AS1.8

In the extratropical atmosphere, Rossby waves (RWs) and internal gravity waves (GWs) propagating from the troposphere mediate a coupling with the middle atmosphere by influencing the dynamics herein. In the current generation chemistry-climate models (CCMs), RW effects are well resolved while GW effects have to be parameterized. Here, we analyze orographic GW (OGW) interaction with resolved dynamics in a comprehensive CCM on the time scale of days. For this, we apply a recently developed method of strong OGW drag event composites for the three strongest northern hemisphere OGW hotspots. We show that locally-strong OGW events considerably alter the properties of resolved wave propagation into the middle atmosphere, which subsequently influences zonal winds and RW transience. Our results demonstrate that the influence of OGWs is critically dependent on the hotspot region, which underlines the OGW-resolved dynamics interaction being a two-way process.

How to cite: Šácha, P., Kuchař, A., Eichinger, R., Pišoft, P., Jacobi, C., and Rieder, H.: Diverse dynamical response to orographic gravity wave drag hotspots - a zonal mean perspective, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-234, https://doi.org/10.5194/egusphere-egu21-234, 2021.

The planetary wave activity in the stratosphere–mesosphere during the Arctic major Stratospheric Sudden Warming (SSW) in February 2018 is discussed on the basis of the microwave radiometer (MWR) measurements of carbon monoxide (CO) above Kharkiv, Ukraine (50.0°N, 36.3°E) and the Aura Microwave Limb Sounder (MLS) measurements of CO and temperature. From the MLS temperature zonal analysis, eastward and westward migrations of wave 1/wave 2 spectral components were differentiated, to which less attention was paid in previous studies. Abrupt changes in zonal wave spectra occur with the zonal wind reversal on 10 February 2018. Eastward wave 1 and wave 2, observed before the SSW onset, disappear during the SSW event, when westward wave 1 becomes dominant. This is consistent with previous studies showing that westward wave 1 in the mesosphere is present after the onset of major SSW events with an elevated stratopause. Analysis of the wavelet power spectra of mesospheric CO variations show statistically significant periods in a band of 20–40 days using both MWR and MLS data. Approximately 10-day periods appear only after the SSW onset. Since the propagation of upward planetary waves is limited in the easterly zonal flow after the zonal wind reversal, forced planetary waves may exist after the onset of SSW due to the instability of the zonal flow in the mid-latitude mesosphere.

This work was partly supported by the projects 19BF051-08, 20BF051-02 Taras Shevchenko National University of Kyiv and by the International Center of Future Science, Jilin University (JLU), under the contract with the JLU. This work also contributed to the State Institution National Antarctic Scientific Center of the Ministry of Education and Science of Ukraine research objectives and to Project 4293 of the Australian Antarctic Program. 

How to cite: Milinevsky, G., Wang, Y., Klekociuk, A., Evtushevsky, O., Han, W., Grytsai, A., Antyufeyev, O., Shi, Y., Ivaniha, O., and Shulga, V.: Planetary waves spectrum in stratosphere-mesosphere during SSW 2018, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1723, https://doi.org/10.5194/egusphere-egu21-1723, 2021.

A short term variability of migrating and non migrating tide is investigated in the stratosphere from the regular Canadian Middle Atmosphere Model (CMAM) and reanalysis ERA-interim temperature and wind dataset during winter of 2006 to 2010. Short term variability of tides is examined by ±10 day’s window size from Earth’s surface to 1hPa pressure level. To examine the short term variability of migrating and non migrating tide in stratosphere, we applied the fast fourier transform method to the CMAM30 and ERA-interim observation. The results reveal that tide changes with amplitude of 1-2K regularly on short timescales (21days) in stratosphere. Similar variability occurs in ERA-interim reanalysis observation. Non-migrating tide DS0 shows strong winter features with finer variation during 2009 and 2010 at 65°N. The short term variability of DE3 tide in stratosphere during 2008 and 2010 may be driven by zonal mean wind and non linear interaction with planetary wave. Amplitude of DW1 shows day to day variabilities clearly during winter of 2006, 2008 and 2009 at 0.7hPa over the equator and mid-latitude while the peak of DW1 is absent at 1hPa and 10hPa from CMAM temperature data set. Short term tidal variability in the stratosphere is not related to a single source. It depends on ozone density, zonal mean wind, and wave-wave non linear interactions. By using smaller window size, short term variabilities and finer variation of non migrating tides and SPW1 are understood. These results will be compared to results from satellite temperature data set, particularly FORMOSAT-3/COMSIC, for investigating short term tidal variability in the stratosphere.

How to cite: Debnath, S. and Das, U.: Short term tidal variability in stratosphere using ERA-interim and CMAM temperature data and comparison with satellite retrievals, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4633, https://doi.org/10.5194/egusphere-egu21-4633, 2021.

Large-scale atmospheric circulation has been represented mostly by interaction between the mean flow and planetary waves (PWs). Although the importance of gravity waves (GWs) has been recognized for long time, contribution of GWs to the large-scale circulation is receiving more attention recently, with conjunction to GW drag (GWD) parameterizations for climate and global weather forecasting models that extend to the middle atmosphere. As magnitude of GWD increases with height significantly, circulations in the middle atmosphere are determined largely by interactions among the mean flow, PWs and GWs. Classical wave theory in the middle atmosphere has been represented mostly by the Transformed Eulerian Mean (TEM) equation, which include PW and GW forcing separately to the mean flow. Recently, increasing number of studies revealed that forcing by combined PWs and GWs is the same, regardless of different PW and GW forcings, implying a compensation between PWs and GWs forcing. There are two ways for GWs to influence on PWs: (i) changing the mean flow that either influences on waveguide of PWs or induces baroclinic/brotropic instabilities to generate in situ PWs, and (ii) generating PWs as a source of potential vorticity (PV) equation when asymmetric components of GWD exist. The fist mechanism has been studies extensively recently associated with stratospheric sudden warmings (SSWs) that are involved large amplitude PWs and GWD. The second mechanism represents more directly the relationship between PWs and GWs, which is essential to understand the dynamics in the middle atmosphere completely (among the mean flow, PWs and GWs). In this talk, a recently reported result of the generation of PWs by GWs associated with the strongest vortex split-type SSW event occurred in January 2009 (Song et al. 2020, JAS) is presented focusing on the second mechanism.  

How to cite: Chun, H.-Y., Song, B.-G., and Song, I.-S.: Generation of planetary waves by gravity-wave drag in the middle atmosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8680, https://doi.org/10.5194/egusphere-egu21-8680, 2021.

Planetary waves and gravity waves are the key drivers of middle atmospheric circulation and variability. While planetary waves are well resolved in climate models, inaccuracies in representation of gravity waves in climate models persist. Inaccuracies in representation of gravity waves limit our understanding of the planetary wave-gravity wave interactions that can be crucial during the Antarctic polar vortex breakdown. Moreover, "missing" gravity wave drag around 60^oS in the upper stratosphere is considered to be responsible for the "cold-pole" bias in comprehensive climate models that employ parameterizations to appproximately represent the gravity wave drag.

We illustrate the strength of the high-resolution ERA-5 reanalysis in resolving a broad spectrum of gravity waves in southern hemisphere midlatitudes and to estimate their contribution to the momentum budget around 60^oS. We find that most of the resolved mountain waves excited over the Andes and Antarctic peninsula propagate away from their source and deposit momentum around 60^oS over the Southern Ocean. Further, a composite analysis around 60^oS during the vortex breakdown period using ERA-5 reveals considerably large fractional contribution of resolved + parameterized GWD towards the vortex deceleration. Upto 30 days prior to the breakdown, a balance between the Coriolis acceleration and the planetary wave deceleration provides a weak net deceleration of the mean winds, following which, they provide a net acceleration of the mean winds. The gravity waves, however, provide a steady deceleration of the mean winds throughout the breakdown period. The resolved drag in ERA-5 accounts for as much as one-fourth of the zonal wind deceleration at 60^oS and 10 hPa, while the parameterized drag in ERA-5 accounts for more than one-half of the zonal wind deceleration.  The findings establish the crucial role of gravity waves in wintertime stratospheric circulation and opens avenues for further stratospheric gravity wave analysis using ERA-5.

How to cite: Gupta, A., Birner, T., Doernbrack, A., and Polichtchouk, I.: Importance of gravity wave forcing for springtime southern polar vortex breakdown as revealed by ERA5, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15949, https://doi.org/10.5194/egusphere-egu21-15949, 2021.

Every spring, the stratospheric polar vortex transitions from its westerly wintertime state to its easterly summertime state due to seasonal changes in incoming solar radiation, an event known as the "final stratospheric warming" (FSW). While FSWs tend to be less abrupt than reversals of the boreal polar vortex in midwinter, known as sudden stratospheric warming (SSW) events, their timing and characteristics can be significantly modulated by atmospheric planetary-scale waves. Just like SSWs, FSWs have been found to have predictable surface impacts. While SSWs are commonly classified according to their wave geometry, either by how the vortex evolves (whether the vortex displaces off the pole or splits into two vortices) or by the dominant wavenumber of the vortex just prior to the SSW (wave-1 versus wave-2), little is known about the wave geometry of FSW events. We here show that FSW events for both hemispheres in most cases exhibit a clear wave geometry. Most FSWs can be classified into wave-1 or wave-2 events, but wave-3 also plays a significant role in both hemispheres. Additionally, we find that in the Northern Hemisphere, wave-2 events are more likely to occur later in the spring, while in the Southern Hemisphere, wave-1 or wave-2 events show no clear preference in timing. The FSW enhances total column ozone over the pole of both hemispheres during spring, but the spatial distribution of ozone anomalies can be influenced by the wave geometry and the timing of the event. We also describe the stratosphere's downward influence on surface weather following wave-1 and wave-2 FSW events. Significant differences between the tropospheric response to wave-1 and wave-2 FSW events occur over North America and over the Southern Ocean, while no significant differences are found over the North Atlantic region, Europe, and Antarctica. 

How to cite: H. Butler, A. and I.V. Domeisen, D.: The wave geometry of final stratospheric warming events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1324, https://doi.org/10.5194/egusphere-egu21-1324, 2021.

Large-scale overturning mass transport in the stratosphere is commonly explained through the action of potential vorticity (PV) rearrangement in the flank of the stratospheric jet. Large-scale Rossby waves, with their wave activity source primarily in the troposphere, stir and mix PV and an overturning circulation arises to compensate for the zonal torque imposed by the breaking waves. In this view, any radiative heating is relaxational and the circulation is mechanically driven. Here we present a fully thermodynamic analysis of these phenomena, based on ERA-Interim data. Streamfunctions in a thermodynamic, log(pressure) – temperature space are computed. The sign of a circulation cell in these coordinates directly shows whether it is mechanically driven, converting kinetic energy to potential and thermal energy, or thermally driven, with the opposite conversion. The circulation in the lower stratosphere is found to be thermodynamically indirect (i.e., mechanically driven). In the middle and upper stratosphere thermodynamically indirect and direct circulations coexist, with a prominent semiannual cycle. A part of the overturning in this region is thermally driven, while a more variable indirect circulation is mechanically driven by waves. The wave driving does not modulate the strength of the thermally direct part of the circulation. This suggests that the basic overturning circulation in the stratosphere is largely thermally driven, while tropospheric waves add a distinct indirect component to the overturning. This indirect overturning is associated with poleward transport of anomalously warm air parcels.

How to cite: Nycander, J., Ruggieri, P., and Ambaum, M.: Thermodynamic cycles in the stratosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3113, https://doi.org/10.5194/egusphere-egu21-3113, 2021.

A new, low-order model of the variability of the Arctic polar vortex has been derived in the context of a shallow-water contour dynamics representation of quasigeostrophic shallow water flow on a polar f-plane. The model consists of a single linear wave mode propagating on a near-circular patch of constant potential vorticity (PV). The PV jump at the vortex edge serves as an additional degree of freedom.  The wave is forced by surface topography, and interacts with the vortex through a simplified parameterization of diabatic wave/mean flow interaction.

The resulting system of three coupled ODEs depends on four non-dimensional parameters, and the structure of the steady state solutions can be determined analytically in some detail. The system exhibits a range of dynamical behaviour closely related to that of the Holton-Mass model, including multiple steady states corresponding to weak and strong vortex states, and dynamically active limit cycles.

One key insight from the model is that, in dynamically active parameter regimes, the time-mean state of the vortex is predominantly controlled by the properties of the Rossby wave mode, while the strength of the topographic forcing plays a far weaker role.

How to cite: Hitchcock, P.: A minimal model of stratospheric vacillations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6867, https://doi.org/10.5194/egusphere-egu21-6867, 2021.

The stratosphere, the layer of the atmosphere at heights between 10-50 km, is an important source of variability for the weather and climate at the Earth’s surface on timescales of weeks to decades. Since the stratospheric circulation evolves more slowly than that of the troposphere below, it can contribute to predictability at the surface. Our synthesis of studies on the coupling between the stratosphere and the troposphere reveals that the stratosphere also contributes substantially to a wide range of climate-related extreme events. These extreme events include cold air outbreaks and extreme heat, air pollution, wildfires, wind extremes, and storm clusters, as well as changes in tropical cyclones and sea ice cover, and they can have devastating consequences for human health, infrastructure, and ecosystems. A better understanding of the vertical coupling in the atmosphere, along with improved representation in numerical models, is therefore expected to help predict extreme events on timescales from weeks to decades in terms of the event type, magnitude, frequency, location, and timing. With a better understanding of stratosphere-troposphere coupling, it may be possible to link more tropospheric extremes to stratospheric forcing, which will be crucial for emergency planning and management.

How to cite: Domeisen, D. I. V. and Butler, A. H.: Stratospheric drivers of extreme events at the Earth’s surface, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2774, https://doi.org/10.5194/egusphere-egu21-2774, 2021.

Major sudden stratospheric warmings (SSWs) are extreme wintertime circulation events of the Arctic stratosphere that are accompanied by a breakdown of the polar vortex. The stratospheric anomalies can propagate downward to the lower stratosphere and influence the weather of the troposphere and the surface for up to two months after the onset of SSW events. Therefore, SSWs can be an important source of predictability on subseasonal to seasonal (S2S) time scales over the Northern Hemisphere (NH) mid- and high- latitudes. However, SSWs themselves are difficult to forecast, with a predictability limit of around one to two weeks. Therefore, understanding the dynamical process that leads to the vortex breakdown is crucial to improve the predictability of SSWs, and ultimately, the weather at the Earth’s surface. To this end, we employ a mode decomposition diagnosis to analyze Ertel's potential vorticity (PV) equation by decomposing each term using empirical orthogonal functions (EOFs) of PV to study the vortex weakening process. With this method, a principal component (PC) tendency equation can be derived, which includes the evolution of the linear and nonlinear PV advection terms and indicates how they contribute to the vortex weakening. The results show that the linear advection term is the main contributor to the increase of PC tendency in the early stage of the warming and contains distinct signals that indicate the weakening of the vortex as early as 25 days before the onset of SSWs using ERA-interim daily data. Our results indicate that both the lead times of the onset of SSW events as well as the type of the event may be extended beyond the current predictability limit, promising to provide longer lead times for the prediction of surface weather. 

How to cite: Wu, Z., Jiménez-Esteve, B., de Fondeville, R., Székely, E., Obozinski, G., and Domeisen, D.: Extended-range predictability of sudden stratospheric warming events suggested by mode decomposition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2607, https://doi.org/10.5194/egusphere-egu21-2607, 2021.

Extreme stratospheric events, e.g strong vortex events and sudden stratospheric warming (SSW) events, are often the main focus of stratospheric predictability studies. Other than strong vortex and SSW events, strong vortex acceleration and deceleration events are related but less studied events. A better understanding of the mechanisms of acceleration and deceleration events would also contribute to the understanding of SSWs and strong vortex events in the stratosphere. As SSWs tend to be less predictable than strong vortex events, it is hypothesized that the predictability of acceleration and deceleration events might differ as they are related to opposite mechanisms. We identify wind acceleration and deceleration events using the daily mean of the zonal mean zonal winds at 60°N and 10 hPa from the ERA-interim reanalysis for the winters of 1998/99-2018/19. Acceleration and deceleration events are defined as a wind change over a 10-day window above the 60th percentile of the magnitude of all identified events. To evaluate the predictability of the events, the ECMWF S2S hindcasts are verified against ERA-interim data. As expected, the predictability of the events increases with decreasing lead time (as the model initialisation date approaches the event onset date). We also find that all 4 types of events, namely acceleration, deceleration, strong vortex and SSW events, show the same predictability behavior, that is, that the predictability of an event is independent of its nature but dependent only on its magnitude. We discuss the difficulties of the model in predicting events associated with strong wind changes by investigating the heat flux-wind relationship in the model. A better understanding of the predictability and dynamical variability in the stratospheric polar vortex by the model could provide a better understanding of the mechanisms of stratospheric events, thus potentially also improving surface weather predictability.

How to cite: Wu, R. W.-Y. and Domeisen, D. I. V.: Predictability of the stratospheric polar vortex in the ECMWF S2S reforecasts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7212, https://doi.org/10.5194/egusphere-egu21-7212, 2021.

How to cite: Afargan-Gerstman, H., Polkova, I., Papritz, L., Ruggieri, P., King, M. P., Athanasiadis, P., Baehr, J., Wulff, O., Sprenger, M., and Domeisen, D. I. V.: Stratospheric modulation of cold air outbreaks and winter storms in the North Atlantic region and impacts on predictability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4629, https://doi.org/10.5194/egusphere-egu21-4629, 2021.

The influence of El Niño Southern Oscillation (ENSO) and the Stratospheric Polar Vortex (SPV) on the zonal asymmetries in the Southern Hemisphere atmospheric circulation during spring and summer is examined. The main objective is to explore if the SPV can modulate the ENSO teleconnections in the extratropics. We use a large ensemble of seasonal hindcasts from the European Centre for Medium-Range Weather Forecasts Integrated Forecast System to provide a much larger sample size than is possible from the observations alone.

We find a small but statistically significant relationship between ENSO and the SPV, with El Niño events occurring with weak SPV and La Niña events occurring with strong SPV more often than expected by chance, in agreement with previous works. We show that the zonally asymmetric response to ENSO and SPV can be mainly explained by a linear combination of the response to both forcings, and that they can combine constructively or destructively. From this perspective, we find that the tropospheric asymmetries in response to ENSO are more intense when El Niño events occur with weak SPV and La Niña events occur with strong SPV, at least from September through December. In the stratosphere, the ENSO teleconnections are mostly confounded by the SPV signal. The analysis of Rossby Wave Source and of wave activity shows that both are stronger when El Niño events occur together with weak SPV, and when La Niña events occur together with strong SPV.

How to cite: Osman, M., Shepherd, T., and Vera, C.: The Combined Influence of the Stratospheric Polar Vortex and ENSO on Zonal Asymmetries in the Southern Hemisphere Upper Tropospheric Circulation during Austral Spring and Summer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5311, https://doi.org/10.5194/egusphere-egu21-5311, 2021.

A flow regime index is constructed based on the November-December standard deviation of the Ertel’s potential vorticity (EPV) in the northern upper stratosphere at 1500 K (~40 km). The index reveals two flow regimes in both the stratosphere and the troposphere. In the stratosphere, the two flow regimes involve zonally asymmetric variability that is manifested by a modulation of the Aleutian High and distinct early-to-late winter development of the polar vortex. During the wide-jet regime, an anomalously strengthened, upright polar vortex is found in middle winter, which involves an equatorward shift of the surf zone in the middle to upper stratosphere, a poleward movement of the polar vortex axis, and a sharpening of the polar vortex edge, suggesting a dominant effect of Rossby wave breaking. During the narrow-jet regime, the vortex weakens at least a month earlier in association with enhanced large-scale PV mixing.

The upper stratospheric flow regimes also have detectable signal in the vicinity of the tropospheric westerly jets in middle winter. The tropospheric responses are also zonally asymmetric. During the wide-jet regime, the largest response is found over the North Pacific with a weakened, poleward shifted westerly jet over north America.  The circulation anomalies during the narrow-jet regime are most strong over the North Atlantic with a weakened, and equatorward shifted westerly jet there. The flow regimes also differ distinctively in their impacts on high-frequency variability downstream of the westerly jets and associated temperature variability. Given the flow regimes in the upper stratosphere leads the tropospheric response by one to two months, improved representation of upper stratospheric variability in climate models may offer more skillful prediction of long-range surface weather forecasts.

How to cite: Lu, H., Gray, L., Martineau, P., King, J., and Bracegirlde, T.: Regime Behaviour in the Upper Stratosphere as a Precursor of Stratosphere- Troposphere Coupling of the Northern Hemisphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8534, https://doi.org/10.5194/egusphere-egu21-8534, 2021.

Both sudden stratospheric warming (SSW) events and tropospheric blocking events can have a significant influence on winter extratropical surface weather. Upward propagating planetary waves from the troposphere can interact with the stratospheric mean flow and disrupt the stratospheric polar vortex, which is associated with an SSW event. Blocking has often been suggested as one of the tropospheric precursors for anomalous upward propagating wave activity flux. It remains an open question to what extent upward wave activity caused by blocking is related to SSW events. In the present study, we examine the evolution of the Eliassen-Palm fluxes during blocking events that precede SSWs. We use Global Navigation Satellite System radio occultation measurements for this analysis to provide accurate and vertically well-resolved information on the wave coupling between these two phenomena in the upper troposphere and stratosphere. First results will be presented and discussed.

Keywords: sudden stratospheric warming, Eliassen-Palm flux, blocking

How to cite: Yessimbet, K. and Steiner, A.: Investigating the connection between tropospheric blocking and sudden stratospheric warming events using GNSS radio occultation observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6400, https://doi.org/10.5194/egusphere-egu21-6400, 2021.

The two most intense wildfires of the last decade that took place in Canada in 2017 and Australia in 2019-2020 were followed by large injections of smoke in the stratosphere due to pyroconvection. It was discovered that, after the Australian event, part of this smoke self-organized as anticyclonic confined vortices that rose against the Brewer-Dobson circulation in the mid-latitude stratosphere up to 35 km (Khaykin et al., 2020, doi: 10.1038/s43247-020-00022-5).  Based on CALIOP lidar observations and the ECMWF ERA5 reanalysis, we analyze the Canadian case and find, similarly, that the large plume which penetrated the stratosphere on 12 August 2017 and reached 14 km got trapped thereafter within a meso-scale anticyclonic structure which travelled across the Atlantic. It then broke into three offsprings that could be followed until mid-October 2017, each performing  round the world journeys and rising up to 23 km for one of them. We analyze the dynamical structure of the vortices produced by these two wildfires in the ERA 5 and demonstrate how they are maintained by the assimilation of data from instruments measuring the signature of the vortices in the temperature and ozone field. We propose that these vortices can be seen as bubbles of very low potential vorticity carried vertically by their internal radiative heating across the stratosphere against the stratification. We will also present elements of a theory and first numerical simulations explaining the dynamics of such structures  and discuss possible occurrences after other forest fires and volcanic eruptions in the past as well as  future likely impacts. This new phenomenon in geophysical fluid mechanics has, to our knowledge, no reported analog (see reference: https://acp.copernicus.org/preprints/acp-2020-1201/).

How to cite: Legras, B., Lestrelin, H., Podglajen, A., and Salihoglu, M.: Rising smoke-charged vortices in the mid-latitude stratosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9309, https://doi.org/10.5194/egusphere-egu21-9309, 2021.

A known adverse side effect of stratospheric aerosol modification (SAM) by artificial aerosol injections is the alteration of the quasi-biennial oscillation (QBO), which is caused by the stratospheric heating associated with an artificial aerosol layer. Multiple studies found the QBO to slow down or even completely vanish for point-like injections of SO₂ at the equator. The reason for this was found to be a modification of the thermal wind balance and an acceleration of the residual circulation leading to a stronger tropical upwelling. For other injection strategies, different responses of the QBO have been observed in model simulations. It has not yet been presented a theory which is able to explain those differences in a comprehensive manner. This is further complicated by the fact that the simulated QBO response is highly sensitive to the used model even under identical boundary conditions.

Therefore, within our study we investigated the response of the QBO to continuous artificial aerosol injections for three different injection strategies (point-like injection at the equator, point-like injection at 30°N and 30°S simultaneously, and areal injection into a 60° wide belt along the equator), and 3 different injection rates (5, 10, 25 Tg(S) yr ^-1). For each injection scenario we ran 10-year AMIP-style simulations with the general circulation model MAECHAM5, which was coupled interactively to the aerosol microphysical model HAM.

Our simulations show that the QBO response significantly depends on the injection location. Based on thermal wind balance, we demonstrate that this dependency is explained by differences in the meridional structure of the aerosol-induced stratospheric warming, i.e. the location and meridional extension of the maximum warming, rather than its absolute magnitude. Additionally, we tested two different injection species, SO₂and H₂SO₄, since the injection of H₂SO₄has been recently proposed as an alternative to an injection of SO₂ as first studies indicate that an injection of H₂SO₄ may be more efficient than an injection of SO₂. Our simulations indicate that the QBO response is qualitatively similar for both investigated injection species, but quantitatively stronger for an injection of H₂SO₄.

How to cite: Franke, H. and Niemeier, U.: Differences in the QBO response to artificial stratospheric aerosol injections depending on injection strategy and species, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8173, https://doi.org/10.5194/egusphere-egu21-8173, 2021.

The wintertime stratosphere is dominated by the polar vortex, a strong westerly wind, which surrounds the cold polar region. In the northern hemisphere the polar vortex can vary a lot during the winter and these variations affect the surface weather, e.g., in Europe and North America. Earlier studies have shown that the northern polar vortex is modulated by different terrestrial drivers and two solar-related drivers: electromagnetic radiation and energetic particle precipitation. Solar radiation varies in concert with the sunspot cycle by affecting the upper atmosphere at lower latitudes. Energetic electron precipitation (EEP) is driven by the solar wind and affects the polar stratosphere and mesosphere by forming ozone depleting NOx and HOx compounds. However, it is unclear how the effects of these solar-related and other, terrestrial drivers compare to each other. In this study we examine the effects of two solar-related drivers (solar radiation and EEP) and three terrestrial drivers (Quasi-Biennial Oscillation (QBO), El-Nino Southern Oscillation (ENSO) and volcanic aerosols) on the northern polar vortex. We use a new composite dataset including ERA-40 and ERA-Interim reanalysis of atmospheric variables and the multilinear regression analysis to estimate atmospheric responses to these five drivers in years 1957 – 2017. We confirm the findings of earlier studies that westerly QBO wind, cold ENSO, volcanic aerosols and increased EEP are associated with a stronger polar vortex. Furthermore, we find that EEP produces the strongest and most significant effect on the northern polar vortex among the studied variables. Only in December the effect of QBO is comparable to the EEP effect. We also find that EEP effect is strong and significant in the easterly QBO phase, while in the westerly phase it does not stand out from the effects of other drivers.

How to cite: Salminen, A., Asikainen, T., Maliniemi, V., and Mursula, K.: Solar-related and terrestrial drivers modulating the northern polar vortex, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7766, https://doi.org/10.5194/egusphere-egu21-7766, 2021.

The stratospheric polar vortex in the Southern Hemisphere plays an important role in the intensity of the stratospheric ozone destruction during austral spring, which started in the late 1970s. The so-called ozone hole has in turn influenced the evolution of weather patterns in the Southern Hemisphere in the last decades (WMO, 2018). The Northern Hemisphere polar vortex is less stable because of larger dynamical activity in winter. It is thus less cold and polar arctic ozone losses are less important. The seasonal and interannual evolution of the polar vortex in both hemispheres has been analyzed using meteorological fields from the European Center for Meteorology Weather Forecasts ERA-Interim reanalyses and the MIMOSA model (Modélisation Isentrope du transport Méso-échelle de l’Ozone Stratosphérique par Advection, Hauchecorne et al., 2002). This model provides high spatial resolution potential vorticity (PV) and equivalent latitude fields at several isentropic levels (675K, 550K and 475K) that are used to evaluate the temporal evolution of the polar vortex edge. The edge of the vortex is computed on isentropic surfaces from the wind and gradient of PV as a function of equivalent latitude (e.g. Nash et al, 1996; Godin et al., 2001). On an interannual scale, the signature of some typical forcings driving stratospheric natural variability such as the 11-year solar cycle, the quasi-biennial oscillation (QBO), and El Niño Southern Oscillation (ENSO) is evaluated. The study includes analysis of the onset and breakup dates of the polar vortex, which are determined from the wind field along the vortex edge. Several threshold values, such as 15.2m/s, 20m/s and 25m/s following Akiyoshi et al. (2009) are used. Results on the seasonal and interannual evolution of the intensity and position of the vortex edge, as well as the onset and breakup dates of the Southern and Northern polar vortex edge over the 1979 – 2020 period will be shown.

References:

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How to cite: Lecouffe, A., Godin-Beekmann, S., Pazmiño, A., and Hauchecorne, A.: Evolution of the stratospheric polar vortex in the Southern and Northern Hemispheres over the period 1979 – 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9523, https://doi.org/10.5194/egusphere-egu21-9523, 2021.

The stratospheric Brewer-Dobson circulation (BDC) is an important element of climate as it determines the transport and distributions of key radiatively active atmospheric trace gases, which affect the Earth’s radiation budget and surface climate.
Here, we evaluate the inter-annual variability and trends of the BDC in the ERA5 reanalysis and inter-compare with the ERA-Interim reanalysis for the 1979–2018 period. We also assess the modulation of the circulation by the Quasi-Biennial Oscillation (QBO) and the El Niño-Southern Oscillation (ENSO), and the forcings of the circulation by the planetary and gravity wave drag. A comparison of ERA5 and ERA-Interim reanalyses shows a very good agreement in the morphology of the BDC and in its structural modulations by the natural variability related to QBO and ENSO. Despite the good agreement in the spatial structure, there are substantial differences in the strength of the BDC and of the natural variability impacts on the BDC between the two reanalyses, particularly in the upper troposphere and lower stratosphere (UTLS), and in the upper stratosphere. Throughout most regions of the stratosphere, the variability and trends of the advective BDC are stronger in the ERA5 reanalysis due to stronger planetary and gravity wave forcings, except in the UTLS below 20 km where the tropical upwelling is about 40 % weaker due to a weaker gravity wave forcings at the equatorial flank of the subtropical jet. In the extra-tropics, the large-scale downwelling is stronger in ERA5 than in ERA-Interim linked to significant differences in planetary and gravity wave forcings. Analysis of the BDC trend shows a global acceleration of the annual mean residual circulation with an acceleration rate of about 1.5 % per decade at 70 hPa due to the long-term intensification in gravity and planetary wave breaking, consistent with observed and future climate model predicted BDC changes.

How to cite: Diallo, M., Ern, M., and Ploeger, F.: The advective Brewer-Dobson circulation in the ERA5 reanalysis: climatology, variability, and trends, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11920, https://doi.org/10.5194/egusphere-egu21-11920, 2021.