Abstract. Stratospheric extreme events during Northern winter and spring have been shown to sometimes enhance the sub-seasonal predictability of large-scale tropospheric circulation patterns such as the North Atlantic oscillation (NAO) and Greenland/European blocking. However, it remains unclear whether event-to-event differences in the observed tropospheric evolution after individual sudden stratospheric warmings (SSWs) represent a robust difference in the tropospheric response to the events, or whether such differences in tropospheric evolutions are simply caused by tropospheric variability. To make progress on this question, we robustly quantify the tropospheric response with an ensemble simulation strategy in a controlled model environment. We construct a model climatology using the ICON global numerical weather prediction (NWP) model, representing a wide range of realistic stratosphere–troposphere evolutions during winter months, but under controlled boundary conditions to exclude confounding factors like teleconnections of tropical origin. The simulations reproduce key aspects of observed stratosphere-troposphere coupling, providing a consistent framework to assess event-specific tropospheric responses. We produce spin-off ensembles for selected SSW events; the corresponding ensemble means help robustly quantify the tropospheric response to these SSWs. We find pronounced and robust event-to-event differences in the tropospheric response to SSWs. We further show that the flow anomalies in the lower stratosphere during the second week are well correlated with the surface response 3–7 weeks after the SSW. Moreover, our results indicate that the formation of wave reflection surfaces within the lower stratosphere may prevent the establishment of persistent lower-stratospheric anomalies. Overall, our controlled model simulations show that individual SSWs may differ significantly in their likelihood to induce a tropospheric response and that this likelihood is mainly determined by the post-SSW flow evolution within the stratosphere. These results may be relevant for sub-seasonal predictability of surface weather, especially given that the stratospheric part of the response to SSWs is highly predictable.

Sudden Stratospheric Warmings (SSWs) and Polar Stratospheric Clouds (PSCs) exhibit striking inter-hemispheric asymmetries: SSWs are frequent in the Arctic but rare in the Antarctic, while PSCs are more persistent in the Antarctic. Although land-sea thermal contrast and orography (LSCO) have been traditionally invoked to explain these asymmetries, here we show using semi-idealized model experiments that even though LSCO strongly impacts the mean state of SSWs and PSCs, it alone cannot fully account for the observed differences. Using model experiments, we reveal that midlatitude oceanic sea surface temperature (SST) fronts represent a crucial supporting additional driver of the hemispheric stratospheric differences. Like LSCO, SST fronts enhance stratospheric convergence of resolved waves, strengthening the Brewer-Dobson Circulation and inducing high-latitude adiabatic warming. This warming significantly enhances Arctic SSW frequency and strongly suppresses PSC formation. Sub-grid-scale non-resolved wave forcing modulates the stratospheric resolved waves effect. The oceanic impact is dominated by North Pacific SST fronts. Our results highlight the indispensable role of SST fronts in shaping Arctic-Antarctic asymmetries in stratospheric dynamics and associated extremes.

Abstract The Pacific/North American (PNA) pattern is a dominant mode of mid‐latitude climate variability with major impacts on North American weather. Its extended‐range prediction remains challenging, influenced by complex drivers including upstream wave dynamics, tropical forcing, and the stratosphere. This study investigates the notable failure of extended‐range forecasts to capture the extreme negative PNA event of February 2018 using the novel Stratospheric Nudging and Predictable Surface Impacts (SNAPSI) multi‐model ensemble designed to isolate stratospheric sources of predictability. The results reveal that the forecast error stems from an underestimation of several key processes, including a chain of misrepresented factors. The primary bottleneck is the poor simulation of the East Asian trough, a well‐established upstream precursor; models that better represent it produce more accurate PNA forecasts. Biases in both tropical (La Niña) and extratropical North Pacific sea surface temperatures also contribute to the PNA forecast bias. While the Madden–Julian Oscillation has limited direct influence within the SNAPSI forecasts, a supplementary analysis shows that its accurate simulation in earlier weeks is critical for EAT development based on models from the Subseasonal‐to‐Seasonal prediction project database. The stratospheric contribution is initialization‐dependent: for a late‐January forecast, the tropospheric response to the February 2018 sudden stratospheric warming varies widely, limiting its net effect, while for a February initialization, most models produce an excessive negative Arctic Oscillation response, which amplifies a positive PNA bias. The findings highlight priorities for improving predictions: better representing mid‐latitude wave dynamics, refining the representation of stratosphere‐troposphere coupling, and integrating tropical‐extratropical linkages.

Abstract We investigated the effects of both major and minor Arctic sudden stratospheric warming (SSW) events from 2017 to 2020, along with the 2019 Antarctic SSW, on the ionospheric F2 region using Digisonde observations from a low‐mid latitude Indian station located in New Delhi (28.6°N, 77.2°E; geomagnetic latitude 19.2°N, dip latitude 42.4°N). Our study revealed significant ionospheric changes, with electron densities exhibiting variations of more than 245% during these warming events. We therefore investigated the extent of ionospheric changes following the 2017 and 2018 SSW events using ionosonde data for the first 6 months, revealing that the SSW period showed the highest deviation from median values, highlighting its significant impact on the variability of the ionospheric F2 region. In addition to short‐term oscillations and a distinct 9‐day periodicity, 19–25‐day periodicities are also observed in the F2 layer frequency during the 2017–2020 SSW events, indicating persistent secondary planetary wave modulations consistent with previously reported results. However, our observations did not indicate a lunar influence (14‐day periodicity) in the ionosphere during these events. The Equatorial Electrojet (EEJ) exhibited significant fluctuations at the far equatorial station at New Delhi during the 2017 and 2018 SSW events, resulting in the occurrence of Counter Electrojet (CEJ) phenomena.

Abstract The Arctic midlatitude linkage (AML) suggests a connection between Arctic amplification (AA) and cold midlatitude winter weather. This study explores the role of the stratospheric pathway of the AML in a warming climate, assessing its detectability alongside other factors related to climate change. We investigate tropospheric and stratospheric climate change signals using the chemistry–climate model ECHAM/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) and three time slice experiments, representing preindustrial (1850), present (2020), and future (2100) climates. Both climate change simulations reveal increasing wave propagation and wave breaking in the stratosphere, accompanied by an increased frequency of sudden stratospheric warmings (SSWs). The intensified wave activity enters the stratosphere, particularly from the North Pacific and the Atlantic–European region. An evaluation of subseasonal wave activity episodes reveals more frequent tropopause-level wave events during winter. Further analysis of tropospheric baroclinicity reveals that AA suppresses baroclinic wave formation by weakening horizontal temperature gradients in the lower troposphere. In contrast, the enhanced wave generation in the midlatitude upper troposphere could be attributed to temperature changes outside the Arctic, specifically, tropical warming and lower-stratospheric cooling. Additionally, a jet stream analysis shows no pronounced weakening or increased waviness in the polar jet. Thus, the hypothesis that weaker temperature gradients slow down jet streams was not supported in our real-world climate scenarios. Finally, the more frequent SSWs in our model reveal no cooling influence on midlatitude winter weather. Instead, AA appears to diminish the downward influence of SSWs, a previously overlooked effect that stands in contrast to the expectations of the AML hypothesis. Significance Statement The discussion continues on how Arctic warming might influence winter weather in midlatitudes through a stratospheric pathway. This study examines whether indicators for this hypothesis appear in climate model simulations of a warmer world. Although Arctic warming is assumed to increase wave activity, our model shows a reduction near the surface. We find enhanced upper-tropospheric wave activity, but this seems driven by temperature variations in other regions, not by Arctic processes. Jet streams exhibit no general weakening, a fact that does not support the simple assumption of extending the thermal wind balance from the surface to the tropopause. Our results also indicate that Arctic warming weakens the influence of stratospheric events on surface weather, leading to milder cold air outbreaks.

Abstract We investigate the neutral wind and semidiurnal tide (SDT) variations in the mesosphere and lower thermosphere (MLT) during two consecutive minor Southern Hemisphere (SH) sudden stratospheric warmings (SSWs) that occurred unusually early in July–August 2024. Zonal and meridional winds from four meteor radar stations at 50–70°S were analyzed. Zonal winds reversed from eastward to westward between 80 and 100 km altitude during both events, showing a more distinct reversal in the second event. The SDT amplitudes increased and exhibited longitudinal differences around the second event. To elucidate the mechanisms responsible, we analyzed ozone observations from Aura/MLS along with MERRA‐2 shortwave heating. Positive ozone anomalies at 10 hPa (∼32 km) in the SH polar region around each event coincide with enhanced SDT amplitudes from meteor radars. In addition, the shortwave heating rate shows an enhanced 12‐hr component at SH high‐latitudes above 40 km during these events, supporting an ozone‐related radiative contribution to the SDT variability. Using phase‐differences from longitudinally separated meteor radars, we estimated the zonal wavenumber. Based on this analysis, we propose that nonlinear interaction between the quasi‐16‐day zonal wavenumber‐2 planetary wave (Q16DW2) and the migrating semidiurnal tide (SW2) contributed to the observed longitudinal differences in SDT amplitude. Furthermore, nonlinear advection associated with Q16DW2–SDT interactions is examined and shows clear longitudinal differences that lead to longitudinal asymmetry in SDT amplitude. These findings show the strong modulation of the SDT by SH SSWs and underscore the combined roles of ozone variability and nonlinear wave interactions in modulating upper‐atmospheric tidal responses.

Abstract. Sudden Stratospheric Warming events (SSWs) can have a downward impact on the troposphere, but the mechanism remains uncertain. This study focuses on classifying SSWs based on their tropospheric responses and documenting associated dynamical characteristics. Using the ERA5 data and CESM2-WACCM outputs, 52 SSWs are identified in ERA5 and 273 in CESM2-WACCM, with 33 and 119 downward-propagating SSWs (DWs), respectively. The DWs are classified into three types based on cold surges over Eurasia (EA), North America (NA), and both (BOTH), respectively. Both DWs and non-downward-propagating SSWs (NDWs) weaken and deform the polar vortex, but DWs induce stronger negative Northern Annular Mode (NAM) and North Atlantic Oscillation (NAO) responses. For DWs, the anomalous high develops in the polar region, which deflects to lower latitudes, consistent with the frequent appearance of the polar high and the midlatitude blockings. The shape of the anomalous polar high varies with the DW type, and the extension and shift of the anomalous high lead to different surface responses. The DWs are also accompanied by a southward shift of the precipitation belt, especially over the oceanic and coastal regions. The relatively weaker tropospheric impact of NDWs may be partly explained by their weaker stratospheric disturbance amplitude. The three types of DWs differ in spatiotemporal evolutions of the NAM and NAO pattern, different forcing by planetary waves, and varying ratios between displacement and split. This study reveals the diversity of the DWs and distinguishes their potential impacts on both continents in the Northern Hemisphere.

<i>Background</i><i>:</i> Polar vortex splits, a subset of sudden stratospheric warming, can drive extreme midlatitude cold outbreaks by coupling stratospheric disruptions downward to the troposphere. However, surface impacts vary widely, with some events producing severe, persistent cold and others remaining benign, highlighting the need to distinguish underlying dynamical pathways. <i>Purpose</i><i>:</i> This study aims to quantify the spectrum of surface cold impacts from historical polar vortex splits and to elucidate the key tropospheric and stratospheric mechanisms that differentiate high-impact synergistic (wave-amplified) events from low-impact zonal-background events. <i>Methods</i><i>:</i> Thirty synthetic vortex split events (1958–2023) were identified from reanalysis data and composited into synergistic and zonal categories. Lagged composites (Days –10 to +20 relative to onset) of potential vorticity, geopotential height, temperature, sea-level pressure, zonal winds, Eliassen-Palm flux, wave amplitude, jet latitude, blocking index, and storm-track activity were analyzed to reveal dynamical contrasts. Novelty: The work provides the first systematic, quantitative comparison of synergistic versus zonal split composites, explicitly linking tropospheric–stratospheric wave interference, jet buckling, persistent blocking, and focused wave breaking to explain heterogeneous surface outcomes. <i>Findings:</i> Synergistic splits produce 4–5× stronger cold anomalies (peak –10.5°C vs. –2.0°C), greater spatial extent (14.4% NH coverage), and longer persistence (~4 days) than zonal splits, driven by constructive wave reinforcement (1.8–5.3× amplification), southward jet displacement (~2°), sustained Greenland blocking (≥4 days), enhanced downstream storm tracks (correlation –0.69), and EP-flux divergence/convergence patterns favoring prolonged negative NAM/NAO responses. <i>Conclusion:</i> Tropospheric planetary wave preconditioning and synergistic coupling, rather than the stratospheric split alone, governs the severity of surface cold extremes. <i>Recommendation:</i> Incorporate real-time wave-precursor diagnostics into forecasting systems and expand analyses with large-ensemble simulations to assess future changes in split-related extreme weather risk.

Abstract The rapid development of artificial intelligence weather forecasting models (AIWFMs) has revolutionized weather prediction. GraphCast, a leading AIWFM, outperforms state‐of‐the‐art physics‐based numerical models at the surface and in the lower troposphere. However, it remains unclear how well GraphCast predicts stratospheric variability and represents stratosphere‐troposphere coupling, an important source of surface predictability on subseasonal to seasonal (S2S) timescales. In this study, we evaluate GraphCast's predictions of the stratosphere and its ability to capture stratosphere‐troposphere coupling, focusing on sudden stratospheric warmings (SSWs) in the Northern Hemisphere (NH). We initialized the model across different winter phases and at various lead times before SSW onsets to assess its predictions. Our results show that GraphCast predicts the stratospheric polar vortex strength up to 2 weeks, although errors increase rapidly with height and exhibit larger winter‐to‐winter variability than the physics‐based model. More importantly, GraphCast fails to predict any SSW events with a 1‐week lead time. The primary reason is its inability to accurately simulate wave‐mean flow interactions in the stratosphere, as it fails to satisfy the Transformed Eulerian Mean momentum budget, despite simulating the wave activity flux similar to reanalysis. Furthermore, GraphCast cannot accurately predict the downward propagation of stratospheric anomalies, resulting in larger surface errors after SSW events compared to normal winters. Our study highlights some issues with GraphCast in representing stratosphere‐troposphere coupling and obeying known dynamical relationships. We emphasize the need for a comprehensive evaluation of AIWFMs throughout the atmospheric column and across various dynamical interactions, which is essential to improve these models and extend their applications.

Abstract Sudden stratospheric warmings (SSWs) strongly impact tropospheric weather, yet their accurate prediction remains a significant challenge. This study investigates the predictability of SSWs in the Whole Atmosphere Community Climate Model with different vertical resolutions. Results show that SSW onset can be successfully captured up to 5 days in advance with 70 vertical layers, notably shorter than ∼10‐day lead time achieved by operational models. In contrast, when vertical layers are increased to 138, the lead time can be extended to 10 days, with the improvement attributed to better prediction of planetary wave vertical propagation and the resultant wave‐mean flow interactions. Further analysis reveals that the SSW predictability is more sensitive to the vertical resolution between 0.1 and 100 hPa than in the troposphere or mesosphere and beyond. This study demonstrates that enhancing vertical resolution alone can substantially improve SSW prediction skill, despite using coarse horizontal resolution (∼1°) and simplified initialization.

Abstract Using a single annual frequency with each year defined from July 1 to June 30 of the following calendar year, this study fits an ellipse to the yearly phase space trajectory of the daily time series of MU (stratospheric mass‐weighted zonally integrated zonal wind at 60°N) and M (total polar stratospheric air mass over 60–90°N) to represent the annual evolution of the Northern Hemisphere stratospheric polar vortex (SPV) in individual years. The utility of this elliptical representation rests on two principal capabilities. First, interannual variability in the SPV's annual trajectory, as well as its sub‐seasonal and seasonal anomalies, can be captured by yearly variations in the six ellipse parameters. Second, the temporal lead of winter maxima in MU with respect to winter minima in M derived from the elliptical representation provides the general lead time information of MU anomalies with respect to M anomalies from sub‐seasonal to seasonal scales. This serves as direct evidence indicating that sub‐seasonal and seasonal anomalies are closely linked to annually varying SPV trajectories. Moreover, the annually varying ellipse parameters and their derived quantities contain statistically significant information on the timing of sudden stratospheric warmings (SSWs). By distilling the SPV's complex seasonal evolution into six ellipse parameters, this framework offers a concise empirical basis for long‐lead forecasts of MU and M throughout the cold season, as well as timing of SSW events, provided that these ellipse parameters can be predicted before the cold season.

Abstract This study investigates tropical total column ozone (TCO) variability during Sudden Stratospheric Warming (SSW) events, using data from the NCEP-NCAR Reanalysis 1 and NOAA/CIRES/DOE 20th Century Reanalysis (V3) datasets, supplemented by NOAA 10.7 cm solar radio flux data, spanning 1981–2015. Key parameters include TCO, air temperature, zonal and meridional winds, downward shortwave radiation flux (DSWRF), and upward longwave radiation flux (ULWRF). Methods included lagged composite analyses over a 91-day window (± 45 days from SSW onset) to compute TCO anomalies and raw variable averages, alongside mixed-effects modeling to capture fixed and random effects. Results show NH tropical TCO depletions of -2 to -3 DU at SSW onset (25°–30°N), recovery beginning after 20 days, while SH depletions range from − 4 to -10 DU (1988 minor event) to -2 to -4 DU (2002 major event), limited by sparse SH SSW events. Correlation analyses of Major SSW events reveal strong positive correlations between TCO and DSWRF (> 0.8) and temperature (0.4–0.8), with a delayed negative correlation for zonal wind (-0.8 at 25 days post-onset). Mixed-effects Model 1, with random intercepts and slopes (MSE = 3.4874), outperforms Model 2 (MSE = 12.423) via five-fold cross-validation, with temperature and DSWRF as key predictors. TCO underestimation in epoch 40 suggests unmodeled factors like stratospheric aerosols or planetary wave activity. These findings highlight the interplay of radiative, thermal, and dynamical processes during SSWs, with Model 1 showing promise for NH ozone forecasting.

Abstract This study examines the combined effects of sudden stratospheric warming events (SSWs) and El Niño and Southern Oscillation (ENSO) on late winter (January–March) climate in China. It is found that SSWs significantly modulate El Niño’s impact over China. During El Niño winters with SSWs, southern China tends to experience notable warming; conversely, without SSWs, the surface air temperatures there are generally colder. However, SSWs do not effectively alter La Niña’s impact; southern China remains anomalously colder during La Niña winters even when SSWs occur. The net warming effect of SSWs over much of China during La Niña, defined as the difference between scenarios with and without SSWs, is found considerably weaker than during El Niño. These discrepancies arise from the contrasting tropospheric pathway of ENSO teleconnection over the East Asia–Pacific region. Specifically, El Niño influences the occurrence of SSWs primarily by enhancing the tropospheric planetary wave 1. This enhancement corresponds to an emergence of a dipole pattern of geopotential height anomalies, characterized by an anomalous low over northeastern Eurasia and an anomalous high over China. In contrast, La Niña contributes to SSW occurrences via an increase in the tropospheric wave 2, which corresponds to only an anomalous low over northeastern Eurasia. The consistently significant high over China both before and after SSWs during El Niño, but much weakened or even absent during La Niña, is mainly responsible for the stronger net warming in late winter over China in El Niño years.

Abstract A strong westward zonal wavenumber‐2 quasi‐4‐day wave (Q4DW) during the 2018/2019 Northern Hemisphere sudden stratospheric warming (SSW) is both captured by Aura Microwave Limb Sounder (MLS) observations and our new whole neutral atmosphere data assimilation system. The Q4DW during this SSW is characterized by a double‐peak altitudinal structure in temperature, geopotential height, and neutral winds ranging from ∼40 km up to the lower thermosphere. The Eliassen‐Palm flux diagnostics show that the wave source at ∼55 km over 45°N–75°N, the excitation, propagation, and amplification of which are controlled by the critical layer and atmospheric barotropic/baroclinic instability in the polar stratosphere related to SSW. The first Hough‐mode decomposition analysis of Q4DW indicates that the enhancement of the Rossby (2, −3) normal mode is mainly responsible for the amplification of Q4DW, the latitudinal structure of which is distorted by the anomalous background winds during this SSW. In the ionosphere, a simultaneous quasi‐4‐day oscillation (Q4DO) is found in the Wuhan University total electron content (TEC) product near 10:00–12:00 LT at magnetic latitudes of ∼+15° and ∼−25° with maximum amplitudes of ∼1.1 TECU and 1.2 TECU, respectively. Besides, the Q4DO also displays significant interhemispheric asymmetry and longitudinal variations. Interestingly, the secondary wave components ( s = 4, T = 10.7 hr and s = 0, T = 13.7 hr) in neutral winds from the nonlinear interactions between the Q4DW and the migrating semidiurnal tide are detected in the dynamo region, which may play a dominant role in generating Q4DO in the F‐region ionosphere.

Abstract During austral summer of 2002/2003, the TIMED/TIDI meridional wind observations show that after the attenuation of the climatological quasi‐2‐day wave (Q2DW) with westward zonal wavenumber 3 (W3) and a period of 47 hr (hrs) in January, a 53 hr abnormal W3 Q2DW event occurred in February with symmetric latitudinal structure about the equator. Our analysis indicates that this abnormal W3 Q2DW is generated by the nonlinear interaction between the climatological W3 Q2DW and a ∼ zonally symmetric oscillation associated with a sudden stratospheric warming (SSW). The SSW occurring in boreal winter modulated the meridional circulation through interhemispheric coupling, which influenced the background wind in the summer hemisphere and favored the amplification of a symmetric Q2DW mode in February consequently. The diagnostic analysis reveals that this February W3 Q2DW was mainly amplified near the stratopause at low latitudes of the Southern Hemisphere, where the background atmospheric conditions are suitable for the amplification of the W3 Q2DW. Thus, both the nonlinear interaction and favorable background atmospheric conditions contribute to the occurrence of the abnormal W3 Q2DW in February.

Abstract. Several studies highlight the relevance of considering polar winter stratospheric information such as the occurrence of Sudden Stratospheric Warmings (SSWs) for skillful Subseasonal to Seasonal (S2S) surface climate predictions. However, current S2S forecast systems can only predict these events about two weeks in advance. A potential way of increasing their predictability is to improve the models' representation of the triggering mechanisms of SSWs. Traditional theories indicate that SSWs follow sustained wave dissipation in the stratosphere, but the relative role of tropospheric versus stratospheric conditions in the enhancement of stratospheric wave activity remains unclear. This study aims to quantify the role of the stratospheric state in wave activity preceding SSWs by analyzing three recent SSWs: the boreal SSWs of 2018 and 2019 and the austral minor SSW of 2019, using specific sets of S2S experiments. These ensembles follow the SNAPSI (Stratospheric Nudging And Predictable Surface Impacts) guidelines and include free-evolving atmospheric runs and nudged simulations, where the zonally-symmetric stratospheric state is nudged to either observations of a certain SSW or a climatological state. Our results show that the models struggle to capture the strong enhancement of wave activity preceding the 2018 SSW, limiting predictability beyond 10 d. In contrast, both SSWs of 2019 are better predicted, consistent with a more accurate simulation of the wave activity. Nudging the zonal mean stratospheric state does not drastically influence the upward wave activity flux or tropospheric circulation anomalies prior to these SSWs, but it has some impact on the stratospheric wave activity, although this modulation depends on the event characteristics. The boreal 2019 SSW appears to be primarily driven by tropospheric processes. In contrast, stratospheric contributions may have also played an important role in triggering the boreal 2018 SSW and the austral 2019 SSW. Understanding these variations is key to improving SSW predictability in S2S models.

Abstract Using ensemble forecasts from the Global Ensemble Forecast System and controlled integrations with the Weather Research and Forecasting (WRF) model, we investigate how the North American stratospheric sub‐vortex, closely linked to the 2019 sudden stratospheric warming (SSW), influences a large‐scale precipitation event over the eastern United States in late January 2019 and further quantify its contribution to the precipitation anomalies of the event. This precipitation event is directly driven by a tropospheric cyclone and a downstream anticyclone, which jointly transport warm, moist air from the Gulf of Mexico into the eastern U.S. Through a series of WRF experiments, we indicate that a nudged run (NOBS), in which the stratospheric circulation is nudged toward the observed evolution represented by the ERA5 reanalysis data set, successfully captures the evolution of the tropospheric cyclone and anticyclone, whereas a parallel run (NCLM) nudged toward the ERA5 winter climatology fails to capture them. The NOBS run accurately simulates the sub‐vortex over North America, which facilitates the southward invasion of cold air. Moreover, in the NOBS experiment, the well‐captured stratospheric sub‐vortex enhances downward coupling through the phase‐locking mechanism, directly promoting the development of the tropospheric cyclone and indirectly influencing the downstream anticyclone. Our results indicate that the stratospheric anomalies contribute to approximately 37% of the total precipitation during the event. This finding underscores the importance of the sub‐vortex's geometry in the lowermost stratosphere for driving this precipitation event and highlights the need to focus on polar vortex geometry to fully understand regional stratosphere‐troposphere coupling.

Abstract January 2024 represented an extreme manifestation of the long-term winter warming trend in Iran, characterized by widespread warmth and dryness. During 14–15 January 2024, surface temperatures exceeded climatological means by up to ~ 4 °C, accompanied by reduced mid-tropospheric humidity and below-average precipitation across much of the country, indicating enhanced drought conditions. Synoptic analyses revealed a weakened and eastward-displaced Siberian High, negative sea-level pressure anomalies over northern Eurasia, and a pronounced mid-tropospheric ridge over Iran. At upper levels, strengthening and eastward displacement of the subtropical jet, together with a reversal of 10-hPa zonal winds, confirmed the occurrence of a major split-type Sudden Stratospheric Warming (SSW). The resulting polar vortex split promoted large-scale subsidence and reduced cold-air advection over Iran, leading to adiabatic warming and suppressed precipitation. Statistical analyses further indicate that tropical–extratropical interactions, particularly the Indian Ocean Dipole and the El Niño–Southern Oscillation, modulated the regional temperature response. Overall, this event demonstrates that warm-side surface responses following SSWs constitute an important but underexplored component of winter climate variability over West Asia, capable of amplifying ongoing regional warming and producing contrasting winter anomalies across Eurasia.

The main objective of this work is to characterize Sudden Stratospheric Warming (SSW) conditions and their impact on local weather forecasting and climate change, using SSW definition criteria. The SSWs strongly affect Arctic vortex structure and midlatitude weather conditions. This work evaluates the frequency, amplitude, and dynamical–thermal characteristics of SSWs under historical and Representative Concentration Pathway (RCP) 4.5 scenarios, focusing on stratospheric air temperature (Ts) and zonal wind speed (Uh) at the 10° N and 60° N latitudes. The fifth-generation ECMWF atmospheric reanalysis (ERA5) is employed as the reference dataset. Simulations of five Coupled Model Intercomparison Project Phase 5 (CMIP5) models, represented by M1 to M5, are analyzed. The primary group of models included 1) the Australian Community Climate and Earth-System Simulator, version 1.3 (ACCESS1-3, M1), 2) the Hadley Center Global Environmental Model, version 2—Carbon Cycle (HadGEM2-CC, M2), and 3) the Max Planck Institute Earth System Model—Medium Resolution (MPI-ESM-MR, M3). The analysis period covers SSW events related to the Quasi-Biennial Oscillation (QBO) in the Northern Hemisphere (NH) from 1980 to 2100. The key findings indicate that while M1, M2, and M3 simulate SSW occurrence correctly for the 21st century, they exhibit significant systematic deficiencies in capturing the structural dynamics of SSW events. Specifically, the M1, M2, and M3 models underestimate the polar stratospheric temperature amplitude (Tamp) by approximately 75–80% and zonal wind amplitude (Uamp) by more than 60% compared to the ERA5 analysis. Furthermore, ERA5 exhibits a strong negative correlation (R ≈ −0.8) between Uh and Ts that is not estimated accurately using the present models. The importance of the horizontal resolution of the models and wave–mean flow interactions in determining SSW intensity and occurrence is also found to be a critical metric. Results suggest that SSW definition criteria affect Arctic and midlatitude weather system prediction at a rate of 61–82%. It is concluded that the primary configurations of CMIP5 models for accurately capturing the dynamical structure and evolution of QBO–SSW interactions are needed, and that they affect future projections of SSW events.

Abstract To elucidate dynamics of significant downward propagation (SDP) events of zonal wavenumber 1 (WN1) planetary waves from the stratosphere after sudden stratospheric warming (SSW) events and their influence on the extratropical troposphere, a case study of an SDP event in March 2023 and composite analyses for SDP events with and without SSW using JRA-3Q reanalysis are conducted. The March 2023 event is characterized by the equatorward propagation of enhanced WN1 components in the troposphere, which follows the WN1 downward propagation after the SSW. Exceptionally warm anomalies in East Asia including Japan in early March are associated with the equatorward propagation. A statistical investigation on the timing of all SDP and SSW events reveals that an SDP event is significantly more likely to occur after an SSW. The composite of the SDP event with SSW is characterized by stratospheric easterlies, in contrast to that without SSW, in which westerlies prevail in the stratosphere. Downward propagated WN1 components in the troposphere propagate equatorward and produce temperature anomalies in the extratropics, depending on the WN1 phase at high latitudes for both SDP events. When the WN1 ridge is positioned around the date line in SDP events with SSW, cold anomalies tend to cover East Asia including Japan. The WN1 component during the March 2023 event has the largest amplitude among SDP events with SSW and a ridge location around 90°W, which is far apart from the rest. These peculiar characteristics of the WN1 component would contribute to the extraordinary warm anomalies near Japan.