Orals: Thu, 7 May, 10:45–12:28 | Room M1
Using ERA5 data from 1979 to 2024, this study classifies 173 wintertime Cold Air Outbreak (CAO) events in midlatitude Asia, based on their temporal phasing relative to pulse-like intensifications of warm air mass transport into the polar stratosphere above 400 K (PULSEs). Two PULSE-related types are identified: PULSE_lead (18.0%), where the PULSE precedes the CAO peak, and PULSE_lag (21.4%), where it follows. PULSE_lead events exhibit more persistent and widespread cold anomalies across Eurasia. The phasing is found to be governed by the planetary-wave driven coupling between the poleward stratospheric warm branch and equatorward tropospheric cold branch of the isentropic meridional mass circulation at 60°N, respectively dominated by warm air transport over the Northwestern Pacific and cold air transport over Asia. PULSE_lead events are preceded by rapid propagation of wavenumber-2 energy into the stratosphere, simultaneously intensify both branches. In contrast, PULSE‐lag events are triggered by a stronger Ural ridge and downstream energy dispersion, with delayed wavenumber‐1‐dominated upward wave flux strengthening the stratospheric warm branch only after the CAO. While PULSE_lag events are mainly caused by tropospheric processes, a downward impact from the stratosphere is found for PULSE_lead. The precursory PULSE induces a stratospheric mass deficit over the East Asian trough region, resulting in barotropic low anomalies, which helping maintain the trough and prolong the CAO. Furthermore, PULSE_lead events have detectable stratospheric polar vortex anomalies 2 weeks in advance. This study clarifies that though most Asian CAOs have a lagged stratospheric response, a significant subset is preceded by active stratospheric forcing.
How to cite: Yu, Y., Ding, Z., Chen, H., Shen, X., and Cai, M.: Cold Air Outbreaks in Midlatitude Asia With and Without Precursory Pulse in the Stratospheric Poleward Warm Air Transport, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1847, https://doi.org/10.5194/egusphere-egu26-1847, 2026.
Weak stratospheric polar vortex (WSPV) events are dynamically connected with the variations in the tropospheric circulation, serving as crucial harbingers for surface cold extremes in the Northern Hemisphere. Although WSPV events are usually featured with either displaced or split stratospheric polar vortex pattern, a notable portion of WSPV events experiences both patterns successively, leading to inconclusive surface impacts of different WSPV events. Here, we propose a novel method to quantitatively identify WSPV events with vortex transition (namely, mixed-type WSPV events) by performing clustering analysis on WSPV days based on 42-yr ERA5 reanalysis, and further examine their climatological features, surface impacts and tropospheric precursors. Results show that the mixed-type WSPV events are usually featured with a routine vortex evolution from displacement to split. In contrast to comparatively weak tropospheric response to pure displaced- and split-type events, the mixed-type WSPV events feature the longer persistence of stratospheric circulation anomalies and are followed by stronger negative Arctic Oscillation-like surface signatures, further contributing to more robust cold anomalies over northern Eurasia and the central U.S. 10–39 days after event onset. Moreover, mixed-type events are typically induced by upward propagated wave activity flux into the stratosphere contributed by the synergistic enhancement of tropospheric planetary wavenumbers 1 and 2. The enhancement of tropospheric planetary wavenumbers 1 and 2 is associated with deepening of the Aleutian Low and strengthening of the dipole over northern Scandinavia-eastern Siberia, respectively. This tropospheric configuration can sevrve as a vital precursor pattern for mixed-type WSPV events, hinting at extreme cold events with far-reaching societal impacts.
How to cite: Zhang, M., Yang, X.-Y., and Huang, Y.: A strong stratospheric harbinger for cold extremes: Weak polar vortex transition from displacement to split pattern, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2667, https://doi.org/10.5194/egusphere-egu26-2667, 2026.
The relationship between minor sudden stratospheric warmings (SSWs) in the Southern Hemisphere (SH) and polar vortex parameters is poorly understood and requires new approaches. A key issue is identifying possible tropospheric and stratospheric precursors to minor SSWs in the SH. Such precursors could include unique blocking structures over the Southern Ocean and Rossby wave trains from the Indian Ocean-El Niño Dipole. Unlike the Northern Hemisphere, where such precursors are frequently studied, precursors in the Southern Hemisphere are less well known. They are likely related to oceanic or ocean-atmosphere transitions rather than topography. We analyzed the parameters of the polar vortex geometry during minor and major sudden stratospheric warmings in the Southern Hemisphere, as well as total ozone anomalies, to determine whether they could provide early warning signals for SSWs by identifying changes in vertical transport. We analyzed changes in vortex area based on potential vorticity at 60°S, which reveals vortex compression or expansion. We also explored the pathways of downward influence, specifically, whether the surface signal of minor SSWs in the Southern Hemisphere is predictable and whether it depends on the vortex's vertical structure, particularly its downward propagation velocity. Unlike the Northern Hemisphere, where an evident downward influence is observed, the signal in the SH is noisy, possibly due to factors such as vortex depth or the phase of the quasi-biennial oscillation (QBO). A search for the possible influence of the QBO on the occurrence and parameters of minor SSWs in the SH was conducted. The downward propagation rate of geopotential height anomalies and the SAM index response after a warming event were examined. Possible links between minor SSWs in the SH and predictable surface impacts were discussed. Unlike the Northern Hemisphere, where SSWs cause extremely low surface temperatures, a search was conducted for links between the SH minor SSW events and extremely high precipitation in Chile, New Zealand, and Australia.
How to cite: Milinevsky, G., Grytsai, A., Yu, R., Evtushevsky, O., Zazubyk, D., Klekociuk, A., and Yukhymchuk, Y.: Searching for the links between minor sudden stratospheric warmings in the Southern Hemisphere and polar vortex parameters, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8539, https://doi.org/10.5194/egusphere-egu26-8539, 2026.
Teleconnections are crucial for regional climate variability and prediction. However, they often appear to be unstable over time, known as nonstationarity. The recent reversal of the well-established ENSO–stratospheric polar vortex (SPV) teleconnection illustrates this puzzle. As this teleconnection is a key pathway for ENSO to influence wintertime circulation in the mid-to-high latitudes, its apparent breakdown questions its reliability as a source of predictability. Here we demonstrate that this nonstationarity is more a statistical artifact than a dynamical shift. By distinguishing the underlying physical linkage from statistical association, we reveal that the observed reversal is driven by an extreme winter outlier and a persistent weakening trend. Much of this weakening can be attributed to the confounding influence of the Quasi-Biennial Oscillation (QBO), whose intermittent alignment with ENSO introduces spurious low-frequency fluctuations in the ENSO–SPV statistical relationship. A physically motivated toy model confirms that such apparent nonstationarity can arise even when the underlying ENSO–SPV linkage remains unchanged, emerging from chance alignment between ENSO and QBO. After accounting for these effects, the ENSO–SPV linkage is substantially more stable than suggested by the raw statistical relationship. Our findings suggest that caution is needed when interpreting time-varying fluctuations in short climate records as structural changes.
How to cite: Shen, X., Kretschmer, M., Shepherd, T. G., and Scaife, A. A.: Stability Behind the Nonstationarity: The Case of the Recent Reversal in the ENSO–Stratospheric Polar Vortex Teleconnection, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8711, https://doi.org/10.5194/egusphere-egu26-8711, 2026.
Prior work has shown that the Northern Hemisphere storm tracks are modulated by the tropical Madden-Julian oscillation (MJO) and that the modulation is strongest during the easterly phase of the stratospheric quasi-biennial oscillation (Guo et al., 2017; Wang et al., 2018). Prior work has also identified a ~27-day solar rotational modulation of the MJO and its eastward propagation (Hoopes et al., 2024). Here, lagged composite analyses of storm tracks relative to 97 strong solar UV peaks and 90 strong solar UV minima occurring during the northern cool season over a 66-year period demonstrate a significant weakening and southward shift of the storm tracks, in both the North Pacific and North Atlantic, near and following UV peaks. Evidence is presented supporting the hypothesis that reduced MJO convection in the Indian Ocean region prior to solar UV peaks produces a positive Rossby wave source that results in a cyclonic circulation anomaly in the Northwest Pacific, thereby causing the weakening and southward shift of the storm tracks.
1 Guo, Y., Shinoda, T., Lin, J., and Chang, E. K. M. (2017). Journal of Climate, 30, 4799-4818. https://doi.org/10.1175/1520-0469(2004)061%3C0023:TEOVIJ%3E2.0.CO;2.
2 Wang, J., Kim, H.-M., Chang, E. K. M., & Son, S.-W. (2018), Journal of Geophysical Research: Atmospheres, 123, 3976-3992. https://doi.org/10.1029/2017JD027977
3 Hoopes, C. A., Hood, L.L., & Galarneau, T. J., Jr. (2024). Geophysical Research Letters, 51, e2023GL107701. https://doi.org/10.1029/2023GL107701
How to cite: Hoopes, C. A., Hood, L. L., and Galarneau, Jr., T. J.: Short-Term Solar Influences via the MJO on the Northern Hemisphere Storm Tracks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13230, https://doi.org/10.5194/egusphere-egu26-13230, 2026.
For the next several decades, the Antarctic ozone hole will remain an annual phenomenon. As concentrations of stratospheric chlorine gradually decrease, so will the severity of ozone depletion within the ozone hole. Chemical influences on the ozone hole are relatively well-understood and readily modelled. However, the dynamical state of the polar stratosphere is considerably more challenging to evaluate. Dynamical conditions exert a strong influence on the springtime progression of the ozone hole, affecting the strength and structure of the polar vortex, transport of ozone, and temperatures across the polar cap. In this work, we share a new diagnostic metric, the Mesospheric Parcel Altitude (MPA), which traces the descent of mesospheric air into the springtime polar vortex. The MPA captures the dynamical state of the vortex interior and serves as a directly observable proxy for horizontal ozone transport. With this novel metric, we can more accurately attribute the chemical and dynamical drivers of uniquely long/short-lived ozone holes.
How to cite: Kessenich, H., Seppälä, A., Smale, D., Rodger, C., and Weber, M.: A novel diagnostic metric for quantifying Antarctic ozone hole dynamics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15198, https://doi.org/10.5194/egusphere-egu26-15198, 2026.
The tropical Cold-point tropopause temperature (CPT) controls the amount of water vapor that enters the stratosphere, as air masses that cross through the equatorial tropopause are subjected to freeze-drying.
GNSS radio-occultation (GNSS-RO) provide temperature profile measurements with high vertical resolution and global coverage, enabling the monitoring of the CPT evolution outside of the few tropical regions covered by radiosondes. Reanalyses are all known to have a modeled CPT that is on average too warm, compared to GNSS-RO measurements.
The reanalysis CPT warm bias maximizes near the Equator, hinting at a possible role of equatorial waves. However, to date the reanalysis CPT biases have only been studied from a zonal-mean and long-term perspective, without looking at the effects of equatorial waves.
Observed equatorial CPT shows peaks in the wavenumber-frequency spectrum coinciding with equatorial wave’s theoretical dispersion curves. This means that equatorial waves that propagate through the equatorial tropopause are modulating CPT variability. The observational study of CPT wave spectrum by Kim and Son (2012) used the COSMIC RO mission, and had a relatively limited space-time resolution: 10°N-10°S meridional average and 3-day running mean, i.e. only showing the symmetric part of the spectrum. Meanwhile, efforts to compare reanalyses’ wave spectra used data at the standard 100 hPa level close to the tropical tropopause, with no observational dataset as reference.
In our study, we showcase a framework to inter-compare CPT wavenumber-frequency spectra from various reanalyses to that of observed CPT from GNSS-RO. We combine multiple GNSS-RO mission data and grid them on 5° x 5° longitude-latitude daily resolution for the years 2007-2018. Model-level CPT from ERA5, ERA-Interim, JRA55 and MERRA-2 reanalyses, are interpolated/averaged onto the same 5° x 5° daily grid from GNSS-RO, enabling a 1-to-1 comparison on the same space-time grid, at the cold-point. Our goals using this dataset are: a comparison between purely observational and reanalyses’ CPT spectra that is as fair as possible, with a better resolution and longer time-period than previous studies, and the separation of the symmetric and anti-symmetric parts of the spectra. This provides valuable information about what types of CPT variability are most troubling to reproduce by the reanalyses.
Observational CPT wavenumber-frequency spectra of power above background from GNSS-RO show well-defined spectral peaks near the MJO domain and the theoretical dispersion curves of Kelvin and equatorial Rossby waves in the symmetric spectrum, as well as mixed Rossby-gravity waves in the anti-symmetric part.
We show the importance of sampling reanalysis data at the observation locations only, as even at synoptic-scales and frequencies of around a week, this can influence spectral power. Reanalyses increasingly struggle at shorter and faster space-and-time-scales, more markedly in the anti-symmetric part of the spectrum.
How to cite: Pilch Kedzierski, R., Davis, S., Tegtmeier, S., Wargan, K., and Weissmann, M.: A framework to compare GNSS-RO and reanalysis equatorial wave spectra of Cold-point tropopause temperature, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19363, https://doi.org/10.5194/egusphere-egu26-19363, 2026.
Vertical diffusion in the free atmosphere due to turbulence has received limited attention in the literature, but its parameterization has significant effects on the models' general circulation. In the new cycle 50r1 of the ECMWF Integrated Forecasting System (IFS), scheduled to become operational in February and already used in the ERA6 reanalysis currently in production, the turbulence scheme was updated to reduce vertical momentum and heat diffusivities in the lower stratosphere. This change is motivated by persistent biases in IFS seasonal forecasts in that region, including biases in the amplitude and vertical descent of the simulated Quasi-Biennial Oscillation (QBO) (ECMWF Newsletter No.185, Autumn 2025).
In this study, we use a recently developed idealized two-dimensional (longitude–altitude) tropical channel model to investigate, in a controlled framework, the impact of these recent changes in the vertical diffusion scheme on the QBO. The model is based on the Weather Research and Forecasting (WRF) model and reproduces, in two dimensions, the canonical wave–mean-flow interaction regime of the QBO following Holton, Lindzen, and Plumb: two monochromatic, planetary-scale gravity waves diabatically forced in the lower model layer propagate upward and force the mean flow as they dissipate. We compare a cycle-49–like long-tail Richardson-number closure (Viterbo, 1999, fLTG) with a cycle-50–like blended stability function that reduces vertical mixing over a finite layer in the lower stratosphere and relies on a cycle-38–like short-tail Richardson-number closure (Beljaars and Holtslag, 1991, fMO).
The new reduced diffusion formulation substantially modifies the simulated QBO, with larger amplitudes in the lower stratosphere and a longer period. This shows how an idealized framework can help identify key sensitivities in a global forecasting system.
How to cite: Bremaud, V., Podglajen, A., Van Niekerk, A., Hertzog, A., and Plougonven, R.: Sensitivity of the QBO to the turbulent diffusion scheme in an idealized model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14795, https://doi.org/10.5194/egusphere-egu26-14795, 2026.
The Quasi-Biennial Oscillation (QBO) is a dominant mode of stratospheric zonal wind variability. Observations indicate that the QBO influences tropical phenomena such as convection, precipitation, and the Madden–Julian Oscillation (MJO), yet climate models often fail to capture these relationships. This study examines the QBO’s impact on high clouds in CMIP6 historical simulations and MODIS observations, given that cloud feedback remains a major source of uncertainty in climate sensitivity estimates.
The QBO modulates dynamic and thermodynamic properties near the tropical tropopause layer, such as temperature, static stability, and vertical wind shear, all linked to cloud formation. Building on recent findings that highlight the major cloud-controlling factors (CCFs) for high clouds, we apply CCF analysis to assess QBO-driven changes in high-cloud amount and interpret these changes in terms of contributions from controlling factors.
Results confirm that the QBO westerly (QBOW) phase is associated with reduced tropical mean high-cloud cover, with strong zonal asymmetry in observations. CMIP6 models successfully capture the reduction in tropical high clouds associated with QBOW, but with a strong inter-model spread. Among the analysed CCFs, upper-tropospheric temperature and relative humidity contribute most to this reduction, followed by static stability. Inter-model differences primarily arise from uncertainty in the high-cloud sensitivity to upper troposphere temperature. The strong inter-model spread highlights that improved constraints on high‑cloud sensitivity to upper‑tropospheric thermodynamics could help enhance how models capture QBO‑related cloud responses.
How to cite: M. Jaison, A., Ceppi, P., and Wilson Kemsley, S.: The role of QBO in tropical high-cloud variability in CMIP6 models and observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20757, https://doi.org/10.5194/egusphere-egu26-20757, 2026.
The stratospheric Quasi-Biennial Oscillation (QBO) is characterized by descending bands of wind and temperature anomalies in the tropical stratosphere with a mean period of 17 ∼28 months. Numerous studies have argued that the QBO has a significant impact on tropical tropospheric climate. However, the observational support for such an impact is complicated by the competing signatures of internal tropospheric climate phenomena. Here we apply an observationally-based, “physical-kernel” methodology that identifies the “direct” component of the tropospheric response that arises from the combination of 1) the influence of the QBO on upper tropospheric static stability and 2) the physical linkages between upper tropospheric static stability, vertical motion, and clouds. Consistent with previous analyses, the results suggest that the westerly phase of the QBO is linked to robust decreases in vertical motion and cloud fraction over the Indian/western tropical Pacific Oceans; in contrast to previous analyses, they indicate only weak direct linkages between the QBO and tropical climate elsewhere. It is argued that the methodology provides a refined estimate of how the QBO directly influences tropical climate variability, with implications for its impacts on the Madden-Julian Oscillation.
How to cite: Thompson, D. and Chen, Y.-J.: A novel methodology for probing the influence of the QBO on tropical tropospheric climate, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21625, https://doi.org/10.5194/egusphere-egu26-21625, 2026.
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.
Winter surface weather during the cold season is frequently discussed in relation to polar vortex variability, particularly in the stratosphere. However, winter weather events characterised by cold outbreaks, snowfall, and blizzard conditions exhibit substantial variability in surface expression across different circulation settings. This motivates a detailed diagnostic analysis of how tropospheric and stratospheric polar vortex variability relate to winter surface weather.
In this study, several winter periods over the Northern Hemisphere are analysed using ERA5 reanalysis data, with the objective of examining the relationship between vortex variability and surface weather characteristics. The analysis focuses on the correspondence between tropospheric circulation regimes, the vertical structure of vortex variability, and the resulting diversity of winter surface weather outcomes.
The analysis follows a troposphere-first approach, in which surface and lower-tropospheric circulation is examined prior to assessing the stratospheric state. Winter surface weather is described in terms of near-surface temperature, snowfall-related precipitation, and associated large-scale circulation regimes.
Tropospheric dynamics are diagnosed using sea-level pressure, 500 hPa geopotential height, 850 hPa temperature advection, upper-tropospheric jet structure, and potential vorticity near the dynamical tropopause, providing a framework for identifying jet displacement, blocking, and cyclone pathways. Stratospheric variability is examined using zonal-mean zonal wind at 60°N and 10 hPa, polar-cap temperature, geopotential height, and potential vorticity across multiple stratospheric levels (10–50 hPa), together with eddy heat flux diagnostics to characterise wave forcing and vertical structure.
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.
[1] R. S. Williams et al., “Future Antarctic Climate: Storylines of Midlatitude Jet Strengthening and Shift Emergent from CMIP6,” Journal of Climate, vol. 37, no. 7, pp. 2157–2178, Apr. 2024, doi: 10.1175/JCLI-D-23-0122.1.
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.
