Extreme stratospheric ozone-loss events, such as the Arctic spring of 2020, can emerge spontaneously in free-running chemistry–climate models during winters and springs with a strong and persistent polar vortex. While ozone is known to potentially affect stratospheric variability, its specific role in the subseasonal-to-seasonal (S2S) predictability of the stratosphere remains unclear. The first step consists of assessing its role in the prediction of internally generated extreme depletion events.

Here we analyse a suite of targeted hindcast experiments for several extreme ozone-loss winters identified in a 200-year free-running WACCM integration. Hindcasts are initialized from January to April to examine how predictability evolves through the winter season, and paired experiments compare configurations with fully interactive ozone to those in which the radiative transfer scheme uses a prescribed climatological ozone distribution.

Preliminary results show that early-winter vortex conditions are not a good predictor of the occurrence of extreme depletion events in late winter-spring, highlighting the limited predictability of the polar vortex, even under the strong vortex conditions that are conducive to ozone depletion in spring. For the most pronounced ozone-loss case, differences between interactive and prescribed ozone ensembles indicate that ozone–radiative feedbacks can, under certain conditions, support the persistence of a strong, cold vortex into late winter and spring, thereby maintaining the dynamical environment in which severe ozone depletion can occur. At the same time, the impact of interactive ozone on S2S skill varies with initialization date and event characteristics.

These findings provide first insights into how chemistry–dynamics coupling affects the predictability of extreme stratospheric states and point to the value of interactive ozone schemes in S2S prediction systems.

How to cite: Hu, W., Chiodo, G., and Chrysanthou, A.: The value of interactive ozone in predicting extreme stratospheric ozone-loss events in the Arctic: insights from targeted WACCM hindcasts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-571, https://doi.org/10.5194/egusphere-egu26-571, 2026.

Orographic gravity waves (OGWs) are ubiquitous in our atmosphere and play an important role in the energy transport both horizontally as well as vertically to the higher levels. Due to their scales, they have to be parameterized in the models.

In this work we are trying to show how the differences in OGW drag between the models in CMIP6 initiative influence the resolved waves propagation and subsequently also the polar vortex.

Taking OGW drag over the maximum in the mid-latitudes in the lower stratosphere, we can show high correlations with Eliassen-Palm flux divergence, but also the zonal winds. Using different descriptive measures of poler vortex such as sudden stratospheric warming frequency (SSW) or Northern Hemisphere annular mode (NAM) we also try to connect this relationship to the strength of the polar vortex.

How to cite: Hájková, D., Šácha, P., and Kuchař, A.: Orographic gravity wave drag in CMIP6 and its influence on the polar vortex, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1002, https://doi.org/10.5194/egusphere-egu26-1002, 2026.

Sudden stratospheric warmings (SSWs) are among the most dramatic regime transitions in the winter stratosphere, yet their onset remains difficult to diagnose and predict. We explore SSWs from a critical-transition perspective using Eigen Microstate Theory (EMT), which provides an entropy-based measure of how the circulation reorganizes during the event life cycle. In reanalysis composites of major SSWs, we identify a robust, non-monotonic entropy evolution: it rises during vortex deceleration, reaches a maximum prior to onset, and then collapses sharply as the vortex breaks. This “order–disorder–order” sequence provides direct empirical evidence that SSWs exhibit signatures of phase transitions and criticality in the real atmosphere. 

To connect this statistical signature to dynamics, we analyze a one-dimensional wave–mean-flow interaction model that captures the nonlinear feedbacks underpinning vortex destabilization. The model reproduces the same entropy peak and collapse when the system is driven toward instability, supporting the interpretation of eigen-microstate entropy as an order parameter for an intrinsically nonequilibrium transition. Across both reanalysis and model experiments, supported by analytical considerations, the entropy shows a pronounced response as the system approaches loss of stability and provides a clearer precursor than conventional single-series early-warning indicators such as lag-1 autocorrelation (AR1). These results suggest a physically interpretable, entropy-based diagnostic of SSW criticality with potential value for subseasonal prediction.

How to cite: Zhao, D. and Zhang, Y.: Sudden stratospheric warmings as nonequilibrium transitions: evidence from eigen-microstate entropy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8570, https://doi.org/10.5194/egusphere-egu26-8570, 2026.

Variability in the stratospheric polar vortex can exert significant impacts on the tropospheric circulation and thereby influence mid-latitude winter weather. A notable winter-time phenomenon are stratospheric wave reflection events, characterized by upward-propagating Rossby waves that are reflected downward by the stratosphere. Previous studies have established a strong link between reflected waves over Canada and cold spells over North America.

Recent work on stratosphere-troposphere coupling during wave reflection has mainly focused on events in the North Pacific and North American region. However, wave reflection can occur in different regions, be triggered by distinct vortex states, and lead to different surface impacts. Therefore, identifying and characterizing different types of reflection events will help improve our understanding of stratosphere-troposphere coupling and identify conditions under which the stratosphere may provide enhanced predictability for winter weather.

To identify distinct types of wave reflection, we apply cluster analysis to spatial patterns of daily meridional eddy heat flux anomalies at the 100hPa level, which in the zonal mean is proportional to the vertical component of the Eliassen-Palm flux. The analysis reveals several modes of wave propagation that differ in region and magnitude and are associated with distinct zonally asymmetric vortex states. One specific type is associated with regionally reflected waves over Europe and a shift of the polar vortex towards Europe. During these events, surface temperatures are anomalously low across Europe. We compare these European reflection events with the more frequently studied North Pacific/North American reflection events. In addition, we examine how the frequency of these events may change under climate change, as previous studies have indicated a persistent shift of the Arctic polar vortex towards the Eurasian continent.

By expanding the understanding of spatial patterns of stratospheric wave reflection events, their regional influence on the tropospheric circulation, and potential future changes in their frequency, this work aims to advance the foundation for improved predictability of mid-latitude winter weather.

How to cite: Dworzak, J. and Domeisen, D. I. V.: Types of Stratospheric Wave Reflection Events and their Surface Impacts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9980, https://doi.org/10.5194/egusphere-egu26-9980, 2026.

Stratospheric NO₂ originates from the decomposition of N₂O after its transport from the troposphere into the stratosphere. Due to anthropogenic activities the tropospheric mixing ratios of N₂O increased from pre-industrial levels of about 265 ppb to about 339 ppb in 2025. Alone during the time of our ground based DOAS measurements the increase of tropospheric N₂O was around +9%, which should result in a similar NO2-increase. In order to test this hypothesis we investigated stratospheric NO₂ column densities from long-term zenith DOAS measurements (1995 – 2026) in Kiruna (northern Sweden). We also compare the ground-based data to satellite observations from several UV/vis sensors (GOME-1, SCIAMACHY, OMI, GOME-2AB, TROPOMI). Good agreement of the relative temporal variations is found between both data sets, but systematic deviations occur for the absolute values, which can be explained by differences in the solar zenith angles during the measurements and the analysis details. Interestingly, no clear trend in the stratospheric NO₂ columns during the whole time series is found, which is in contradiction to the above assumption. Moreover, a strong year-to-year variability of up to about +/-10% is found. Both findings indicate that the stratospheric NO₂ amount is influenced by more complex processes, most probably related to variations of the Brewer-Dobson circulation. We investigate such influences by comparing our long term data sets of stratospheric NO₂ to variables describing the entry of tropospheric air into the stratosphere and the strength of the Brewer-Dobson circulation. 

How to cite: Wagner, T., Gu, M., Enell, C.-F., Platt, U., Raffalski, U., and Richter, A.: Year to year variability of stratospheric NO2 (1995 to 2026) above Kiruna, Northern Sweden, derived from ground-based and satellite DOAS observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10798, https://doi.org/10.5194/egusphere-egu26-10798, 2026.

The Madden-Julian Oscillation (MJO) has been demonstrated to play an important role in the occurrence of sudden stratospheric warming (SSW) events, suggesting possible extratropical impacts of MJO via a stratospheric pathway. However, the existence of this stratospheric pathway is determined by the horizontal and vertical propagation of Rossby waves, which is closely related to both the MJO convection itself and the extratropical basic state. Our studies suggest that the El Niño-Southern Oscillation (ENSO) significantly regulates the MJO-SSW relationship, which is robust during La Niña winters but almost nonexistent during El Niño winters. Further analysis indicates that ENSO influences the extratropical response to MJO, which facilitates the amplification and vertical propagation of the wavenumber 2 component of planetary waves during La Niña winters. Moreover, we have identified a pronounced intensification of the MJO‐SSW relationship in the past two decades, probably due to the prolonged duration of MJO‐related enhanced convection during P7 and the shifts in the extratropical basic state.

How to cite: Ma, J., Chen, W., and Yang, R.: The MJO-SSW Teleconnection: ENSO Modulation and a Recent Intensification over the Past Two Decades, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12252, https://doi.org/10.5194/egusphere-egu26-12252, 2026.

Sudden stratospheric warmings (SSWs) are known to be associated with the presence of a preconditioned polar vortex state that facilitates the upward propagation of planetary waves from the troposphere. Previous studies have suggested that this pre-warming state differs depending on the SSW type, with displacement events characterized by a weakened, funnel-shaped vortex, and split events associated with a narrower and more vertically aligned vortex displaced towards the pole. More recent evidence, however, indicates that the majority of SSWs, largely independent of their type, are preceded by enhanced tropical stratopause wave driving, which shifts the zero-wind line poleward, displaces the vortex towards higher latitudes and promotes the focusing of wave activity into the polar stratosphere and mesosphere.

Despite this progress, there remains an open debate as to whether SSWs require an anomalously strong pulse of tropospheric wave activity, or whether climatological tropospheric forcing is sufficient when combined with a favorable stratospheric state. In this study, we address this question by applying a Euclidean-distance-based analogue method to identify pre-warming polar vortex states using daily ERA5 reanalysis data. Analogous vortex configurations are objectively defined based on their similarity to a reference pattern and are subsequently classified according to whether they precede an SSW or not.

Our results show that approximately 75% of SSWs occurring between December and February during the period 1980–2021 (18 out of 23 events) are preceded by a recurrent preconditioned state characterized by a poleward-displaced vortex north of 60°N. This preconditioning phase persists over a variable number of consecutive days and terminates with a strong stratopause-level vortex deceleration, accompanied by the development of easterly winds that subsequently propagate downward through the stratosphere, marking the onset of vortex decline. A key distinction between cases that do and do not lead to an SSW lies in the strength of lower-tropospheric wave activity. Thus, while wave forcing is enhanced relative to climatology in both cases, it is stronger in SSW events, supporting the idea that both a favorable preconditioned vortex state and anomalously strong tropospheric wave forcing are necessary ingredients for SSW generation. Finally, split SSWs tend to be associated with stronger and more persistent tropospheric wave activity, both during the establishment of the preconditioning state and throughout the subsequent vortex breakdown.

How to cite: Peña-Ortiz, C., Gallego, D., and Álvarez-Castro, C.: Analogue-based identification of preconditioned polar vortex states preceding sudden stratospheric warmings, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12813, https://doi.org/10.5194/egusphere-egu26-12813, 2026.

The Quasi-Biennial Oscillation (QBO) is a pattern of alternating zonal wind regimes in the tropical stratosphere which are forced by upward propagating waves. It extends from around 17km to 50km above the surface, leaving a gap, the “buffer zone”, between its lower limit and the effective source of the waves that provide the forcing (the upper troposphere). The explanation for this buffer zone has been a subject of recent research, notably by Match and Fueglistaler (2019, 2020). They use an idealised one-dimensional model of the QBO to conclude that the buffer zone is formed by mean-flow damping (likely due to horizontal momentum fluxes). We explore this mechanism in two- and three-dimensional models. By imposing mean-flow damping of various shapes and sizes on an idealised 2D (height-latitude) QBO model, we can induce formation of a buffer zone, as well as interesting behaviour not found in the 1D model. We also investigate whether the same behaviour occurs in a 3D GCM, where the horizontal momentum fluxes are due to resolved waves, rather than being imposed as an ad hoc damping. Our results thus far seem to strengthen recent theories on the buffer zone formation mechanism, and will contribute to better understanding of the dynamics at work.

How to cite: Maas, M., Ming, A., and Haynes, P.: The QBO Buffer Zone: Insights from a Hierarchy of Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13869, https://doi.org/10.5194/egusphere-egu26-13869, 2026.

How to cite: Polifronie, E. M. and Cheval, S.: Tropospheric and stratospheric polar vortex variability and winter surface weather, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16471, https://doi.org/10.5194/egusphere-egu26-16471, 2026.

Many past studies based on climate reanalysis data have strongly indicated that energetic electron precipitation (EEP) from space into the polar atmosphere leads to mesospheric and stratospheric ozone loss. This in turn affects radiative balance in the atmosphere and leads to thermal changes, which enhance the stratospheric polar vortex.

Here we study the EEP influence on the atmosphere and climate system using the SOCOL3-MPIOM chemistry-climate model. We run idealized 300-year long timeslice simulations with (experiment run) and without (control run) EEP forcing. The control run captures the internal variability of the climate system without EEP forcing, while the experiment run depicts the variability of the climate system when it is forced with EEP. The EEP forcing employs a parameterization to represent the influx of NOx molecules through the model top created by low-energy auroral precipitation. We also include the direct ionization produced by EEP evaluated from a new data composite based on POES satellite observations. The model is repetitively forced each year throughout the entire simulation with the EEP forcing observed during winter 2003/2004.

Here we discuss the preliminary findings from these long model runs and show that they confirm the EEP-driven ozone loss and subsequent enhancement of the stratospheric polar vortex.

How to cite: Asikainen, T., Salminen, A., Vokhmyanin, M., Arsenovic, P., and Sukhodolov, T.: Impacts of energetic electron precipitation on the atmosphere: results from a 300-year chemistry-climate model experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18784, https://doi.org/10.5194/egusphere-egu26-18784, 2026.

The Southern Hemisphere (SH) stratospheric polar vortex (SPV) is a key dynamical component of the atmospheric circulation, exerting a strong influence on tropospheric circulation and near-surface climate through stratosphere–troposphere coupling. Despite its importance, the future evolution of the SH SPV remains highly uncertain, as projections depend sensitively on the magnitude and character of Antarctic climate change. To address this uncertainty, storyline approaches offer a physically plausible framework that explores contrasting Antarctic climate futures in a structured way, without collapsing distinct responses into an ensemble mean.

Building on the storyline framework of Williams et al. [1], this study investigates the influence of two opposing scenarios of Antarctic climate change on the large-scale circulation of the SH using the ICON (ICOsahedral Nonhydrostatic) atmospheric model. Two physically motivated future scenarios are considered: one characterised by strong Antarctic sea-ice loss combined with an earlier breakdown of the SH stratospheric polar vortex, and one featuring weaker sea-ice loss and a delayed vortex breakdown. These contrasting boundary conditions provide a controlled framework to assess how different Antarctic climate pathways can force distinct stratospheric and tropospheric circulation responses within a single model.

For each storyline, 30-year simulations are analysed for present-day (1985–2014) and end-of-century (2070–2099) conditions. The analysis focuses on changes in stratospheric variability and stratosphere–troposphere coupling, with particular attention to the frequency and near-surface impacts of SPV weakening and strengthening events, thereby assessing the circulation pathways through which contrasting Antarctic boundary conditions affect the SH larg-scale circulation within one model.

How to cite: Richter, K., Gatti, L., Handorf, D., and Köhler, R.: Southern hemispheric large-scale circulation changes in two opposing storylines of future Antarctic climate change, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19297, https://doi.org/10.5194/egusphere-egu26-19297, 2026.

Most climate models that simulate the Quasi-Biennial Oscillation (QBO) substantially underestimate tropical zonal wind amplitudes in the lower stratosphere, which limits their ability to represent global QBO teleconnections. To investigate the causes of this long-standing bias and to identify the key requirements for simulating a realistic QBO, we use an idealized atmospheric model with simplified physics based on the GFDL dry spectral dynamical core. The model incorporates empirically derived latent heating from observed tropical precipitation to represent the effects of tropical convection on the generation of resolved waves that drive the QBO.

We perform an extensive set of sensitivity experiments that systematically vary tropical heating, parameterized gravity wave drag, gravity wave drag strength and formulation, vertical resolution, and horizontal resolution. The results demonstrate that high vertical resolution (L80) is the most critical factor for reproducing realistic QBO amplitudes in the lower stratosphere. Parameterized gravity wave drag is also essential, as tropical heating alone is insufficient to sustain a robust QBO. In contrast, increasing horizontal resolution beyond moderate values provides little benefit, with simulations at T42 resolution already producing a reasonable QBO.

How to cite: Reichler, T. and Johns, Z.: Insights from Idealized Modelling into the Quasi-Biennial Oscillation , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3643, https://doi.org/10.5194/egusphere-egu26-3643, 2026.

Sudden Stratospheric Warmings (SSWs) represent the dominant mode of variability in the winter polar stratosphere and play a key role in modulating tropospheric circulation and surface climate. SSWs can influence surface temperature extremes, with important implications for regional climate variability. However, how SSW characteristics and their surface impacts may change under strong greenhouse gas forcing remains an open question, and current projections show substantial inter-model uncertainty. In this study, we examine the response of SSW-related surface temperature extremes in the Northern Hemisphere to an abrupt quadrupling of CO₂ concentrations (abrupt-4xCO₂) relative to preindustrial conditions, using simulations from CMIP6 models. To better characterize the model uncertainty, we separate the models into two groups according to their projected change in SSW frequency under 4xCO₂ forcing: models exhibiting a decrease in SSW frequency and models showing an increase.

Models with a decrease in SSW frequency project a strengthened polar vortex and a more persistent SSW signal in the lower stratosphere under 4xCO₂ with respect to preindustrial conditions. Consequently, they show a stronger stratosphere–troposphere coupling and a more pronounced surface response following SSW events. SSWs for this group of models are associated with an increased probability and longer duration of cold spells over Scandinavia, and to a lesser extent over northern Siberia, under 4xCO₂ conditions. In contrast, models with increasing SSW frequency exhibit weaker persistence of the stratospheric signal and reduced surface impacts as a response to 4xCO₂. The results highlight contrasting responses between the two model groups, suggesting that projected changes in polar vortex strength and stratosphere-troposphere coupling play an important role in shaping future SSW impacts at surface extreme events.

How to cite: De Maeseneire, D., Ayarzagüena, B., and Calvo, N.: Are SSW impacts on surface temperature extremes changing due to increasing CO2 concentrations?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10386, https://doi.org/10.5194/egusphere-egu26-10386, 2026.