer the second question first.

Fast, zonally oriented Rossby wave dispersion occurs in phase 5. (Amplified Jet)

An extended Pacific Ocean jet stream and southward shifted storm track is observed in phase 7 (strong flat jet over the pacific)

A retracted Pacific Ocean jet stream is a key feature in the total field is observed in phase 3 (Weak pacific jet)

Phase 1 has enhanced subtropical jets and closed lows in the subtropics.

Angular momentum being added usually results in a stronger jetstream I think.

As for how angular momentum is measured and what level then I think it is from the winds at all levels up to 10hPa with those at the jet stream level being important.

GP will probably have a few more details.

That slightly tricky to answer. Lets try to answer the second question first.

Fast, zonally oriented Rossby wave dispersion occurs in phase 5. (Amplified Jet)

An extended Pacific Ocean jet stream and southward shifted storm track is observed in phase 7 (strong flat jet over the pacific)

A retracted Pacific Ocean jet stream is a key feature in the total field is observed in phase 3 (Weak pacific jet)

Phase 1 has enhanced subtropical jets and closed lows in the subtropics.

Angular momentum being added usually results in a stronger jetstream I think.

As for how angular momentum is measured and what level then I think it is from the winds at all levels up to 10hPa with those at the jet stream level being important.

GP will probably have a few more details.

Thanks for your reply BF.

So at present we would like to see the GWO in phases 1-2 which supports higher latitude blocking.

Do other teleconnection factors such as MJO phase and ENSO state affect angular momentum?

C

equally the jet stream across india is not a weak flabby one which might increase asian mountain torque. The strong jet in the western pacific along with OLR charts suggest strong trade winds with a stationary high to the north east of Australia and convectional activity to the north west of Australia. So back to phase 1-2 fairly quickly I think afterwards.

Interesting....

A strong +ve mountain torque is underway over Asia, mainly 30N (worth keeping an eye on stratospheric temperatures within the next 10 - 12 days for signs of a rise),

http://www.cdc.noaa.gov/map/images/reanaly...ltaum.90day.gif

Frictional torque has receeded somewhat and the mountain torque is the main thing supporting a westerly motion within the overall ocean-atmosphere framework. The global wind oscillation is in stage 4. Within 5-10 days of strong +ve torques, we generally see the atmosphere respond by removing westerlies with easterlies (note the impacts of the last +ve mountain torque). With a strong Nina base state now embedded, we should see GLAAM drop sharply.

This will serve to force tendency in global angular momentum downwards and back towards phases 1-2, which correspond to a weak and amplified polar flow over the Atlantic.

Within the tropics, we see the latest MJO wave shift eastwards, although this has detatched itself from the main body of convection centred in the west Pacific around the Philippines. Compare here the MJO plot (phase 5/6) with tropical convection centred more in phase 4 area:

http://www.bom.gov.au/bmrc/clfor/cfstaff/m...Last40days.html

http://www.cdc.noaa.gov/map/images/olr/olr.anom.gif

Composite reanalysis for the centre of this convection:-

analogues>

expected outcome>

(I would adjust these anologues somewhat to take account of increased upper level zonal flows forcing the core ridge eastwards towards the UK)

If you perm the expected GWO signal and tropical convection signal, it is overwhelmingly for high pressure solution in the east Atlantic and NW Europe sector. This contradicts somewhat some of the longer range NWP guidance which suggests an unsettled zonal pattern.

East asian mountain torque looks to be dropping off and I cannot quite see where the next reduction in angular momentum is going to come from. High pressure looks to cover the big mountain ranges in the short term, so I could see a bit of a stall in phase 4 or 5 as a result. Stratospheric modelling would appear to me to be divergent with weather modelling so am undecided beyond about 5 days out as to whether it will move into phase 7 and 8 at that point. I suppose the MJO going into phase 7 and 8 could disrupt the trade winds somewhat which might help to break the pattern. SOI (Southern Oscillation Index) has remained positive which would tend to indicate continued la nina conditions (east to west trade winds) over riding the MJO convective activity.

Evidence suggests a strong alignment of strengthened sub-tropical ridges across the Northern Hemisphere which is helping to force westerly addition to the polar flow and easterly addition to sub-tropical flows:-

http://www.cdc.noaa.gov/map/images/reanaly...leaam.90day.gif

There is a large +ve frictional torque in the mid latitudes and corresponding -ve torque in the sub-tropics:-

http://www.cdc.noaa.gov/map/images/reanaly...ltauf.90day.gif

Overall the balance of frictional torques is negative, which is indicative of falling tendency in global momentum, usually a precursor to negative mountain torques which looks to be ocurring:-

http://www.cdc.noaa.gov/map/images/reanaly...ltaum.90day.gif

Within the tropics, wavetrak images indicate easterly trades setting up:-

http://cimss.ssec.wisc.edu/tropic/real-tim...s/wavetrkP.html

which is consistent with latest OLR data showing strong -ve anomalies (convection) in the western Pacific and supression in the central Pacific,

http://www.cdc.noaa.gov/map/images/olr/olr.anom.gif

and also likely forcing -ve tendency in momentum over the Tropics.

Coriolis torques are also adding easterly flow to the atmopshere:-

http://www.cdc.noaa.gov/map/images/reanaly...ltauc.90day.gif

Put all these together, and there are good grounds for driving tendency in angular momentum down (although I'm dubious about the speed - maybe another day or two of twists and turns), but the net result will be to force the GWO into phase 8/1/2:-

http://www.cdc.noaa.gov/map/images/reanaly...d.sig.90day.gif

http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif

Zonal wind anomalies remain close to normal over the polar field at 200 hPa to 500 hPa, but some suggestion that these will increase over time as downwelling waves increase speeds from the cooling stratosphere:-

http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif

I'm not sure what impact this will have as the polar vortex has shown a marked tendency to spit into two - over the Hudsons and Siberia - one to watch although it will not be until after mid month that we see an increase in organised polar westerlies.

Composite reanalysis for the centred tropical forcing (convection) suggests a band of high pressure in the Atlantic and retracting ridge over western US into the Pacific:-

Composite reanalysis for GWO phase 2 looks a good solution for polar flow and tendency for ridging in the Atlantic:-

CPC 8-14 day analogues (2nd panel):-

http://www.cpc.ncep.noaa.gov/products/pred...4analog.off.gif

Roll these forward 10 days .....

... another signal for high pressure blocking out the mid latitudes over the E Atlantic, and not also the tendency for the Pacific ridge to retrograde which will send an amplification (trough led) downstream with a ridge likely building behind, possibly working towards southern Scandinavia over time and possibly, possibly retrogressing slighty NW towards Shetland.

The composites for GWO phase 2 for me best capture where I think we are heading in the 10-20 day timeframe - mid Atlantic ridge to develop and transfer slightly eastwards and northwards towards Ireland (winds from the NW initially), taking up residence over the UK and southern Scandinavia. Therefore remaining largely dry and temperatures close to or below average.

interesting posts as usual from our lrf gurus, even if some of it is difficult for me to get hold of.

I tend to agree with the summary by Stewart that high pressure is likely to be not far from the UK, maybe over or just ese of UK but then I'm only a beginner at this.

Seems not much chance to me, at the moment, from what I read in the signals of any northern blocking until into February at the earliest?

Tendency in angular momentum is down once more, mainly due to a strengthening easterly trade wind surge and slight easterly mountain torque over the sub-tropics. Latest wavetrak image depicts this over the Pacific and Indian Ocean sectors:

http://cimss.ssec.wisc.edu/tropic/real-tim...s/wavetrkP.html

We still have some poleward positive momentum transport supporting anomalous westerly flow 50-55N, but also hints of increased sub-tropical flow as a result of the tropical winds picking up. Frictional torques are suggesting a slight northward shift in the jet towards the sub-polar region which, in combination with the sub-tropical flow is supportive of the amplified pattern being programmed right now t144 onwards.

http://www.cdc.noaa.gov/map/images/reanaly...ltauf.90day.gif

Overall, the general direction of travel is positive for cold returning via height rises over the UK and Scandinavia.

Very interesting wind profile showing up at the moment with a strong alignment of easterly winds across the tropics. Latest wavetrak satellite imagery shows the easterly wind surge across Pacific and Atlantic:

This is supporting strong sub-tropical ridge development - note how the Azores / Bermuda ridge is showing up as a large well defined feature in the modelling.

Angular momentum flux convergence plot shows how these easterly wind anomalies are being propagated both poleward and equatorward:

Notice the tendency for easterly wind anomaly transport 35-45N and also poleward eddies fluxing easterly winds above 60N (some suggestion of weak blocking).

The overall hemispheric pattern is being dominated by a Pacific sector amplification with the northward displaced sub-tropical ridges fuelling thermal gradients and strong jet flow (although I'm pretty sure that the Atlantic signal is to slow velocities).

What's of interest is which way this jet flow migrates. The extended range modelling has been toying with the idea of shifting south and flooding the North Atlantic with cold air and this is worth keeping an eye on. Ofcourse the alternative is a northward shift with a strong polar vortex.

The global wind oscillation should provide a good guide to where the solution lies. Currently in phase 3/4 following a backward movement yesterday, I was surprised at this given the strength of the easterly surge over the tropics. If we see the GWO and tendency in angular momentum fall back (move towards phases 1/2), then southward movement is on the cards. If it hovers around phases 3-4-5, then something akin to the modelling of the next 120 hours is likely.

http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif

http://www.cdc.noaa.gov/map/images/reanaly...d.sig.90day.gif

One area I am particularly interested in is Iceland. Several model runs (e.g. today's parallel GFS) have tried to develop heights over this area associated with a split jet and southward displacement as surface depressions steer a path south-east. With strong (but not anomalously so just yet) upper level zonal winds, some shifting of the position of this ridge towards Svalbard / Scanindavia is possible (an unusual way to develop such a block). Several historical analogues make such an evolution plausible.

One aspect of the modelling which seems rock solid is the strong Azores ridge and high velocity jet flow coming off the US (but decelerating) given the upstream +PNA ridge - but where is that jet going ?

A little more coherent signal emerging I think which replaces an incorrect call for high pressure solution (failed to pick up a strong +ve momentum transport signal and corresponding increase in the jet).

For the last few days, we have been monitoring an increase in easterly winds across the Tropics. These now extend in a broad belt 0-30N for large parts of the Indian, Pacific, Atlantic Oceans as well as now starting to show over Africa. This is finally being registered in a large -ve tendency in angular momentum:

http://www.cdc.noaa.gov/map/images/reanaly...d.sig.90day.gif

This will start to have an impact on the +PNA ridge, serving to retrogress it and shift the mean position further north. This will have an impact on the downstream longwave pattern, with the Azores ridge likely getting shifted westwards away from the SW approaches - so definately no high pressure build over the Continent, in fact quite the reverse.

Frictional torques can be seen as a good indicator of the tendency in angular momentum over various parts of the globe. The current increase in polar flow was well shown by +ve frictional torques and corresponding -ve torques to the south.

http://www.cdc.noaa.gov/map/images/reanaly...ltauf.90day.gif

These have started to recede over the mid and sub-tropical latitudes - indicative of the jet flow becoming less strong, especially if the wavelength is changed by a retracting Pacific Jet. This may take some time, and residual flow will remain as sub-tropical ridges will continue. However, the net impact of the increasing easterly trade wind surge over the Tropics will I think lead to the sub-tropical ridges (and jet streams) being flattened somewhat and shifted southwards. The Pacific Jet is already strong, flat and southward displaced.

Global Wind Oscillation is continuing its painfully slow orbit at low amplitude towards phase 0/1. The only things preventing a full crash towards a strong Nina type (phases 1,2,3) pattern are the frictional torques (receeding) and +ve mountain torque over Asia (strong Asian jet). There is a corresponding strong -ve mountain torque over North America reflecting the inverted pressure pattern across the mountain ranges and this is adding to net easterly flow.

http://www.cdc.noaa.gov/map/images/reanaly...ltaum.90day.gif

The bottom line here is that we are very close to global momentum crashing into deep negative territory although the Asian mountain torque will likely remain for a time (all eyes on the stratosphere) and prevent a strong Nina type pattern from establishing. Contrary to what you might think, I would argue that at this time, a strong 'attractor' state of the GWO phase 1-2 (falling momentum) would be very good for any prospects of amplified flow. During January this teleconnects to height rises across the north -east Atlantic (clue for southward displaced jet). In February this teleconnects to a similar pattern to that observed in Fbruary with a substantial mid-Atlantic ridge and deep trough over NW Europe which is a logical evolution if the upstream pattern regresses.

So whilst it may take some time for these changes to take effect, I can pretty much support the idea of the jet to become more southward displaced and a more amplified flow to take effect towards the end of the month. In the meantime, a strong signal for an unusual phase of unsettled weather (temperatures close to seasonal average) but trending cooler in the longer term once again.

thanks GP, as ever some of your theory is above my old head but much of it I understand and agree with your final paragraph. Unsettled but overall not mild. I feel there is a short 'window' as I suggested last evening around 15-20 January but then believe we have to wait for early February for most support for a noticably colder interlude.

Some very interesting aspects to the angular momentum budgets tonight.

Flux convergence reanalysis identifies positive transport across the high latitudes which is supportive of negative (easterly wind) anomalies developing and converse developments over the sub-tropics. This is perhaps a first indicator of easterly propagation shifting poleward - the first stage of a -NAO phase.

Frictional torques mirror these fluxes. Over the high latitudes negative torques are adding easterly winds and there has been an end to positive torques over the mid latitudes signalling an end to high velocity jet flow and cue for amplification. Over the sub-tropics we are starting to see +ve torques adding westerlies beneath the sub-tropics and weakening the sub-tropical ridges further north. The Pacific flow will be first impacted and then the Atlantic where we should see the Pacific ridge pulled west and north and subsequently, a series of discontinuous Atlantic ridges develop angling the flow ever more NW then NNW.

The GWO has started to shift back to phase 3/4 although I'm suspecting a dramatic fall is just around the corner tied into this being a positive momentum phase which tend to last 15-30 days, and likely ending within the next 10 days with a real Nina 'kick' coming to the atmosphere.

This I think will send us into a strong amplitude phase 1/2 orbit late Januay / early February - teleconnecting to a mid or high latitude Atlantic ridge and NW European trough, possibly with a developing ridge over the top.

I think the atmosphere continues to slowly evolve towards a very cold pattern and NWP tools are picking up on a signal for the longwave pattern to plunge the upper trough SE into Europe with height rises above.

Frictional torques (wind additions due to longitudinal pressure differentials) are suggesting a negative tendency to angular momentum. The westerly additions appear to be in the tropics and sub-tropics whilst negative (easterly winds) are being added to the higher and mid latitudes. This negative trend is important as this is usually a precursor to convection returning to the Indian Ocean and shift in the GWO towards phases 0-1-2.

Overall tendency in angular mometumn is reflecting this frictional torque and is down:

The GWO has responded and is moving towards phase 0

http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif

Easterly wind propagation from the tropics should continue to see the GWO shift in a more decisive fashion towards phase 1-2 in the next week. GWO composites with February's with these phases suggest a strong tendency for amplification in the mid Atlantic:

NWP tools are starting to hint at this development. It's noticeable that any hints of ridges developing as disturbances off the Gulf Coast are translating to either Scandinavian or Icelandic blocking. The latter ties in particularly well with GWO composites allowing for the general context with a trough located over NW Europe and seasonal trend for the ridge in the Atlantic to get pulled westwards towards Newfoundland, assisted by that warm pool of water.

Upper level zonal winds are suprisingly close to normal 200-500 hPa:

http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif

... which should assist with any attempts to build blocking structures to our north.

So for me, there is an increasing trend for a more amplified upper flow triggering a mid Atlantic ridge and height rises to our north. Current guidance suggest that Scandinavia will be the most appropriate location, and this is most probable given the strength of the flow to the north but I'm not sure, and equally this could evolve over the next few days to being shown more towards Iceland. The most likely timescale for this development to a much more sustained colder spell is the end of Jan into early February. Before that, the jet being aligned NW-SE with disturbances shifting it ocassionally west look about right.

I think we are looking at three dominoes toppling before we can reasonably expect a cold pattern to re-establish.

Domino 1 - the amplification of the Pacific Jet has been achieved.

Domino 2 - the breakdown and retrogression of the Pacific West Coast Ridge is now being consistently modelled within the t120-168 timeframe.

Domino 3 - may prove to be the most robust, the weakening of the Azores Ridge.

Full disc satellite imagery shows the size of the ridge in the Atlantic stretching from the Canaries across to Bermuda and Azores. Wavetrak data shows strong low level easterly winds on the SE flank of the ridge.

http://cimss.ssec.wisc.edu/tropic/real-tim...s/wavetrkA.html

Until this ridge (and specifically the strong easterly flow underneath) is weakened, it will be difficult to achieve any sustained amplification along the polar front.

So what might reduce the Azores Ridge ? Analysis of the longevity of Azores ridges (lat 25-40N) through frictional torques tells us that the ridge is enhanced when frictional torques are decreasing and weakening when torques are increasing.

The 'wavelength' of the frictional torques is about about a month or just less. This means that our current ridge enhancement / falling torques should last until the last week of January.

The GWO composites for phases 1 and 2 (where we are currently heading for) argue that the ridge will also come under some pressure to shift north and north-west over time:

Tendency in angular momentum also indicates some increase in westerly wind motion within the latitude of the sub-tropical ridges and negative tendency above it which should signal the weakening of the ridges although I suspect this is restricted to the Pacific sector at the moment. Given the favourable development of the GWO and natural wavelength, we should continue to see longer range tools advertising a weakening of the strength of the Azores ridge.

can't argue with that reasoning Stewart and we, to my amateur eye, need the 30mb Stratospheric temperature to start climbing on the actual chart not the forecast one.

I still reckon we are at the end of the first week in February perhaps the second before any really marked northern ridging develops?

There are times when the Global wind oscillation is not the whole story and I suspect we have such a period coming up. It is not that its wrong just that there can be some delayed actions from the patterns that occur as a result.

First let us have a quick review of where we think we are. GWO shows we are drifting into phase 1, perhaps spiking as ed berry puts it is a better word although.

Long Paddocks SOI figures show that the pacific trade winds have been somewhat intermittent, showing that the la nina phase at the moment is pretty weak.

1-Jan-2009 1012.26 1002.65 23.56 12.82 14.72

2-Jan-2009 1011.35 1003.55 15.03 12.60 14.81

3-Jan-2009 1012.38 1004.40 15.88 12.42 14.85

4-Jan-2009 1012.21 1004.95 12.49 12.91 14.89

5-Jan-2009 1013.74 1005.60 16.63 13.70 14.95

6-Jan-2009 1013.60 1005.70 15.50 14.38 14.93

7-Jan-2009 1013.45 1006.10 12.91 14.76 14.76

8-Jan-2009 1010.79 1005.40 3.68 14.37 14.57

9-Jan-2009 1008.14 1004.25 -3.39 13.78 14.37

10-Jan-2009 1008.39 1003.60 0.85 13.33 14.22

11-Jan-2009 1011.14 1003.20 15.69 13.47 14.21

12-Jan-2009 1011.61 1006.60 1.89 13.27 13.96

13-Jan-2009 1011.89 1007.60 -1.50 12.88 13.74

14-Jan-2009 1012.16 1006.90 3.07 12.94 13.56

15-Jan-2009 1013.66 1007.35 8.01 13.15 13.38

16-Jan-2009 1014.60 1006.00 18.80 13.44 13.33

17-Jan-2009 1014.59 1007.30 12.63 13.25 13.28

18-Jan-2009 1012.88 1007.60 3.16 12.60 13.20

This also coincides with a pick up in MJO activity as the wave finally comes round again.

Phase 8 where the MJO is currently is usualy the end of the cycle and we might expect la nina and the trade winds to pick up again.

Understanding what is going on Torque wise is rather more tricky. Moutain torque does not suggest any spectacular change in angular momentum, however as GP points out the frictional torque has picked up. Gut feeling tells me that the spike into DWO 1 will lead to a signifcsnt drop in angular momentum coming up (Read a blocked pattern). Normally I would guess that the current pattern is here to stay, but I don't think so and I think most of the models may be perfroming poorly at more than a week out. Ed berry suggest a transitional change and when he talks about winter storm developments focusing on the Plains I cannot help thinking mountain torque and sinking angular momentum.

Now for the fly in thointment so to speak and here I am looking at the stratosphere forecasts.

That looks a lot like a split stratospheric vortex and if that comes off then all bets are off. Current vorticity charts suggest a deep low for the central plains of the US, meaning winter will reallly start over there and a lot of energy will go into the jet stream as a result. High pressure ridging north in the Atlantic would not be an unexpected result although it does depend on how the jet reacts (it could just blast across the atlantic).

Despite my inital argument that the stratosphere was spoiling things there is some symmetry between what the GWO tells us and the ECM stratospheric forecasts. Cooler northerly perhaps on its way then ?

There are times when the Global wind oscillation is not the whole story and I suspect we have such a period coming up. It is not that its wrong just that there can be some delayed actions from the patterns that occur as a result.

Now for the fly in thointment so to speak and here I am looking at the stratosphere forecasts.

That looks a lot like a split stratospheric vortex and if that comes off then all bets are off.

Very much looks that way Mark - the upper atmosphere starting to influence the pattern although this is in advance of any potential impacts of the now impending MMW.

Both ECM and GFS 70hPa layers (indicative of 100 - 10 hPa layers during this period) show splitting in the vortex across the North Pole with one vortex centred over the Davis Straights, the second into Siberia at t168:

Zonal wind means are decreasing rapidly at the 200 hPa, 500 hPa and 1000 hPa layers which will weaken any upper level support for sustained polar westerlies:

http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif

whilst the ECM (arguably a better model for the stratosphere) programs a rapid weakening of zonal winds at 65N:

Some keen eyed will see the QBO running at 10m/s and projected extent of the warming event (negative blue wind anomaly).

So some support there for blocking developing across the Pole and towards Svalbard. I stress this is not a direct response to the warming event although it does make the life of any downwelling wave that much easier and we might see a more rapid 21 day propagation.

The GWO has evolved as expected so not such a clear case for the flat jet programmed into the future and I'm still thinking that the Atlantic ridge is going to come under some pressure to shift NW and become interrupted. Blocking looks a reasonable prospect on this evidence, it's just the strength of upper level flow across 50N - 55N which could hold thinks up here, but generally the trend is our friend right now.

The trend continues to be our friend.

ECM zonal wind forecasts suggest a rapidly shifting dynamic within the upper layers within t192 (this is yesterday's forecast and therefore we can reasonably assume on today's output that this has continued to t168):

http://strat-www.met.fu-berlin.de/cgi-bin/...t=all&var=u

Note two key levels - 500 hPa and 200 hPa and the differences between 50N and 80N.

There is a shift programmed here with negative zonal wind anomalies (not mean values just yet) propagating downwards over the polar field.

NCEP zonal wind anomalies identify mean values of 8m/s and 12m/s at these two layers which best represent the key drivers of overall patterns. These translate to slight negative and nuetral anomalies:

http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif

On current projections, it would appear that we are seeing a very rapid downwelling of waves through the atmosphere and there are now close to normal zonal wind anomalies prevelent. Crucially, we are now starting to repeat the conditions which allowed what was a cold SSTA-driven pattern in December to manifest itself. This underlying pattern is still at hand and it may well be that the reversal in zonal winds will start to favour a weakened jet and a ridge in the NW Atlantic with cut off low beneath it, in addition to some form of ridge over the NE. So I ask, are we going to get ambushed from both NE and NW vectors in the coming weeks ?

06Z GFS spread for 500hPa values identifies some uncertainty over the pressure over the NW Atlantic indicated by some of the ensembles building pressure off Newfoundland with a cut off low beneath:

Easterly wind propagation from the tropics should continue to see the GWO shift in a more decisive fashion towards phase 1-2 in the next week. GWO composites with February's with these phases suggest a strong tendency for amplification in the mid Atlantic:

NWP tools are starting to hint at this development. It's noticeable that any hints of ridges developing as disturbances off the Gulf Coast are translating to either Scandinavian or Icelandic blocking. The latter ties in particularly well with GWO composites allowing for the general context with a trough located over NW Europe and seasonal trend for the ridge in the Atlantic to get pulled westwards towards Newfoundland, assisted by that warm pool of water.

Upper level zonal winds are suprisingly close to normal 200-500 hPa:

http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2009.gif

... which should assist with any attempts to build blocking structures to our north.

So for me, there is an increasing trend for a more amplified upper flow triggering a mid Atlantic ridge and height rises to our north. Current guidance suggest that Scandinavia will be the most appropriate location, and this is most probable given the strength of the flow to the north but I'm not sure, and equally this could evolve over the next few days to being shown more towards Iceland. The most likely timescale for this development to a much more sustained colder spell is the end of Jan into early February. Before that, the jet being aligned NW-SE with disturbances shifting it ocassionally west look about right.

The GWO continues to advertise itself as an excellent long range forecasting tool.

No updates to some of the angular momentum budgets since Saturday so we working on some assumptions here.

Current outgoing longwave radiation anomaly plots depict a reasonably strong convective signal in the Indian Ocean, eastern Pacific, Africa and tropical Atlantic. Anticyclones are lining up above these convectively active areas consistent with the GWO evolving through phase 2/3 and frictional torque is likely be strongly positive at 30N adding westerly flow and positive momentum transport at 40-50N. Net result, fast velocity sub-tropical flow and meridional polar flow.

Both the MJO and GWO are in phase 2/3 transition and I strongly suspect that we are seeing one of those phases where the tropical signal (MJO) and extratropical signals (GWO) are harnessed and phasing in sync. to promote a strong wind signal. Composites for GWO phase 3 and MJO phase 3 teleconnect to an upper ridge in the Mid Atlantic and positive height anomalies over Greenland with a deep upper trough over Scandinavia:

This looks reasonably conistent with the extended range modelling taking us to day 14 and I support the notion of the model guidance to draw the ridge forming over Scandinavia towards Iceland and then southern Greenland. From previous postings you'll understand my reasons for supporting a continued pulling of the ridge back keeping a deep trough in place over Scandinavia and the trend in the models supports this. t240 ensemble means for the GEM and GFS advertise well this pattern:

This is interesting as it opens up an attack from the SW approaches as low pressure attempts to force its way through beneath a block over Greenland towards the Scandinavian trough. Here lies an opportunity for heavy snowfall and the precursor easterly will be crucial here - we need as much cold as possible over the UK and near continent as the lows attack, ideally with as much blocking as possible towards Iceland - which is very possible I think.

Beyond the 14 day period, we should start to see the early developments of the MMW on the upper troposphere. Standby for height anomalies resembling this as the AO starts to tank....

So here we go, the real deal is about to begin.

The atmosphere is pretty much were it was in mid to late December, albeit with global momentum rammed significantly lower c/o strengthened easterly wind addition consistent with a maturish La Nina base state. Recent negtaive mountain and frictional torques have reinforced this with the GWO in high amplitude phase 3.

Lates tropwave imagery identifies a growing easterly wind anomaly through the Tropics and across large parts of the Equator. As a result, anticyclones are now lining up across the Tropics (denoted below by +ve (brown) OLR anomalies) to sustain a strong westerly component across 30N. An increase in sub-tropical momentum and weakened sub-tropical ridges are likely results days 6-10. This shown nicely by frictional torques which are increasing across 30N:

Based on where the overall base state is at:

http://www.cdc.noaa.gov/map/images/aam/glaam.gif

.... a solidly low AAM position, I suspect that we will continue to see these easterly winds across the Tropical and Equatorial Pacific, west Indian Ocean and Atlantic although some convective flare ups towards Indonesia and the Philippines are possible.

The crucial point about the comparison with late December is that the addition of westerly winds through frictional torques happend then as now. However, momentum fluxing and frictional torques transfering poleward stoked up the polar jet (allied to stratospheric anomalies) cutting short the potential of that cold event. Crucially, zonal wind anomalies across the polar field are entirely different now compared to late December which should work to deflect any tendency to shift zonal winds poleward over the next 10-15 days and this I think will be entirely attributable to the stratospheric warming.

The Major Mid Winter Warming (MMW) event centred on 23rd January has shown signs of propagating efficiently thrpugh the lower stratoshpere to a point where it's first impacts are being felt on the tropopause. Pictured below is the mean zonal wind anomaly over time. Note the development of classic 'propagating node' (discontinuous green) and the hatched line getting as far down as the 100 hPa layer.

Based on previous warming events such as this, we are likely to see a further two to three such 'nodes', each with increasing influence bringing about ever more pronounced high latitude blocking. On current form, we are likely to see the first real appreciable wave hit somewhere around 10th February with a peak in blocking 21-28 February.

Tendency in Global Angular Momentum is likely to peak within the next 5 days as frictional torques transfer into mountain torques driving the GWO into phase 3, possibly 4.

However, a sharp decline is likely to ensue as these additions are rapidly removed from the atmosphere. Considering the very low base state that we are are currently in, I suspect that a very high amplitude phase 1-2 orbit is very possible in the GWO as tendency in momentum drops through the floor. This should time itself with first tangible signs of the MMW. The implications for the Northern Hemisphere are huge with high impact weather events very likely.

GWO composites for phase 2/3 provide a rough guide as to where the atmosphere is already at and where it will likely remain over the next 20 days:

... suggesting a north Atlantic ridge and deep Scandinavian trough. The effects of the MMW should be to super-charge or hyper enhance this pattern - therefore more emphasis on a ridge building over Greenland I think. The CPC 8-14 day forecast reasonably captures my thoughts w/r/t/ the longwave pattern...

In terms of the 'actual' weather, I'm thinking sustained and quite severe cold (cold intensifying as we progress through Feb) with a polar trough centred just to the east of the UK feeding in cold north or north-easterly winds with snow and very low night time temperatures throughout the outlook period.

very interesting read, since finishing my course become a lot easier to understand. thanks for taking the time to do that

Thanks GP. Your Teleconnection forecasts have been excellent this winter, here's hoping this one is on the money.

Top post informative and succinct as ever, lets just hope this thread doesn't deteriorate into "whens it going to snow" .

Mods I posted a link to this thread in the mod. discus. thread can replies in here be monitored and moved/ deleted to keep this valuable knowledge resource intact.

Thanks GP.

Certainly mouth watering prospects during this month and your post has certainly made me very happy.

Ditto me too! Thanks GP- great post and rubber stamps the increasing train of thought over the last week or so.

On that basis, easy to forgive an imminent snow event maybe being too brief for some of us!

From what I have gathered the increased easterly winds in the northern hemisphere at 200mb in the polar latitudes have caused the sudden stratospheric warming, or as you like to call it the Major-Mid Winter Warming; to my knowledge it hasn't been this strong during this time of year since 1978.

This would set up high latitude blocking over the Northern Atlantic and hopefully some more winter cold outbreaks.

By the way, awesome discussion.

I still learning how to put the pieces of the puzzle together like the momentum fluxes and frictional torques. I hope that these discussions continue.

I'm posting this here in response to the discussion regarding this paper http://ams.allenpress.com/archive/1520-046...69-22-6-597.pdf .....in the Model Output Thread.

I personally think that Winter SW warming events are "linked" with cold winters and that recent "bad" winters were ruined by the strong omnipresent PV.

However this paper begins by talking about a mid-Jan 63 warming as contibuting to that phenomenal winter that I remember so well.

However the Autumn of 62 had been mostly very cold and the extreme cold synoptics were in place by Xmas day. By mid-Jan we were 3 weeks into an uninterrupted regime of extreme cold.

If the Warming takes 2 weeks to alter the synoptics, then we had already had 5 uniterupted weeks of record-breaking cold, by the time the "cause" arrived!

Perhaps that winter was caused by other factors.

Or perhaps the SSW is not the direct cause? Could it be a parrallel effect caused by a deeper, unknown, cause that promotes cold NH winters?

Could it be a "marker" rather than a simple cause? Or a self reinforcing "product-&-further-cause" of cold.

Notwithstanding my queries about it "causing" the 62/63 winter I still think it's the best explanation we have for "good" and "bad" winters.

Please keep up the info. GP et al.

Len

I'm posting this here in response to the discussion regarding this paper http://ams.allenpress.com/archive/1520-046...69-22-6-597.pdf .....in the Model Output Thread.

However this paper begins by talking about a mid-Jan 63 warming as contibuting to that phenomenal winter that I remember so well.

Len

re the above link Len.

I'm not convinced by that article, although I am fairly well convinced by the general argument that events in the Troposphere are linked to what has happened in the Troposphere. Their argument, if it is to try and explain the severe winter of 1962-63, falls down as by mid January the severe cold had been in existence over much of the UK since just after Xmas.

So here we go, the real deal is about to begin.

In terms of the 'actual' weather, I'm thinking sustained and quite severe cold (cold intensifying as we progress through Feb) with a polar trough centred just to the east of the UK feeding in cold north or north-easterly winds with snow and very low night time temperatures throughout the outlook period.

Reanalysis of the first week of February identifies the following H5 anomaly:

Note the marked -ve anomaly over Europe and +ve anomaly over Iceland and southern Greenland. I've depicted this to demonstrate the link between the 500 hPa Geopotential height anomaly and real weather, and to emphasise the link between forecast patterns and verification. This chart delivered the snowfall that many of us have seen and a CET which currently stands at 0.5C.

As anticipated, the GWO has exhibited a high amplitude quasi stationary oscillation in phase 2/3. This correlates well with composites for these two stages, allowing for a weaker zonal wind anomaly across the polar field allowing for the ridge to pull northwards.

The major stratospheric warming is continuing to propagate down through the tropopause. Note however that this propagation is somewhat discontinuous with sudden increases in zonal winds occuring within the tropopause ahead of downwelling waves. This laddering down of the effects of the warming event is not unusual. It is this switching ahead of the downwelling that is IMO behind the model fragility within the extended range. I had hoped that the ECM as the best upper atmosphere modeller would be able to cope but unfortunately, it would appear vulnerable. Beware, another episode of laddering is likely so perhaps the models to switch like a shoal of fish in a stream and pumping more energy into the polar jet before twigging just what is coming down from the 'headwaters'. I fully expect the huge negative momentum anomaly to have an impact on the Northern Hemisphere's weather patterns for a period over February and at least the first half of March. However, this will intensify and ease off reflecting the nature of downwelling waves.

The effects of the stratospheric warming show nicely in terms of a negative momentum anomaly within the upper sigma levels and positive anomaly in the lower layers - the prelude to downwelling.

Mountain torques (pressure differential across mountain ranges) have worked to add westerly wind anomalies to the circulatory system (once again, wavebreaking a possible trigger for further stratospheric warming). Note the cycle of positive torques is more or less tied into the cycle of torques over the tropics (purple line) and the addition of westerly winds is now looking like reversing in a natural rubbing out out of these westerlies coinciding with the tropical signal. A fall is likely adding easterly wind anomalies to the circulatory system.

These positive mountain torques have added westerly motion to the mid and high latitudes. However, frictional torques (pressure differential between latitudinal air masses) have helped to keep lid on these westerly additions by counteracting and adding easterly winds.

Tendency in earth angular momentum is now down:

..and the overall signal here is for tendency in anglar momentum to decrease, especially as decreasing mountain torques take effect. This will drive the GWO through phases 3-2-1 and back again 1-2-3 as easterly wind additions are rubbed out.

The composites for GWO phases 1-2-3 once again capture the overall likely longwave pattern - mid Pacific and Atlantic ridges (some suggestion for retrogression) and Scandinavian trough, possibly bridging over as the trough becomes cut off.

However, as we have seen already, we need to tweak these anomalies to allow for pressue to rise to our north and north-west in the face of increasing signals for blocking related to downwelling waves (remember the laddering though).

Overall, the thoughts discussed at the end of January remain unchanged - below average to continue with more progressive blocking to the second half of the month - winds from a north-east and northerly vectors. Both GEM t240 00z ensemble mean height anomaly, CPC and ECM mean height anomaly depict this well.

thanks Stewart for another fascinating read, most of which I THINK I understood.

Interesting that several of us, from not the quite same direction, tend to feel that the models are not really showing what is 'around the corner' so to speak.

I agree that cold or at least below normal is the most likely temperature level for the Uk as a whole, and into March as you say. How far in I have no idea just yet.

thanks again Stewart.

Very interesting GP.

When can we expect this to be reflected in the models?

Events in the stratosphere are pretty unusual and the tropospheric response should be linked to the specifics of the type of event going on. To my way of thinking there are three distinct types of event that can occur in the stratosphere.

1) A split Vortex with both displaced southwards much like we have now.

2) A single vortex displaced southwards, much like we will have in a few days.

3) A complete disruption of the vortex which I think may be on the cards after that.

Lets look at now.

Temps and the vortex

Signals for high pressure as a result

Lets move on 5 days and we see one part of the vortex collapsing as they both rotate around. Unfortunately that collapsing bit moves nearer to the UK and seems likely to give us some more atlantic type weather untill early next week.

Further out and that second part of the vortex collapses leaving just one part displaced southwards. Vorticty charts suggest here we could be in for another cold period with a risk of snow.

Even further out and we have to start looking upward in the stratosphere for clues and you need to go almost all the way up to the 5hPa layer before you start to see any kind of vortex reforming.

Giving very little vorticity in the stratosphere for a period. Meaning other signals will take priority over the stratosphere.

Those signals will most likely involve the GWO which you would expect to head into phase 1 and 2 but seems to be stubornly stuck in 3 4 and 5.

I am not really convinced about mountain torque reducing angular momentum which if I understand things correctly means as a cyclone crosses one of the great mountain ranges it transfers momentum to the earth ,taking it away from the atmosphere. My take on this would be although the mourtain torque is falling it is still positive and as such we still have some time before angular momentum falls away. Perhaps this is something for Glacier Point to elaborate on.

Cyclically it looks like warm water ought to be on the decline in the Pacific, but we do have a stalled MJO in phase 6 so maybe this will come into play. SST's I think will have their day this year if things really calm down so we ought to be thinking about high pressure maybe to the north of the warm spots in the atlantic.

The upshot of this is that models are likely to struggle a bit with the transitioning phases in the stratosphere and globally we might expect some high impact weather this year as a result of this event.

Promising posts thanks GP and BF. I will keep an eye on the potential vorticity charts in the next few days to see how they change. So for those who can look further than the short term variable model outputs still a lot to look forward to with some potential wintry weather possible into March. I didn't think this SSW would give up that easily without something more to show so fingers crossed!

c

Promising posts thanks GP and BF. I will keep an eye on the potential vorticity charts in the next few days to see how they change. So for those who can look further than the short term variable model outputs still a lot to look forward to with some potential wintry weather possible into March. I didn't think this SSW would give up that easily without something more to show so fingers crossed!

c

Yes indeed. That was a very interesting post from Brickfielder and adds yet another perspective onto things in terms of how events may transpire. I certainly agree with the notion that the effects of the SSW may well be long lasting beyond the remit of cold and wintry weather. So the post mortem on the event so eagerly awaited in certain quarters on the model thread may well be a very long time coming!