Sudden Stratospheric Warming Research Articles

Tropospheric links to uncertainty in stratospheric subseasonal predictions

Abstract. Variability in the stratosphere, especially extreme events such as sudden stratospheric warmings (SSWs), can impact surface weather. Understanding stratospheric prediction uncertainty is therefore crucial for skillful surface weather forecasts on weekly to monthly timescales. Using ECMWF subseasonal hindcasts, this study finds that stratospheric uncertainty is most strongly linked to tropospheric uncertainty over the North Pacific and Northern Europe, regions that can modulate but also respond to stratospheric variability, suggesting a two-way propagation of uncertainty. A case study of the 2018 SSW event shows an initial poleward and upward propagation of uncertainty from tropical convection, followed by a downward propagation where ensemble members that accurately predict the SSW are also better at predicting its downward impacts. These findings highlight the locations in the troposphere that are linked to stratospheric uncertainty and suggest that improved model representation of tropospheric mechanisms linked to polar vortex variability could enhance both stratospheric and extratropical surface prediction.

Sporadic E Layer Intensification in the Winter of 2009 Examined by FORMOSAT‐3/COSMIC RO Data and GAIA Model

AbstractThis study examines the role of winds in wintertime sporadic E layer intensification (WEsLI) in 2009 from a global viewpoint. Previous studies showed that sporadic E layer (EsL) intensity had increased for 20–30 days in some winters, although intense EsLs do not form generally in winter. A recent study found that vertical ion convergence (VIC) driven by intensified migrating semidiurnal (SW2) tides caused WEsLI at middle latitudes in 2009. However, no studies have investigated the global distributions and generation mechanisms of WEsLI in 2009. Herein, we employed FORMOSAT‐3/COSMIC radio occultations to investigate the global distributions of WEsLI in 2009. Distributions of VIC driven by winds obtained from the Ground‐to‐topside model of Atmosphere and Ionosphere for Aeronomy were compared with global WEsLI distributions to elucidate the role of winds in WEsLI. We found that WEsLI in 2009 occurred at geomagnetic low/middle latitudes except between W and E. WEsLI was observed below 120 km altitudes, especially at 12–17 local times. WEsLI was attributable to VIC driven by SW2 tides, migrating diurnal tides, and eastward propagating diurnal tides with wavenumber 3. Tidal amplifications were possibly related to mesospheric/stratospheric atmospheric variations such as sudden stratospheric warming, zonal mean zonal winds, and quasi‐biennial oscillations. WEsLI in 2009 is further evidence of the coupling between EsLs and mesospheric/stratospheric atmospheric variations through tidal modifications.

Predictability of European winter 2022/23

AbstractThe boreal winter of 2022/23 was notable as a third consecutive winter in which La Niña had an influence on the European weather. The GloSea6 seasonal forecast system predicted a blocked circulation pattern in the North Atlantic in early winter (December), and then a transition through mid‐winter (January) into a more zonal pattern in late winter (February), consistent with the canonical La Niña teleconnection pattern seen previously. The seasonal forecast for the UK was an increased likelihood of near average temperatures, and drier‐ and calmer‐than‐average conditions. Both the predicted broad‐scale circulation patterns and UK winter mean weather conditions verified well against observations, and we show that seasonal forecasts of the North Atlantic Oscillation (NAO) over the last 10 winters show similar skill to previously reported hindcasts. Throughout the winter, the Madden–Julian Oscillation (MJO) was particularly active. On three occasions, it exhibited strong phases 6 and 7. There was also a sudden stratospheric warming (SSW) that occurred on 16th February. This was followed by colder conditions and associated impacts similar to the canonical negative NAO response over the UK, although the main impact fell in March and so did not affect the winter (December–January–February) mean conditions.

Mechanisms Underlying the Changes in Sporadic E Layers During Sudden Stratospheric Warming

During sudden stratospheric warming (SSW) events, significant modifications occur, not only in the neutral atmosphere, but also in the ionosphere. Specifically, sporadic E layers in the mesosphere and lower thermosphere regions significantly disrupt satellite communication. Although research has frequently focused on ionospheric alterations during SSW events, detailed studies on sporadic E layers remain limited. Examining these variations during SSW events could enhance our understanding of the interaction mechanisms between the ionosphere and the neutral atmosphere, and provide insights into the patterns of sporadic E layer alterations. This study analyzed the behavior of sporadic E layers during the 2008/2009 winter SSW period using data from three Japanese stations and satellite observations. The principal findings included the following: (1) The enhancement in the critical frequency of the sporadic E layers was most notable following the transition from easterly to westerly winds at 60° N at a 10 hPa altitude, accompanied by quasi 6-day and quasi 16-day oscillations in frequency. (2) The daily average zonal and meridional wind speeds in the MLT region also exhibited quasi 6-day and quasi 16-day oscillations, aligning with the observed periodicities in the critical frequency of the sporadic E layers. (3) Planetary waves were shown to modulate the amplitude of diurnal and semidiurnal tides, influencing the sporadic E layers. Furthermore, a wavelet analysis of foEs data with a time resolution of 0.25 h demonstrated that planetary waves also modulate the frequency of diurnal tides, thereby affecting the sporadic E layers. This research contributes to a deeper understanding of the formation mechanisms and prediction of sporadic E layer behavior.

Signature of a zonally symmetric semidiurnal tide during major sudden stratospheric warmings and plausible mechanisms: a case study

Sudden Stratospheric Warming (SSW) is a winter phenomenon initiated primarily by the enhanced stationary planetary waves (SPWs), characterized by an increase in polar stratospheric temperature by a few tens of kelvin for a few days. Wave-wave non-linear interaction can produce secondary waves, with sum and difference frequencies of the primary wave frequencies. The sun-synchronous semidiurnal tide is a major component at mid and high latitude middle atmosphere, which non-linearly interacts with the dominant SPW in the stratosphere to produce the zonally symmetric semidiurnal tide component (S0), as observed during two boreal SSWs. The zonally symmetric distribution of ozone has also potential to excite the S0 component by absorption of solar ultraviolet radiation as evident during a rare Austral SSW. Overall, the present study sheds light on the dominant generation mechanisms involved in the S0 enhancement during the SSW.

Evidence of the double amplification of travelling planetary waves during the January 2021 sudden stratospheric warming

Abstract The in situ generation and characteristics of planetary waves (PWs) in the mesosphere and lower thermosphere (MLT) during the January 2021 sudden stratospheric warming (SSW) are investigated using the Navy Global Environmental Model. During a SSW, upward-propagating PWs can be absorbed, refracted, or amplified, the latter implying in situ PW generation. Additionally, asymmetric dissipation of GW drag in the MLT due to varying stratospheric winds may also seed PW generation. Ultimately, the interaction of waves with instability can enhance westward-propagating, quasi-stationary, or eastward-propagating components of PWs (WPW, QSPWs, and EPWs, respectively). The propagation pathway of a wave is diagnosed by its refractive index and is dependent on factors such as the baroclinic/barotropic stability of the atmosphere, the wave’s relative phase velocity, and the wavenumber. Through these pathways waves are able to transfer heat and momentum throughout the atmosphere. Under particular conditions, the waves can amplify in situ by extracting energy from the background wind. Our study identifies two scenarios in which WPWs and EPWs were amplified successively. Amplified WPWs propagated upwards and influenced the MLT while amplified EPWs propagated downwards and influenced the upper troposphere.

Tropospheric amplification of stratosphere–troposphere coupling

AbstractDuring northern winter the stratospheric polar vortex is highly variable — even breaking down completely during sudden stratospheric warmings. The strength of the stratospheric polar vortex is observed to be coupled to surface weather, primarily through swings in the phase of the Northern Annular Mode — which is nearly uuivalent to polar‐cap sea‐level pressure. Pressure changes (as a function of height) are largest at the surface, which is difficult to reconcile with the remote effects of stratospheric variability. This conundrum requires some mechanism(s) to amplify the polar sea‐level pressure anomalies. A simple, intuitive explanation has remained elusive. We use Japanese 55‐year Reanalysis (JRA‐55) reanalyses to investigate the reason that stratospheric variability causes large polar sea‐level pressure changes. The main result of this paper is that vertical movement of the polar tropopause is essential in understanding how surface pressure changes are generated. The strength of the polar vortex is strongly coupled to the height of the polar tropopause, resulting in stretching/compression of the polar tropospheric air column. This causes pressure changes that are largest at the surface. For example, the lowering of the polar tropopause during a sudden stratospheric warming leads to a compression and spin down of the tropospheric column and is therefore associated with higher polar sea‐level pressure. This mechanism can be compared to the textbook example of conservation of potential vorticity as an air column moves over a mountain range — except in this case the column depth changes because the stratosphere is changing the height of the tropopause. The observed vertical profile of pressure changes matches closely what would be expected from the stretching/compression mechanism. The higher polar sea‐level pressure associated with a sudden stratospheric warming is due mainly to mass moving into the polar cap within the troposphere, rather than rearranged mass in the stratosphere.

Global-scale gravity wave analysis methodology for the ESA Earth Explorer 11 candidate CAIRT

Abstract. In the past, satellite climatologies of gravity waves (GWs) have initiated progress in their representation in global models. However, these could not provide the phase speed and direction distributions needed for a better understanding of the interaction between GWs and the large-scale winds directly. The ESA Earth Explorer 11 candidate CAIRT could provide such observations. CAIRT would use a limb-imaging Michelson interferometer resolving a wide spectral range, allowing temperature and trace gas mixing ratio measurements. With the proposed instrument design, a vertical resolution of 1 km, along-track sampling of 50 km, and across-track sampling of 25 km in a 400 km wide swath will be achieved. In particular, this allows for the observation of three-dimensional (3D), GW-resolving temperature fields throughout the middle atmosphere. In this work, we present the methodology for the GW analysis of CAIRT observations using a limited-volume 3D sinusoidal fit (S3D) wave analysis technique. We assess the capability of CAIRT to provide high-quality GW fields by the generation of synthetic satellite observations from high-resolution model data and comparison of the synthetic observations to the original model fields. For the assessment, wavelength spectra, phase speed spectra, horizontal distributions, and zonal means of GW momentum flux (GWMF) are considered. The atmospheric events we use to exemplify the capabilities of CAIRT are the 2006 sudden stratospheric warming (SSW) event, the quasi-biennial oscillation (QBO) in the tropics, and the mesospheric preconditioning phase of the 2019 SSW event. Our findings indicate that CAIRT would provide highly reliable observations not only of global-scale GW distributions and drag patterns but also of specific wave events and their associated wave parameters. Even under worse-than-expected noise levels of the instrument, the resulting GW measurements are highly consistent with the original model data. Furthermore, we demonstrate that the estimated GW parameters can be used for ray tracing, which physically extends the horizontal coverage of the observations beyond the orbit tracks.

Observational perspective on sudden stratospheric warmings and blocking from Eliassen–Palm fluxes

Observational perspective on sudden stratospheric warmings and blocking from Eliassen–Palm fluxes

Impact of Polar Vortex Modes on Winter Weather Patterns in the Northern Hemisphere

This study is an additional investigation of stratosphere–troposphere coupling based on the recent stratospheric winter descriptions in five distinct modes: January, February, Double, Dynamical, and Dadiative. These modes, established in a previous study, categorize the main stratospheric winter typologies modulated by the timing of important sudden stratospheric warmings (SSWs) and final stratospheric warmings (FSWs). The novelty of this research is to investigate the Northern Annular Mode, mean sea level pressure (MSLP) anomalies in the Ural and Aleutian regions, and the decomposition of Eliassen–Palm flux into wavenumbers 1 and 2 within each mode. The results show that the January and Double modes exhibit similar pre-warming surface signals, characterized by Ural blocking and Aleutian trough events preceding weak polar vortex events. The January mode displays a positive MSLP anomaly of +395 hPa (−191 hPa) in the Ural (Aleutian) region in December, while the Double mode shows +311 hPa (−89 hPa) in November. These modes are primarily wave-1 driven, generating tropospheric responses via negative Arctic Oscillation patterns. Conversely, the February and Dynamical modes show opposite signals, with Aleutian blocking and Ural trough events preceding strong polar vortex events. In December, the February mode exhibits MSLP anomalies of +119 hPa (Aleutian) and −180 hPa (Ural), while the Dynamical mode shows +77 hPa and −184 hPa, respectively. These modes, along with important SSWs in February and dynamical FSWs, are driven by both wave-1 and wave-2 and do not significantly impact the troposphere. The Radiative mode’s occurrence is strongly related to the Aleutian blocking presence. These findings confirm that SSW timing is influenced by specific dynamical forcing related to surface precursors and underscore its importance in subsequent tropospheric responses. This study establishes a connection between early winter tropospheric conditions and upcoming stratospheric states, potentially improving seasonal forecasts in the northern hemisphere.

Effects of extreme stratospheric polar vortex events on near-surface wind gusts across Europe

Abstract Extreme stratospheric polar vortex (SPV) events can influence winter tropospheric circulation for up to 60 d. Their impacts on air temperature have been extensively studied recently. However, there is a research gap in their effects on wind speeds and its extremes. This study aims to evaluate, for the first time, the impacts of such extreme SPV events on observed and modelled near-surface wind gusts across Europe. We have analysed wind gust data coming from: station-based observations (for the Iberian Peninsula and Scandinavia), the ERA5 reanalysis and the SEAS5 and GloSea6 seasonal forecasting systems. We assess their similarities in reproducing 4 parameters of their corresponding distributions: median, standard deviation, skewness and kurtosis. For all these datasets, the results indicate that extreme positive SPV events are followed by negative wind gust anomalies in Southern Europe and positive in Northern Europe. Whereas, negative SPV events (such as Sudden Stratospheric Warmings) have positive gust anomalies in Southern Europe and negative in the north. A central region shows negligible anomalies in both cases. This highlights the ability of SPV as a predictor for short-medium-term forecasting of extreme wind events, which would have direct applications to many socioeconomic and environmental issues such as the estimation of wind-power generation.

What Drives the Spread and Bias in the Surface Impact of Sudden Stratospheric Warmings in CMIP6 Models?

Abstract This study evaluates the representation of the composite-mean surface response to sudden stratospheric warmings (SSWs) in 28 CMIP6 models. Most models can reproduce the magnitude of the SLP response over the Arctic, although the simulated Arctic SLP response varies from model to model. Regarding the structure of the SLP response, most models exhibit a basin-symmetric negative Northern Annular Mode (NAM)-like response with a cyclonic Pacific SLP response, whereas the reanalysis shows a highly basin-asymmetric negative NAO-like response without a robust Pacific center. We then explore the drivers of these model biases and spread by applying a multiple linear regression (MLR). The results show that the polar cap temperature anomalies at 100 hPa (ΔT100) modulate the magnitude of both the Arctic SLP response and the cyclonic Pacific SLP response. Apart from ΔT100, the intensity and latitudinal location of the climatological eddy-driven jet in the troposphere also affect the magnitude of the Arctic SLP response. The compensation of model biases in these two tropospheric metrics and the good model representation of ΔT100 explain the good agreement between the ensemble mean and the reanalysis on the magnitude of the Arctic SLP response, as indicated by the fact that the ensemble mean lies well within the reanalysis uncertainty range and that the reanalysis mean sits well within the model distribution. The Niño-3.4 SST anomalies and North Pacific SST dipole anomalies together with ΔT100 modulate the cyclonic Pacific SLP response. In this case, biases in both oceanic drivers work in the same direction and lead to the cyclonic Pacific SLP response in models that are not present in the reanalysis. Significance Statement Sudden stratospheric warmings (SSWs) represent an important source of skill for forecasting winter weather on subseasonal-to-seasonal time scales. To what extent SSWs could be used to improve the prediction of surface weather depends on how well stratosphere–troposphere coupling associated with SSWs is represented in climate models. Therefore, we evaluate the representation of stratosphere–troposphere coupling associated with SSWs in 28 state-of-the-art climate models. The representation is found to diverge widely among climate models, and some are biased noticeably from the reanalysis. The models’ spread and bias are largely driven by five major factors and can be reduced substantially by making bias corrections to these factors.

The Impact of Polar Vortex Strength on the Longitudinal Structure of the Noontime Mid-Latitude Ionosphere and Thermosphere

Ground-based ionospheric, CHAMP/STAR, and GOCE satellite neutral density ρ observations under deep solar minimum conditions were used to find whether there is a dependence of longitudinal variations on polar vortex strength. Ionospheric stations at fixed-dipole geomagnetic Φ ≈ 38° and geographic φ ≈ 40°N latitudes located in ‘near-pole’ and ‘far-from-pole’ longitudinal sectors were used in the analysis. No significant longitudinal NmF2 (electron concentration in the F2-layer maximum) dependence on the polar vortex strength was revealed. Geomagnetic control was shown to be responsible for the observed longitudinal NmF2 variations. Satellite-observed longitudinal variations in neutral density did not show any visible reaction to the polar vortex strength. However, the impact of sudden stratospheric warming (SSW) on the upper atmosphere is strong enough to change the neutral density longitudinal distribution. The impact of SSW shows a global occurrence and ‘works’ within 3–5 days in geographic coordinates in the vicinity of the SSW peak. Atomic oxygen values retrieved under ‘strong’ and ‘weak’ polar vortex strengths confirm the results obtained on longitudinal variations in NmF2 and ρ. In conclusion, no visible effects related to ‘strong’ or ‘weak’ polar vortex strengths have been revealed in either NmF2 or satellite neutral density longitudinal variations. Alternatively, such effects may be very small and therefore cannot be confirmed experimentally.

Connection between Winter East Asia Flow Patterns and Stratospheric Polar Vortex Anomalies

Using a reanalysis dataset, this work investigates the possible connection of winter East Asia (EA) flow patterns to stratospheric polar vortex (SPV) anomalies. Cluster analysis is performed on the principal components of daily 500 hPa geopotential height fields to identify five distinct flow patterns. SPV anomalies are considered in terms of the occurrence of major sudden stratospheric warmings (MSSWs). The results reveal that for the 15 days before the MSSWs, one of the five patterns occurs more frequently than usual, whereas another occurs less frequently. The former constructively interferes with the climatological EA trough in the troposphere and strengthens the planetary wave activity (heat flux) in the extratropical troposphere and stratosphere. It has a similar pattern in the 500 hPa height to the composite leading to the MSSWs, implying that such strengthening can contribute to the forcing of the MSSWs. The latter is in the opposite sense (destructive interference) and is disadvantageous before the MSSWs. Evidence of a stratospheric downward influence on the five flow patterns is relatively unclear. These results suggest a potential coupling between flow patterns or weather regimes in different regions through the SPV, as well as warrant further investigation of the downward influence on EA weather regimes.

Sensitivity of Easterly QBO’s Boreal Winter Teleconnections and Surface Impacts to SSWs

Abstract The quasi-biennial oscillation (QBO) is thought to influence boreal winter surface conditions over Asia and around the North Atlantic. Confirming if these responses are robust is complicated by the QBO having multiple pathways to influence surface conditions as well as internal variability. The reanalysis record suggests that sudden stratospheric warmings (SSWs), breakdowns of the polar vortex that can elicit persistent surface impacts, are more frequent during easterly QBO (EQBO). Hence, this modulated frequency of SSWs may account for some of the EQBO surface responses. However, many climate models do not reproduce this QBO–SSW relationship, perhaps because it is noise or because the model QBOs are deficient. We circumvent these issues by using an ensemble of fixed boundary condition branched simulations in which a realistic EQBO is prescribed in control simulations previously devoid of a QBO, allowing us to isolate the transient atmospheric response to EQBO. Imposing EQBO accelerates the tropical upper-tropospheric wind, shifts the subtropical jet poleward, and attenuates the polar vortex. Interestingly, the latter is not entirely dependent on the statistically significant increase in SSW frequency due to EQBO. Corroborating observations, EQBO is associated with warmer surface temperatures over Asia and negative North Atlantic Oscillation (NAO) conditions. We then subsample the branched/control simulations based on which EQBO members have SSWs. The negative NAO response is primarily associated with more frequent SSWs, while the Asia warming develops irrespective of SSWs. These results have implications for wintertime predictability and clarify the pairing of particular QBO teleconnections with certain surface impacts. Significance Statement The QBO is one of the few parts of the Earth system that is predictable months in advance and that also elicits global effects on surface temperature, circulation, and precipitation. Unfortunately, climate models and operational forecast systems do not simulate the QBO well and it is not always clear how robust the global impacts of the QBO are. Here, we impose the QBO in idealized model simulations, which modulates wintertime surface temperature and precipitation over Asia, the North Atlantic, Europe, and Africa in a manner consistent with observations. This work substantiates the importance of climate and forecast models properly simulating the QBO.

Large-ensemble assessment of the Arctic stratospheric polar vortex morphology and disruptions

Abstract. The stratospheric polar vortex (SPV) comprises strong westerly winds during winter in each hemisphere. Despite ample knowledge on the SPV's high variability and its frequent disruptions by sudden stratospheric warmings (SSWs) in the Northern Hemisphere (NH), questions on how well current climate models can simulate these dynamics remain open. Specifically the accuracy in reproducing SPV morphology and the differentiation between split and displacement SSW events are crucial to assess the models in this regard. In this study, we evaluate the capability of climate models to simulate the NH SPV by comparing large ensembles of historical simulations to ERA5 reanalysis data. For this, we analyze geometric-based diagnostics at three pressure levels that describe SPV morphology. Our analysis reveals that no model exactly reproduces SPV morphology of ERA5 in all diagnostics at all altitudes. Concerning the SPV morphology as stretching (aspect ratio) and location (centroid latitude) parameters, most models are biased to some extent, but the strongest deviations can be found for the vortex-splitting parameter (excess kurtosis). Moreover, some models underestimate the variability of SPV strength. Assessing the reliability of the ensembles in distinguishing SSWs subdivided into SPV displacement and split events, we find large differences between the model ensembles. In general, SPV displacements are represented better than splits in the simulation ensembles, and high-top models and models with finer vertical resolution perform better. A good performance in representing the morphological diagnostics does not necessarily imply reliability and therefore a good performance in simulating displacements and splits. Assessing the model biases and their representation of SPV dynamics is needed to improve credibility of climate model projections, for example, by giving stronger weightings to better performing models.

Unexpected Global Structure of Quasi‐4‐Day Wave With Westward Zonal Wavenumber 2 During the February 2023 Unusual Major Sudden Stratospheric Warming With Elevated Stratopause

AbstractDuring February 2023, the quasi‐4‐day wave (Q4DW) with westward zonal wavenumber 2 (W2) reached its largest amplitude of ∼400 m in the Southern Hemisphere (SH) geopotential height observations since 2004, which occurred simultaneously with an Arctic major sudden stratospheric warming (SSW) with an elevated stratopause (ES). However, the Q4DW‐W2 perturbations in the Northern Hemisphere (NH) were unexpectedly suppressed despite the unstable Arctic stratosphere and mesosphere during the 2023 ES‐SSW. Diagnostic analysis shows that the westward winds at ∼54°N–70°N in the upper stratosphere of ∼‐79 m/s during the 2023 ES‐SSW were the strongest during boreal winters over the past two decades, which benefited from the onset of a preceding minor SSW at the end of January. The strongest westward wind generated a wave geometry configuration of full reflection for Q4DW‐W2 in the NH, while the Q4DW‐W2 enhancement in the SH was induced by the in‐situ amplification of the surviving seeding perturbations.

Cyclone‐Like Features Within the Stratospheric Polar‐Night Vortex

AbstractDistinctive synoptic‐scale (∼1,500 km) flow features are identified within the core of the stratospheric polar‐night vortex at stratopause altitudes (∼50 km). Typically they comprise a train or a complex pattern of transient vortices, each characterized by enhanced values of potential vorticity (PV) and relative vorticity but with a weaker thermal signal. In the MERRA‐2 (and two other) reanalysis fields these cyclone‐like features persist for several days, occur episodically, and form essentially within the core of the polar‐night vortex itself. Their origin is plausibly linked to a form of barotropic instability associated with a radiatively‐induced annular ring of enhanced PV. Moreover, their ubiquity and dynamics carries possible implications for: ‐ the structure of the larger‐scale polar vortex and its preconditioning ahead of a Sudden Stratospheric Warming event; the distribution of trace‐constituents within the core; and the features representation in extended range/seasonal prediction and climate models.

Investigation of wave processes during complex sudden stratospheric warmings

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Characterizing the Spatial Distribution of Mixing and Transport in the Northern Middle Atmosphere During Winter

AbstractA three‐dimensional winter (DJF) climatology of Lagrangian diffusivity characterizing eddy mixing and transport in the northern middle atmosphere is presented. To emphasize aspects other than zonal averages, we use the theory of Lagrangian diffusivity (κyy) hitherto not applied in stratospheric contexts to our knowledge. Our formulation of Lagrangian diffusivity requires the calculation of parcel trajectories, which is made on isentropic surfaces. A Lagrangian descriptor is used to estimate the boundary of the stratospheric polar vortex (SPV). To characterize quasi‐geostrophic motions and their influence on the SPV we apply the wave activity flux (W) and Local Wave Activity . Our data set is the ERA5 reanalysis for the period 1979–2013. Results for κyy show important zonal asymmetries. In the lower and middle stratosphere, κyy is highest at midlatitudes, particularly around the prime meridian. This location is surrounded by manifolds associated with hyperbolic trajectories emanating from the outer SPV boundary. κyy is also high within the SPV, and near the locations where the SPV boundary is open. Zonal asymmetries are also clear in W at midlatitudes. The larger values of are at high latitudes and upstream of the opening of the vortex boundary. The role of quasi‐geostrophic waves on the south‐north shift of the midlatitude westerlies is highlighted. In particular, the waves contribute to open the SPV boundary at around 90W. The interannual variability of κyy is explored by contrasting winters with positive‐negative Northern Annular Mode index, and Sudden Stratospheric Warmings of displacement‐split type.