Here are the current Papers & Articles under the research topic Stratosphere - Sudden Stratospheric Warming (SSW). Click on the title of a paper you are interested in to go straight to the full paper.
Defining Sudden Stratospheric Warmings
Published Nov 2015.
Sudden stratospheric warmings (SSWs) are large, rapid temperature rises in the winter polar stratosphere, occurring predominantly in the Northern Hemisphere. Major SSWs are also associated with a reversal of the climatological westerly zonal-mean zonal winds. Circulation anomalies associated with SSWs can descend into the troposphere with substantial surface weather impacts, such as wintertime extreme cold air outbreaks. After their discovery in 1952, SSWs were classified by the World Meteorological Organization. An examination of literature suggests that a single, original reference for an exact definition of SSWs is elusive, but in many references a definition involves the reversal of the meridional temperature gradient and, for major warmings, the reversal of the zonal circulation poleward of 60° latitude at 10 hPa.
Though versions of this definition are still commonly used to detect SSWs, the details of the definition and its implementation remain ambiguous. In addition, other SSW definitions have been used in the last few decades, resulting in inconsistent classification of SSW events. We seek to answer the questions: How has the SSW definition changed, and how sensitive is the detection of SSWs to the definition used? For what kind of analysis is a “standard” definition useful? We argue that a standard SSW definition is necessary for maintaining a consistent and robust metric to assess polar stratospheric wintertime variability in climate models and other statistical applications. To provide a basis for, and to encourage participation in, a communitywide discussion currently underway, we explore what criteria are important for a standard definition and propose possible ways to update the definition.
Optimizing the Definition of a Sudden Stratospheric Warming
Published March 2018.
Various criteria exist for determining the occurrence of a major sudden stratospheric warming (SSW), but the most common is based on the reversal of the climatological westerly zonal-mean zonal winds at 60° latitude and 10 hPa in the winter stratosphere. This definition was established at a time when observations of the stratosphere were sparse. Given greater access to data in the satellite era, a systematic analysis of the optimal parameters of latitude, altitude, and threshold for the wind reversal is now possible. Here, the frequency of SSWs, the strength of the wave forcing associated with the events, changes in stratospheric temperature and zonal winds, and surface impacts are examined as a function of the stratospheric wind reversal parameters. The results provide a methodical assessment of how to best define a standard metric for major SSWs. While the continuum nature of stratospheric variability makes it difficult to identify a decisively optimal threshold, there is a relatively narrow envelope of thresholds that work well—and the original focus at 60° latitude and 10 hPa lies within this window.
What is a Polar Stratospheric Warming?
An simple explanation of a Sudden Stratospheric Warming by NASA.
How Sudden Stratospheric Warming Affects the Whole Atmosphere
Published March 2018.
No abstract, but the intro:
Weather events 10–50 kilometers above Earth’s surface, in the atmospheric layer called the stratosphere, affect weather on the ground as well as weather hundreds of kilometers above. Experiments demonstrate that resolving stratospheric dynamics enables forecasters to predict surface weather farther into the future, particularly during winter in the Northern Hemisphere [Tripathi et al., 2015]. Thus, meteorologists looking to improve their short- and long-term weather forecasts are seeking accurate models representing the way stratospheric disturbances propagate downward into the troposphere, the atmospheric layer closest to Earth’s surface. Chief among these disturbances are common events called sudden stratospheric warmings (SSWs). During SSWs, stratospheric temperatures can fluctuate by more than 50°C over a matter of days. Recent research has conclusively shown the existence of a strong connection between SSWs and extensive changes throughout Earth’s atmosphere. These changes can affect atmospheric chemistry, temperatures, winds, neutral (nonionized particle) and electron densities, and electric fields (Figure 1), and they extend from the surface to the thermosphere (Figure 2) and across both hemispheres. These changes span regions that scientists had not previously considered to be connected.
Forecasting extreme stratospheric polar vortex events
Published Sept 2020.
Extreme polar vortex events known as sudden stratospheric warmings can influence surface winter weather conditions, but their timing is difficult to predict. Here, we examine factors that influence their occurrence, with a focus on their timing and vertical extent. We consider the roles of the troposphere and equatorial stratosphere separately, using a split vortex event in January 2009 as the primary case study. This event cannot be reproduced by constraining wind and temperatures in the troposphere alone, even when the equatorial lower stratosphere is in the correct phase of the quasi biennial oscillation. When the flow in the equatorial upper stratosphere is also constrained, the timing and spatial evolution of the vortex event is captured remarkably well. This highlights an influence from this region previously unrecognised by the seasonal forecast community. We suggest that better representation of the flow in this region is likely to improve predictability of extreme polar vortex events and hence their associated impacts at the surface.
Using large ensembles to quantify the impact of sudden stratospheric warmings and their precursors on the North Atlantic Oscillation
Published Feb 2023
Sudden-stratospheric-warming (SSW) events are often followed by significant weather and climate impacts at the surface. By affecting the North Atlantic Oscillation (NAO), SSWs can lead to periods of extreme cold in parts of Europe and North America. Previous studies have used observations and free-running climate models to try to identify features of the atmosphere prior to an SSW that can determine the subsequent impact at the surface. However, the limited observational record makes it difficult to accurately quantify these relationships. Here, we instead use a large ensemble of seasonal hindcasts. We first test whether the hindcasts reproduce the observed characteristics of SSWs and their surface signature. We find that the simulations are statistically indistinguishable from the observations, in terms of the overall risk of an SSW per winter (56 %), the frequency of SSWs with negative NAO responses (65 %), the magnitude of the NAO responses, and the frequency of wavenumber-2-dominated SSWs (26 %). We also assess the relationships between prior conditions and the NAO response in the 30 d following an SSW. We find that there is little information in the precursor state to guide differences in the subsequent NAO behaviour between one SSW and another, reflecting the substantial natural variability between SSW events. The strongest relationships with the NAO response are from pre-SSW sea level pressure anomalies over the polar cap and from zonal-wind anomalies in the lower stratosphere, both exhibiting correlations of around 0.3. The pre-SSW NAO has little bearing on its post-SSW state. The strength of the pre-SSW zonal-wind anomalies at 10 hPa is also not significantly correlated with the NAO response. Finally, we find that the mean NAO response in the first 10 d following wave-2-dominated SSWs is much more strongly negative than in wave-1 cases. However, the subsequent response in days 11–30 is very similar regardless of the dominant wavenumber. In all cases, the composite mean responses are the result of very broad distributions from individual SSW events, necessitating a probabilistic analysis using large ensembles.
The Stratospheric Sudden Warming Event in February 2018 and its Prediction by a Climate System Model
Published Nov 2018
A major stratospheric sudden warming (SSW) event was observed in February 2018 after a 4-year absence since the winter of 2013/2014. Based on the reanalysis data, the polar night jet changed from a very strong state to a moderate state during 12–19 January, and the moderate westerlies directly reversed to easterlies during 5–15 February. The intensified East Asian trough, Alaskan blocking, and East U.S. trough amplified the extratropical climatological wave 2, which propagated upward into the stratosphere, leading to a vortex-splitting SSW event. Predictions of the February 2018 SSW event are explored in hindcasts initialized 0–4 weeks in advance by the Beijing Climate Center Climate System Model. Less than 20% of the 28 ensemble members predict the reversal of [U]60°N, 10hPa in hindcasts initialized 3 or 4 weeks in advance if a 5-day error is allowed, while this ratio increases to 43% in hindcasts initialized 1 week in advance. Based on the climatological occurrence of SSW events in the forecast system, the maximum deterministic predictable limit of this event is 1–2 weeks in this forecast system. The eddy heat flux and its domination by wave 2 can only be predicted within the predictable time limit. A comparison between hindcast members initialized 2 weeks in advance suggests that the extratropical troughs and blockings are responsible for the upward propagation of waves from the troposphere to the stratosphere. The predictable limit of the stratospheric circulation pattern for the February 2018 SSW, 1–2 weeks, also generalizes to other vortex split SSW events such as the January 2009 and February 1999 cases.
A climatology of polar winter stratopause warmings and associated planetary wave breaking
Published May 2013.
This work presents a climatology of synoptic-scale disturbances in the upper stratosphere lower mesosphere (USLM) based on 20.5 years of assimilated data analyses from the U. K. Meteorological Ofﬁce (1991–2012). USLM disturbance criteria are established, based on stratopause warmings at the 2 hPa level, to create climatologies in
both hemispheres that delineate their timing, frequency, and geographic location. USLM disturbances occur on average 2.3 times per winter in the Northern Hemisphere (NH) (November through March) and 1.6 times per winter in the Southern Hemisphere (SH) (May through September), persist on average for 8 days in the NH and only 4 days in the SH, occur most frequently in December (July) in the Northern (Southern) Hemisphere, and
are predominantly located in the longitude sector between 0oE and 90oE in both hemispheres. This is the ﬁrst work to show that all major Sudden Stratospheric Warmings (SSWs) over the 20.5 year data record are preceded by USLM disturbances. One third of USLM disturbances evolve into a major SSW; only 22% of minor SSWs evolve into a major SSW. USLM disturbances and minor SSWs illustrate, at times, similar occurrence statistics, but the minor warming criteria seem to include a more diverse range of dynamical conditions. USLM disturbances are more speciﬁc in their dynamical construct with strong baroclinicity being a necessary condition. Potential vorticity analysis indicates that all USLM events occur with planetary wave breaking and that subsequent baroclinic
instability may lead to the development of USLM disturbances.
A comparative study of the major sudden stratospheric warmings in the Arctic winters 2003/2004 – 2009/2010
Published Sept 2012.
We present an analysis of the major sudden stratospheric warmings (SSWs) in the Arctic winters 2003/04–2009/10. There were 6 major SSWs (major warmings [MWs]) in 6 out of the 7 winters, in which the MWs of 2003/04, 2005/06, and 2008/09 were in January and those of 2006/07, 2007/08, and 2009/10 were in February. Although the winter 2009/10 was relatively cold from mid-December to mid-January, strong wave 1 activity led to a MW in early February, for which the largest momentum flux among the winters was estimated at 60◦N/10 hPa, about 450 m2s−2.The strongest MW, however, was observed in 2008/09 and the weakest in 2006/07. The MW in 2008/09 was triggered by intense wave 2 activity and was a vortex split event. In contrast, strong wave 1 activity led to the MWs of other winters and were vortex displacement events. Large amounts of Eliassen-Palm (EP) and wave 1/2 EP fluxes (about 2–4×105kg s−2) are estimated shortly before the MWs at100 hPa averaged over 45–75◦N in all winters, suggesting profound tropospheric forcing for the MWs. We observe an increase in the occurrence of MWs (∼1.1 MWs/winter) in recent years (1998/99–2009/10), as there were 13 MWs in the 12 Arctic winters, although the long-term average (1957/58–2009/10) of the frequency stays around its historical value (∼0.7 MWs/winter), consistent with the findings of previous studies. An analysis of the chemical ozone loss in the past 17 Arctic winters (1993/94–2009/10) suggests that the loss is inversely proportional to the intensity and timing of MWs in each winter, where early (December–January) MWs lead to minimal ozone loss. Therefore, this high frequency of MWs in recent Arctic winters has significant implications for stratospheric ozone trends in the northern hemisphere.
A Dynamical Model of the Stratospheric Sudden Warming
Published Nov 1971.
The dynamics of the stratosphere sudden warming phenomenon is discussed in terms of the interaction of vertically propagating planetary waves with zonal winds. If global-scale disturbances are generated in the troposphere, they propagate upward into the stratosphere, where the waves act to decelerate the polar night jet through the induction of a meridional circulation. Thus, the distortion and the break-down of the polar vortex occur. If the disturbance is intense and persists, the westerly jet may eventually disappear and an easterly wind may replace it. Then “critical layer interaction” takes place. Further intensification of the easterly wind and rapid warming of the polar air are expected to occur as well as weakening of the disturbance. The model is verified by numerical integrations of the adiabatic-geostrophic potential vorticity equation. Computed results possess features similar to those observed in sudden warming phenomena.
A New Look at Stratospheric Sudden Warmings. Part III: Polar Vortex Evolution and Vertical Structure
Published Mar 2009.
The evolution of the Arctic polar vortex during observed major midwinter stratospheric sudden warmings (SSWs) is investigated for the period 1957–2002, using 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) Ertel’s potential vorticity (PV) and temperature fields. Time-lag composites of vertically weighted PV, calculated relative to the SSW onset time, are derived for both vortex-displacement SSWs and vortex-splitting SSWs, by averaging over the 15 recorded displacement and 13 splitting events. The evolving vertical structure of the polar vortex during a typical SSW of each type is clearly illustrated by plotting an isosurface of the composite PV field, and is shown to be very close to that observed during representative individual events. Results are verified by comparison with an elliptical diagnostic vortex moment technique.
For both types of SSW, little variation is found between individual events in the orientation of the developing vortex relative to the underlying topography; that is, the location of the vortex during SSWs of each type is largely fixed in relation to the earth’s surface. During each type of SSW, the vortex is found to have a distinctive vertical structure. Vortex-splitting events are typically barotropic, with the vortex split occurring near simultaneously over a large altitude range (20–40 km). In the majority of cases, of the two daughter vortices formed, it is the “Siberian” vortex that dominates over its “Canadian” counterpart. In contrast, displacement events are characterized by a very clear baroclinic structure; the vortex tilts significantly westward with height, so that the top and bottom of the vortex are separated by nearly 180° longitude before the upper vortex is sheared away and destroyed.
A sudden stratospheric warming compendium (original paper published Feb 2017)
Associated table: SSWC: Sudden Stratospheric Warming Compendium data set (continually updated)
Major, sudden midwinter stratospheric warmings (SSWs) are large and rapid temperature increases inthe winter polar stratosphere are associated with a complete reversal of the climatological westerly winds (i.e., thepolar vortex). These extreme events can have substantial impacts on winter surface climate, including increasedfrequency of cold air outbreaks over North America and Eurasia and anomalous warming over Greenland andeastern Canada. Here we present a SSW Compendium (SSWC), a new database that documents the evolution ofthe stratosphere, troposphere, and surface conditions 60 days prior to and after SSWs for the period 1958–2014.The SSWC comprises data from six different reanalysis products: MERRA2 (1980–2014), JRA-55 (1958–2014),ERA-interim (1979–2014), ERA-40 (1958–2002), NOAA20CRv2c (1958–2011), and NCEP-NCAR I (1958–2014). Global gridded daily anomaly fields, full fields, and derived products are provided for each SSW event.The compendium will allow users to examine the structure and evolution of individual SSWs, and the variabilityamong events and among reanalysis products. The SSWC is archived and maintained by NOAA’s NationalCenters for Environmental Information (NCEI, doi:10.7289/V5NS0RWP).
Abrupt Stratospheric Vortex Weakening Associated With North Atlantic Anticyclonic Wave Breaking (2018)
Published July 2019.
The sudden stratospheric warming (SSW) of 12 February 2018 was not forecast by any extended-range model beyond 12 days. From early February, all forecast models that comprise the subseasonal-to-seasonal (S2S) database abruptly transitioned from indicating a strong stratospheric polar vortex (SPV) to a high likelihood of a major SSW. We demonstrate that this forecast evolution was associated with the track and intensity of a cyclone in the northeast Atlantic, with an associated anticyclonic Rossby wave break, which was not well forecast. The wave break played a pivotal role in building the Ural high, which existing literature has shown was a precursor of the 2018 SSW. The track of the cyclone built an anomalously strong sea level pressure dipole between Scandinavia and Greenland (termed the S-G dipole), which we use as a diagnostic of the wave break. Forecasts that did not capture the magnitude of this event had the largest errors in the SPV strength and did not show enhanced vertical wave activity. A composite of 49 similarly strong wintertime (November–March) S-G dipoles in reanalysis shows associated anticyclonic wave breaking leading to significantly enhanced vertical wave activity and a weakened SPV in the following days, which occurred in 35% of the 15-day periods preceding observed major SSWs. Our results indicate a particular transient trigger for weakening the SPV, complementing existing results on the importance of tropospheric blocking for disruptions to the Northern Hemisphere extratropical stratospheric circulation.
Blocking high influence on the stratospheric variability through enhancement and suppression of upward planetary-wave propagation
Published July 2011.
Previous studies have suggested the importance of blocking high (BH) development for the occurrence of stratospheric sudden warming (SSW), while there is are cent study that failed to identify their statistical linkage.Through composite analysis applied to high-amplitude anticyclonic anomaly events observed around every grid point over the extratropical Northern Hemisphere, the present study reveals distinct geographical dependence of BH influence on upward propagation of planetary waves (PWs) into the stratosphere. Tropospheric BHs that develop over the Euro-Atlantic sector tend to en-hance upward PW propagation, leading to the warming in the polar stratosphere and, in some occasions, to major SSW events. In contrast, the upward PW propagation tends to be suppressed by BHs developing over the western Pacific and the Far East, resulting in the polar stratospheric cooling. This dependence is found to arise mainly from the sensitivity of the interference between the climatological PWs and upward-propagating Rossby wave packets emanating from BHs to their geographical locations.This study also reveals that whether a BH over the eastern Pacific and Alaska can enhance or reduce the upward PW propagation is case-dependent. It is suggested that BHs that induce the stratospheric cooling can weaken statistical relationship between BHs and SSWs.
Blocking precursors to stratospheric sudden warming events
Published July 2009.
The primary causes for the onset of major, midwinter, stratospheric sudden warming events remain unclear. In this paper, we report that 25 of the 27 events objectively identified in the ERA-40 dataset for the period 1957–2001 are preceded by blocking patterns in the troposphere. The spatial characteristics of tropospheric blocks prior to sudden warming events are strongly correlated with the type of sudden warming event that follows. Vortex displacement events are nearly always preceded by blocking over the Atlantic basin only, whereas vortex splitting events are preceded by blocking events occurring in the Pacific basin or in both basins contemporaneously. The differences in the geographical blocking distribution prior to sudden warming events are mirrored in the patterns of planetary waves that are responsible for producing events of either type. The evidence presented here, suggests that tropospheric blocking plays an important role in determining the onset and the type of warmings.
Changes in Frequency of Major Stratospheric Sudden Warmings with El Niño/Southern Oscillation and Quasi-Biennial Oscillation
Published Oct 2014.
This study investigates observed interannual changes in the Northern winter stratosphere with El Niño/Southern Oscillation (ENSO) and quasi-biennial oscillation (QBO) using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data for 56 years. We focus on changes in occurrence of major stratospheric sudden warmings (MSSWs) as well as in seasonal mean states. Our results reveal complex changes in the MSSW probability with both ENSO and QBO as in the seasonal mean states. However, statistically significant changes at the 90 % confidence level are obtained only for some combi-nations of ENSO and QBO conditions reflecting the limitation of the data period. When the QBO is in a westerly phase, the MSSW probability increases with the ENSO sea-surface temperature condition in the eastern equatorial Pacific, i.e., from ENSO cold (La Niña), through neutral, to warm (El Niño) years. When the QBO is in an easterly phase, on the other hand, the probability significantly increases for La Niña years than for neutral years, whereas the probability is not significantly different between neutral and El Niño years. A characteristic feature is the high MSSW probability for the La Niña and QBO easterly winters, which is consistent with strengthened stationary wave with zonal wavenumber 1 compared to the climatology. These results suggest the importance of taking into account both ENSO and QBO factors, when one examines the frequency of MSSWs in the Northern winter stratosphere.
Characteristics of stratospheric warming events during Northern winter
Published May 2019.
The strong interest in Sudden Stratospheric Warmings (SSWs) is motivated by their role inthe two-way stratospheric-tropospheric dynamical coupling. While most studies only investigate majorSSWs (vortex breakdown), the minor ones (strong vortex deceleration) are overlooked. This work aimsat overcoming this gap by providing a comprehensive description of stratospheric warming eventswithout a priori distinctions between major and minor SSWs, leading to a more complete estimateof the stratospheric variability. Warming events are extracted from reanalysis data sets by means of amidstratospheric polar cap temperature daily index. Events are characterized by a bimodal distributionin amplitude, with a broad peak at small amplitudes (inferior to5K) and a sharp peak at around9K.Due to the intrinsic polar vortex dynamics, the warming amplitude presents a distinct seasonal distribution.Small amplitude warmings mainly occur during early and late wintertime by contrast with thelarger-amplitude ones occurring during midwintertime. From mid-November to mid-March, thelarge-amplitude warmings (i.e., strong warming events, SWEs) include both major and minor SSWs, aswell as Canadian and Final warmings. Although major SSWs belong to the tail of the SWEs distribution,there is no clear distinction between the major and minor SSWs according to the considered properties ofthe events. Such result brings out the idea of “warming continuum.” Furthermore, diagnostics of heat fluxreveal that there is no statistical difference between SWEs with regard to their feedbacks on the planetarywaves and hence on their potential influence into the troposphere.
Defining Sudden Stratospheric Warming in Climate Models: Accounting for Biases in Model Climatologies
Published July 2017.
A sudden stratospheric warming (SSW) is often defined as zonal-mean zonal wind reversal at 10 hPa and 60°N. This simple definition has been applied not only to the reanalysis data but also to climate model output. In the present study, it is shown that the application of this definition to models can be significantly influenced by model mean biases (i.e., more frequent SSWs appear to occur in models with a weaker climatological polar vortex). To overcome this deficiency, a tendency-based definition is proposed and applied to the multimodel datasets archived for phase 5 of the Coupled Model Intercomparison Project (CMIP5). In this definition, SSW-like events are defined by sufficiently strong vortex deceleration. This approach removes a linear relationship between SSW frequency and intensity of the climatological polar vortex in the CMIP5 models. The models’ SSW frequency instead becomes significantly correlated with the climatological upward wave flux at 100 hPa, a measure of interaction between the troposphere and stratosphere. Lower stratospheric wave activity and downward propagation of stratospheric anomalies to the troposphere are also reasonably well captured. However, in both definitions, the high-top models generally exhibit more frequent SSWs than the low-top models. Moreover, a hint of more frequent SSWs in a warm climate is found in both definitions.
Does the Arctic stratospheric polar vortex exhibit signs of preconditioning prior to sudden stratospheric warmings?
Characteristics of the Arctic stratospheric polar vortex are examined using reanalysis data with dynamic time warping (DTW) and a clustering technique to determine whether the polar vortex exhibits canonical signs of preconditioning prior to sudden stratospheric warmings (SSWs). The DTW and clustering technique is used to locate time series motifs in vortex area, vortex edge averaged PV gradients, and vortex edge averaged windspeeds. Composites of the motifs reveal that prior to roughly 75% of SSWs, in the middle to upper stratosphere, PV gradients and windspeeds in the vortex edge region increase, and vortex area decreases. These signs agree with prior studies that discuss potential signals of preconditioning of the vortex. However, similar motifs are also found in a majority of years without SSWs. While such non-SSW motifs are strongly associated with minor warming signals apparent only in the middle and upper stratosphere, only roughly half of these can be associated with later “significant disturbances” (SDs) that do not quite meet the threshold for major SSWs. The median lead time for sharpening vortex edge PV gradients represented in the motifs prior to SSWs and SDs is ~25 days, while the median lead time for the vortex area and edge windspeeds is ~10 days. Overall, canonical signs of preconditioning do appear to exist prior to SSWs, but their existence in years without SSWs implies that preconditioning of the vortex may be an insufficient condition for the occurrence of SSWs.
Dynamics of 2013 Sudden Stratospheric Warming event and its impact on cold weather over Eurasia: Role of planetary wave reflection
Published April 2016.
In the present study, we investigate the impact of stratospheric planetary wave reflection on tropospheric weather over Central Eurasia during the 2013 Sudden Stratospheric Warming (SSW) event. We analyze EP fluxes and Plumb wave activity fluxes to study the two and three dimensional aspects of wave propagation, respectively. The 2013 SSW event is excited by the combined influence of wavenumber 1 (WN1) and wavenumber 2 (WN2) planetary waves, which makes the event an unusual one and seems to have significant impact on tropospheric weather regime. We observe an extraordinary development of a ridge over the Siberian Tundra and the North Pacific during first development stage (last week of December 2012) and later from the North Atlantic in the second development stage (first week of January 2013), and these waves appear to be responsible for the excitation of the WN2 pattern during the SSW. The wave packets propagated upward and were then reflected back down to central Eurasia due to strong negative wind shear in the upper stratospheric polar jet, caused by the SSW event. Waves that propagated downward led to the formation of a deep trough over Eurasia and brought extreme cold weather over Kazakhstan, the Southern part of Russia and the Northwestern part of China during mid-January 2013.
El Niño, La Niña, and stratospheric sudden warmings: A re-evaluation in light of the observational record
Published July 2011.
Recent studies have suggested that El Niño‐Southern Oscillation (ENSO) may have a considerable impact on Northern Hemisphere wintertime stratospheric conditions. Notably, during El Niño the stratosphere is warmer than during ENSO‐neutral winters, and the polar vortex is weaker. Opposite‐signed anomalies have been reported during La Niña, but are considerably smaller in amplitude than during El Niño. This has led to the perception that El Niño is able to substantially affect stratospheric conditions, but La Niña is of secondary importance. Here we revisit this issue, but focus on the extreme events that couple the troposphere to the stratosphere: major, mid‐winter stratospheric sudden warmings (SSWs). We examine 53 years of reanalysis data and find, as expected, that SSWs are nearly twice as frequent during ENSO winters as during non‐ENSO winters. Surprisingly, however, we also find that SSWs occur with equal probability during El Niño and La Niña winters. These findings corroborate the impact of ENSO on stratospheric variability, and highlight that both phases of ENSO are important in enhancing stratosphere‐troposphere dynamical coupling via an increased frequency of SSWs.
Enhanced Stratosphere/Troposphere Coupling During Extreme Warm Stratospheric Events with Strong Polar-Night Jet Oscillation
Published Nov 2018.
Extreme warm stratospheric events during polar winters from ERA-Interim reanalysis and CMIP5-ESM-LR runs were separated by duration and strength of the polar-night jet oscillation (PJO) using a high statistical confidence level of three standard deviations (strong-PJO events). With a composite analysis, we demonstrate that strong-PJO events show a significantly stronger downward propagating signal in both, northern annular mode (NAM) and zonal mean zonal wind anomaly in the stratosphere in comparison with non-PJO events. The lower stratospheric EP-flux-divergence difference in ERA-Interim was stronger in comparison to long-term CMIP5-ESM-LR runs (by a factor of four). This suggests that stratosphere–troposphere coupling is stronger in ERA-Interim than in CMIP5-ESM-LR. During the 60 days following the central date (CD), the Arctic oscillation signal was more intense during strong-PJO events than during non-PJO events in ERA-Interim data in comparison to CMIP5-ESM-LR runs. During the 15-day phase after CD, strong PJO events had a significant increase in stratospheric ozone, upper tropospheric zonally asymmetric impact, and a regional surface impact in ERA-Interim. Finally, we conclude that the applied high statistical threshold gives a clearer separation of extreme warm stratospheric events into strong-PJO events and non-PJO events including their different downward propagating NAM signal and tropospheric impacts.
Identification and Classification of Sudden Stratospheric Warming Events
Published Nov 2012
Analysis of northern hemisphere stratospheric data from 1978-2011 is used to identify and classify Stratospheric Sudden Warming events. A total of 41 events are identified during this 33 year period, resulting in an average occurrence rate of 1.24 events/year. No significant variation in the rate is observed during the period analyzed. The average temperature increase during an SSW event is 12 K and the average duration is 32 days. Each identified event is classified as either a vortex displacement or split event and the ratio of displacement to split events is found to be 0.86.
Life Cycle of the Northern Hemisphere Sudden Stratospheric Warming
Published Feb 2004.
Motivated by recent evidence of strong stratospheric–tropospheric coupling during the Northern Hemisphere
winter, this study examines the evolution of the atmospheric flow and wave fluxes at levels throughout the
stratosphere and troposphere during the composite life cycle of a sudden stratospheric warming. The composite comprises 39 major and minor warming events using 44 years of NCEP–NCAR reanalysis data. The incipient stage of the life cycle is characterized by preconditioning of the stratospheric zonal flow and anomalous, quasi-stationary wavenumber-1 forcing in both the stratosphere and troposphere. As the life cycle intensifies, planetary wave driving gives rise to weakening of the stratospheric polar vortex and downward propagation of the attendant easterly wind and positive temperature anomalies. When these anomalies reach the tropopause, the life cycle is marked by momentum flux and mean meridional circulation anomalies at tropospheric levels that are consistent with the negative phase of the Northern Hemisphere annular mode. The anomalous momentum fluxes are largest over the Atlantic half of the hemisphere and are associated primarily with waves of wavenumber 3 and higher.
More Frequent Sudden Stratospheric Warming Events due to Enhanced MJO Forcing Expected in a Warmer Climate
Published July 2017.
Sudden stratospheric warming (SSW) events influence the Arctic Oscillation and midlatitude extreme
weather. Observations show SSW events to be correlated with certain phases of the Madden–Julian oscil-
lation (MJO), but the effect of the MJO on SSW frequency is unknown, and the teleconnection mechanism, its
planetary wave propagation path, and time scale are still not completely understood. The Arctic stratosphere
response to increased MJO forcing expected in a warmer climate using two models is studied: the compre-
hensive Whole Atmosphere Community Climate Model and an idealized dry dynamical core with and
without MJO-like forcing. It is shown that the frequency of SSW events increases significantly in response to
stronger MJO forcing, also affecting the averaged polar cap temperature. Two teleconnection mechanisms
are identified: a direct propagation of MJO-forced transient waves to the Arctic stratosphere and a nonlinear
enhancement of stationary waves by the MJO-forced transient waves. The MJO-forced waves propagate
poleward in the lower stratosphere and upper troposphere and then upward. The cleaner results of the
idealized model allow identifying the propagating signal and suggest a horizontal propagation time scale of
10–20 days, followed by additional time for upward propagation within the Arctic stratosphere, although
there are significant uncertainties involved. Given that the MJO is predicted to be stronger in a warmer
climate, these results suggest that SSW events may become more frequent, with possible implications on
tropospheric high-latitude weather. However, the effect of an actual warming scenario on SSW frequency
involves additional effects besides a strengthening of the MJO, requiring further investigation.
Multi‐decadal variability of sudden stratospheric warmings in an AOGCM
Published Jan 2011.
The variability in the number of major suddenstratospheric warmings (SSWs) is analyzed in a multi‐centurysimulation under constant forcing using a stratosphereresolving atmosphere‐ocean general circulation model.A wavelet‐analysis of the SSW time series identifiessignificantly enhanced power at a period of 52 years. Thecoherency of this signal with tropospheric and oceanicparameters is investigated. The strongest coherence is foundwith the North Atlantic ocean‐atmosphere heat‐flux fromNovember to January. Here, an enhanced heat‐flux fromthe ocean into the atmosphere is related to an increase inthe number of SSWs. Furthermore, a correlation is foundwith Eurasian snow cover in October and the number ofblockings in October/November. These results suggest thatthe multi‐decadal variability is generated within the ocean‐troposphere‐stratosphere system. A two‐way interactionof the North Atlantic and the atmosphere buffers andamplifies stratospheric anomalies, leading to a coupledmulti‐decadal mode.
No robust evidence of future changes in major stratospheric sudden warmings: a multi-model assessment from CCMI
Published Mar 2018.
Major mid-winter stratospheric sudden warmings (SSWs) are the largest instance of wintertime variability in the Arctic stratosphere. Because SSWs are able to cause significant surface weather anomalies on intra-seasonal
timescales, several previous studies have focused on their potential future change, as might be induced by anthropogenic forcings. However, a wide range of results have been reported, from a future increase in the frequency of SSWs to an actual decrease. Several factors might explain these contradictory results, notably the use of different metrics for the identification of SSWs and the impact of large climatological biases in single-model studies. To bring some clarity, we here revisit the question of future SSW changes, using an identical
set of metrics applied consistently across 12 different models participating in the Chemistry–Climate Model Initiative. Our analysis reveals that no statistically significant change in the frequency of SSWs will occur over the 21st century, irrespective of the metric used for the identification of the event. Changes in other SSW characteristics – such as their duration, deceleration of the polar night jet, and the tropospheric forcing – are also assessed: again, we find no evidence of future changes over the 21st century.
Northern Hemisphere mid‐winter vortex‐displacement and vortex‐split stratospheric sudden warmings: Influence of the Madden‐Julian Oscillation and Quasi‐Biennial Oscillation
Published Nov 2014.
We investigate the connection between the equatorial Madden-Julian Oscillation (MJO) anddifferent types of the Northern Hemisphere mid-winter major stratospheric sudden warmings (SSWs), i.e.,vortex-displacement and vortex-split SSWs. The MJO-SSW relationship for vortex-split SSWs is stronger thanthat for vortex-displacement SSWs, as a result of the stronger and more coherent eastward propagating MJOsbefore vortex-split SSWs than those before vortex-displacement SSWs. Composite analysis indicates that boththe intensity and propagation features of MJO may influence the MJO-related circulation pattern at highlatitudes and the type of SSWs. A pronounced Quasi-Biennial Oscillation (QBO) dependence is found forvortex-displacement and vortex-split SSWs, with vortex-displacement (-split) SSWs occurring preferentially ineasterly (westerly) QBO phases. The lagged composites suggest that the MJO-related anomalies in the Arctic arevery likely initiated when the MJO-related convection is active over the equatorial Indian Ocean (around theMJO phase 3). Further analysis suggests that the QBO may modulate the MJO-related wave disturbances viaits influence on the upper tropospheric subtropical jet. As a result, the MJO-related circulation pattern in theArctic tends to be wave number-one/wave number-two ~25–30 days following phase 3 (i.e., approximatelyphases 7–8, when the MJO-related convection is active over the western Pacific) during easterly/westerly QBO phases, which resembles the circulation pattern associated with vortex-displacement/vortex split SSWs.
Observed connection between stratospheric sudden warmings and the Madden-Julian Oscillation
Published: Sept 2012
Abstract: The effect of the Madden-Julian Oscillation (MJO) on the Northern Hemisphere wintertime stratospheric polar vortex and major, mid-winter stratospheric sudden warmings (SSWs) is evaluated using a meteorological reanalysis dataset. The MJO influences the region in the tropospheric North Pacific sector that is most strongly associated with a SSW. Consistent with this, SSWs in the reanalysis record have tended to follow certain MJO phases. The magnitude of the influence of the MJO on the vortex is comparable to that associated with the Quasi-Biennial Oscillation and El Niño. The MJO could be used to improve intra-seasonal projections of the Northern Hemisphere high latitude circulation, and in particular of the tropospheric Northern Annular Mode, at lags exceeding one month.
Observed Relationships Between Sudden Stratospheric Warmings and European Climate Extremes
2019 paper. NOTE: currently behind a paywall, so abstract only:
Sudden stratospheric warmings (SSWs) have been linked with anomalously cold temperatures at the surface in the middle to high latitudes of the Northern Hemisphere as climatological westerly winds in the stratosphere tend to weaken and turn easterly. However, previous studies have largely relied on reanalyses and model simulations to infer the role of SSWs on surface climate and SSW relationships with extremes have not been fully analyzed. Here, we use observed daily gridded temperature and precipitation data over Europe to comprehensively examine the response of climate extremes to the occurrence of SSWs. We show that for much of Scandinavia, winters with SSWs are on average at least 1 °C cooler, but the coldest day and night of winter is on average at least 2 °C colder than in non‐SSW winters. Anomalously high pressure over Scandinavia reduces precipitation on the northern Atlantic coast but increases overall rainfall and the number of wet days in southern Europe. In the 60 days after SSWs, cold extremes are more intense over Scandinavia with anomalously high pressure and drier conditions prevailing. Over southern Europe there is a tendency toward lower pressure, increased precipitation and more wet days. The surface response in cold temperature extremes over northwest Europe to the 2018 SSW was stronger than observed for any SSW during 1979–2016. Our analysis shows that SSWs have an effect not only on mean climate but also extremes over much of Europe. Only with carefully designed analyses are the relationships between SSWs and climate means and extremes detectable above synoptic‐scale variability.
On the Relationship between ENSO, Stratospheric Sudden Warmings and Blocking
Published June 2014
This paper examines the influence of El Niño–Southern Oscillation (ENSO) on different aspects of major stratospheric sudden warmings (SSWs), focusing on the precursor role of blocking events. The results reveal an ENSO modulation of the blocking precursors of SSWs. European and Atlantic blocks tend to precede SSWs during El Niño (EN), whereas eastern Pacific and Siberian blocks are the preferred precursors of SSWs during La Niña (LN) winters. This ENSO preference for different blocking precursors seems to occur through an ENSO effect on regional blocking persistence, which in turn favors the occurrence of SSWs. The regional blocking precursors of SSWs during each ENSO phase also have different impacts on the upward propagation of planetary-scale wavenumbers 1 and 2; hence, they determine ENSO differences in the wavenumber signatures of SSWs. SSWs occurring during EN are preceded by amplification of wavenumber 1, whereas LN SSWs are predominantly associated to wavenumber-2 amplification. However, there is not a strong preference for splitting or displacement SSWs during any ENSO phase. This is mainly because during EN, splitting SSWs do not show a wavenumber-2 pattern.
Orography and the Boreal Winter Stratosphere: the Importance of the Mongolian mountains
Planetary‐scale wave activity as a source of varying tropospheric response to stratospheric sudden warming events: A case study
Predicting Sudden Stratospheric Warming 2018 and Its Climate Impacts With a Multimodel Ensemble
Published Dec 2018.
Sudden stratospheric warmings (SSWs) are significant source of enhanced subseasonal predictability, but whether this source is untapped in operational models remains an open question. Here we report on the prediction of the SSW on 12 February 2018, its dynamical precursors, and surface climate impacts by an ensemble of dynamical forecast models. The ensemble forecast from 1 February predicted 3 times increased odds of an SSW compared to climatology, although the lead time for SSW prediction varied among individual models. Errors in the forecast location of a Ural high and underestimated magnitude of upward wave activity flux reduced SSW forecast skill. Although the SSW's downward influence was not well forecasted, the observed northern Eurasia cold anomaly following SSW was predicted, albeit with a weaker magnitude, due to persistent tropospheric anomalies. The ensemble forecast from 8 February predicted the SSW, its subsequent downward influence, and a long-lasting cold anomaly at the surface.
Predictability of the coupled troposphere-stratosphere system
Role of gravity waves in vertical coupling during sudden stratospheric warmings
Simulation of the December 1998 Stratospheric Major Warming
1999 paper. Abstract:
An atypically early major stratospheric sudden warming in mid-Dec 1998 resulted in an abnormally warm and weak polar vortex through most of the 1998-99 winter. The first major warming in nearly 8 years, it was only the second major warming observed before the end of Dec, and strongly resembled the previous Dec 1987 major warming in several characteristics atypical of major warmings later in winter. 3D mechanistic model simulations reproduced most characteristics of the Dec 1998 major warming, including the magnitudes of zonal mean easterlies and temperature in- creases and the 3D evolution of the flow, paving the way for more detailed future studies of the mechanisms involved in this unusual event.
Solar and QBO Influences on the Timing of Stratospheric Sudden Warmings
Stratospheric polar vortex splits and displacements in the high‐top CMIP5 climate models
Published Jan 2016.
Sudden stratospheric warming (SSW) events can occur as either a split or a displacement of the stratospheric polar vortex. Recent observational studies have come to different conclusions about the relative impacts of these two types of SSW upon surface climate. A clearer understanding of their tropospheric impact would be beneficial for medium‐range weather forecasts and could improve understanding of the physical mechanism for stratosphere‐troposphere coupling. Here we perform the first multimodel comparison of stratospheric polar vortex splits and displacements, analyzing 13 stratosphere‐resolving models from the fifth Coupled Model Intercomparison Project (CMIP5) ensemble. We find a wide range of biases among models in both the mean state of the vortex and the frequency of vortex splits and displacements, although these biases are closely related. Consistent with observational results, almost all models show vortex splits to occur barotropically throughout the depth of the stratosphere, while vortex displacements are more baroclinic. Vortex splits show a slightly stronger North Atlantic surface signal in the month following onset. However, the most significant difference in the surface response is that vortex displacements show stronger negative pressure anomalies over Siberia. This region is shown to be colocated with differences in tropopause height, suggestive of a localized response to lower stratospheric potential vorticity anomalies.
Sudden Stratospheric Warmings and Anomalous Upward Wave Activity Flux
Sudden Stratospheric Warmings – developing a new classification based on vertical depth, applying theory to a SSW in 2018, and assessing predictability of a cold air outbreak following this SSW
The Downward Influence of Sudden Stratospheric Warmings: Association with Tropospheric Precursors
The early major warming in December 2001 – exceptional?
2002 paper. Abstract:
The early major warming in December 2001 is described and compared to the two other December major warmings in 1998 and 1987, showing a strong tropospheric‐stratospheric coupling in all three cases. We argue that the occurrence of free westward propagating Rossby waves interacting with a forced quasi‐stationary wave number 1 led to these three early events. The possible excitation of these waves is discussed with respect to the tropospheric circulation, which showed strong blockings over the northern Atlantic prior to the early major warmings.
The Effect of Climate Change on the Variability of the Northern Hemisphere Stratospheric Polar Vortex
The effects of different sudden stratospheric warming types on the ocean
The influence of the equatorial upper stratosphere on stratospheric sudden warmings.
2003 paper. Abstract:
A stratosphere mesosphere model is used in a set of idealized experiments to investigate the sensitivity of the Northern Hemisphere winter stratospheric flow to the equatorial zonal winds in the upper and the lower stratosphere. Sensitivity is found to the lower stratospheric winds (below 40 km) only in the early part of the model integration as the model relaxes towards a perpetual winter state. On the other hand the model showed significant sensitivity to the upper stratospheric winds throughout the major part of the integration. This has possible implications for the influence of the quasi‐biennial oscillation (QBO) and solar cycle on the northern winter stratospheric flow. It suggests that the lower stratospheric QBO may influence the early winter, but later in the winter when the flow is highly non‐linear the greatest influence may come from variations in the semi‐annual oscillation (SAO) which dominates the equatorial winds in the upper stratosphere. Possible sources of variations in the SAO include both the influence of the QBO and of the 11‐year solar cycle.
The preconditioning of major sudden stratospheric warmings
The role of North Atlantic-European weather regimes in the surface impact of sudden stratospheric warming events
Revised & Published Aug 2020.
Sudden stratospheric warming (SSW) events can significantly impact tropospheric weather for a period of several weeks, in particular in the North Atlantic–European (NAE) region. While the stratospheric forcing often projects onto the North Atlantic Oscillation (NAO), the tropospheric response to SSW events, if any, is highly variable, and what determines the existence, location, timing, and strength of the downward impact remains an open question. We here explore how the variable tropospheric response to SSW events in the NAE region can be characterized in terms of a refined set of seven weather regimes and if the tropospheric flow in the North Atlantic region around the onset of SSW events is an indicator of the subsequent downward impact. The weather regime analysis reveals the Greenland blocking (GL) and Atlantic trough (AT) regimes as the most frequent large-scale patterns in the weeks following an SSW. While the GL regime is dominated by high pressure over Greenland, AT is dominated by a southeastward-shifted storm track in the North Atlantic. The flow evolution associated with GL and the associated cold conditions over Europe in the weeks following an SSW occur most frequently if a blocking situation over western Europe and the North Sea (European blocking) prevailed around the SSW onset. In contrast, an AT regime associated with mild conditions over Europe is more likely following the SSW event if GL occurs already around SSW onset. For the remaining tropospheric flow regimes during SSW onset we cannot identify a dominant flow evolution. Although it remains unclear what causes these relationships, the results suggest that specific tropospheric states in the days around the onset of the SSW are an indicator of the subsequent tropospheric flow evolution in the aftermath of an SSW, which could provide crucial guidance for subseasonal prediction.
The role of stratosphere‐troposphere coupling in the occurrence of extreme winter cold spells over northern Europe
The role of wave–wave interactions in sudden stratospheric warming formation
Published March 2020.
The effects of wave–wave interactions on sudden stratospheric warming formation are investigated using an idealized atmospheric general circulation model, in which tropospheric heating perturbations of zonal wave numbers 1 and 2 are used to produce planetary-scale wave activity. Zonal wave–wave interactions are removed at different vertical extents of the atmosphere in order to examine the sensitivity of stratospheric circulation to local changes in wave–wave interactions. We show that the effects of wave–wave interactions on sudden warming formation, including sudden warming frequencies, are strongly dependent on the wave number of the tropospheric forcing and the vertical levels where wave–wave interactions are removed. Significant changes in sudden warming frequencies are evident when wave–wave interactions are removed even when the lower-stratospheric wave forcing does not change, highlighting the fact that the upper stratosphere is not a passive recipient of wave forcing from below. We find that while wave–wave interactions are required in the troposphere and lower stratosphere to produce displacements when wave number 2 heating is used, both splits and displacements can be produced without wave–wave interactions in the troposphere and lower stratosphere when the model is forced by wave number 1 heating. We suggest that the relative strengths of wave number 1 and 2 vertical wave flux entering the stratosphere largely determine the split and displacement ratios when wave number 2 forcing is used but not wave number 1.
The Role of Zonal Asymmetry in the Enhancement and Suppression of Sudden Stratospheric Warming Variability by the Madden–Julian Oscillation
The roles of planetary and gravity waves during a major stratospheric sudden warming as characterized in WACCM
The Stratospheric Major Warming of Early December 1987
1989 paper. Abstract:
The stratospheric major warming of early December 1987 is analyzed using NMC observations of temperature and geopotential. This warming is distinguished as the earliest major warming ever recorded in the Northern Hemisphere winter. The observed mean zonal wind reversal and reversed poleward temperature gradient at 10 mb were preceded by the anomalous amplification of a zonal wavenumber 1 planetary wave emanating from the troposphere. This planetary wave event, similarly, is distinguished for having produced the second largest sustained flux of wavenumber 1 activity ever observed to propagate upwards from the troposphere at such an early time in the winter. Within the troposphere, the amplification of wave 1 was accompanied by several simultaneous blocking episodes, but it is unclear whether this blocking caused the anomalous formation of the planetary wave (or vice versa, or neither). Amplification of the planetary wave within the stratosphere led to a significant off-polar displacement of the circumpolar vortex and a reduction in vortex area, as observed in connection with other warmings. However, in the present case there is less significant evidence of a preconditioned stratospheric vortex, except for a small precursor event in late November which may have slightly retarded the normal, climatological expansion of the vortex. Therefore, it appears that this unusually early major warming was mainly attributable to an anomalously large tropospheric forcing.
After the climax of the warming, the midstratosphere vortex was observed to split into a double-vortex pattern. This feature is quite striking when viewed three-dimensionally, as the two 850 K vortex components remained contiguous, respectively, with single vortices in the upper and lower stratosphere. Thereafter, the stratosphere returned to a cold, undisturbed pattern until the beginning of March, when an early final warming occurred. The relatively cold January-February period coincided with the deep westerly phase of the equatorial quasi-biennial oscillation (QBO), as observed in connection with other cold, undisturbed winters. However, the QBO had already attained this phase by early December 1987, suggesting that the phase of the QBO per se is insufficient to prevent the occurrence of a major warming.
The stratopause evolution during different types of sudden stratospheric warming event
Transient Tropospheric Forcing of Sudden Stratospheric Warmings
2012 paper. Abstract:
The amplitude of upward-propagating tropospherically forced planetary waves is known to be of first-order importance in producing sudden stratospheric warmings (SSWs). This forcing amplitude is observed to undergo strong temporal fluctuations. Characteristics of the resulting transient forcing leading to SSWs are studied in reanalysis data and in highly truncated simple models of stratospheric wave–mean flow interaction.It is found in both the reanalysis data and the simple models that SSWs are preferentially generated by transient forcing of sufficiently long time scales (on the order of 1 week or longer). The time scale of the transient forcing is found to play a stronger role in producing SSWs than the strength of the forcing. In the simple models it is possible to fix the amplitude of the tropospheric forcing but to vary the time scale ofthe forcing. The resulting frequency of occurrence of SSWs shows dramatic reductions for decreasing forcingtime scales.
Tropospheric Precursors and Stratospheric Warmings
Tropospheric and Stratospheric Precursors to the January 2013 Sudden Stratospheric Warming
Vortex dynamics of stratospheric sudden warmings: a reanalysis data study using PV contour integral diagnostics
2019 paper. Abstract:
The dynamics of the polar vortex underlying stratospheric sudden warming (SSW)events is investigated in a data-based diagnostic study. Potential vorticity (PV) contour integral quantities on isentropic surfaces are discussed in a unified framework; new expressions for their time evolution, particularly suitable for evaluation from data,are presented and related to previous work. Diagnostics of mass and circulation are calculated from ERA-40 reanalysis data for the stratosphere in case studies of 7 winters with different SSW behaviour. The edge of the polar vortex is easily identifiable in these diagnostics as an abrupt transition from high to low gradients of PV, and the warming events are clearly visible. The amount of air stripped from the vortex as part of a preconditioning leading up to the warming events is determined using the balance equation of the mass integral. Significant persistent removal of mass from the vortex is found, with several such stripping events identifiable throughout the winter, especially in those during which a major sudden warming event occurred. These stripping episodes are visible in corresponding PV maps, where tongues of high PV are being stripped from the vortex and mixed into the surrounding surf zone. An attempt is made to diagnose from the balance equation of the circulation the effect of frictional forces such as gravitywave dissipation on the polar vortex.
Vortex Preconditioning due to Planetary and Gravity Waves prior to Sudden Stratospheric Warmings
What kind of stratospheric sudden warming propagates to the troposphere?
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