AS1.8

Stratospheric dynamics

The variability in the stratosphere is important for many atmospheric phenomena. Examples include the dynamical two-way coupling between the stratosphere and troposphere during sudden stratospheric warming events, the transport of trace gases through the meridional circulation of the stratosphere, or the connection between the Quasi-Biennial Oscillation of the tropical stratosphere and the Madden-Julian Oscillation. This session is interested in all aspects of stratospheric circulation variability, including the mechanisms behind the vertical coupling between the stratosphere and troposphere in tropics and extratropics, the importance of stratospheric dynamics for explaining both short-term atmospheric weather and long-term climate variability, and the role of the stratospheric circulation for the chemical composition of the atmosphere. We welcome abstracts that study this problem from an observational, modelling, or theoretical viewpoint on all temporal and spatial scales.

Convener: Thomas Reichler | Co-conveners: Blanca Ayarzagüena, Bo Christiansen, Seok-Woo Son
vPICO presentations
| Tue, 27 Apr, 13:30–17:00 (CEST)

vPICO presentations: Tue, 27 Apr

Chairpersons: Blanca Ayarzagüena, Bo Christiansen
Waves
13:30–13:32
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EGU21-234
|
ECS
Petr Šácha, Aleš Kuchař, Roland Eichinger, Petr Pišoft, Christoph Jacobi, and Harald Rieder

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, 2020.

13:32–13:34
|
EGU21-1723
Gennadi Milinevsky, Yuke Wang, Andrew Klekociuk, Oleksandr Evtushevsky, Wei Han, Asen Grytsai, Oleksandr Antyufeyev, Yu Shi, Oksana Ivaniha, and Valerii Shulga

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.

13:34–13:36
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EGU21-4633
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ECS
Subhajit Debnath and Uma Das

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.

13:36–13:38
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EGU21-5487
|
ECS
Daria Sobaeva, Yulia Zyulyaeva, and Sergey Gulev

Strong quasi-decadal oscillations of the stratospheric polar vortex (SPV) intensity are in phase with the Pacific decadal oscillation (PDO). A stronger SPV is observed during the positive phase of the PDO, and during the negative phase, the intensity of the SPV is below the mean climate values. The SPV intensity anomalies, formed by the planetary waves and zonal mean flow interaction, lead to the weakening/intensification of the vortex.

This research aimed to obtain the differences in the characteristics and the spatial propagation pattern of the planetary waves in the middle troposphere and lower stratosphere during different PDO phases. We analyzed composite periods of years when the PDO index has extremely high and low values. Two periods were constructed for both positive and negative phases, the first consisting of years with El-Nino/La-Nina events and the second without prominent sea surface temperature anomalies in the tropics. 

During the wintertime in the Northern Hemisphere (December-February), wave 2 with two ridges (Siberian and North American Highs) and two troughs (Icelandic and Aleutian Lows) dominates in the middle troposphere, along with wave 1 dominating in the lower stratosphere. In the middle troposphere, at the positive phase ​​of the PDO, the amplitude of wave 2 is higher than in years with negative values of the PDO index. The differences in the Aleutian Low and the North American High intensity between the two phases are significant at the 97.5% level. In the lower stratosphere, the wave amplitude is lower at the negative phase ​​of the PDO, but we can also talk about a slight shift of the wave phase to the east. The regions of the heavy rains in the tropics during El-Nino events are the planetary waves source. They propagate from low to high latitudes, which results in modifying the characteristics and locations of the intensification of the stationary planetary waves in mid-latitudes.

How to cite: Sobaeva, D., Zyulyaeva, Y., and Gulev, S.: Manifestation of the Pacific Decadal Oscillation in the stationary planetary waves activity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5487, https://doi.org/10.5194/egusphere-egu21-5487, 2021.

13:38–13:40
|
EGU21-8680
Hye-Yeong Chun, Byeong-Gwon Song, and In-Sun Song

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.

13:40–13:42
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EGU21-15949
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ECS
Aman Gupta, Thomas Birner, Andreas Doernbrack, and Inna Polichtchouk

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 60oS 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 60oS. 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 60oS over the Southern Ocean. Further, a composite analysis around 60oS 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 60oS 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.

Warmings
13:42–13:44
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EGU21-1324
Amy H. Butler and Daniela I.V. Domeisen

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.

13:44–13:46
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EGU21-1509
|
ECS
Oscar Dimdore-Miles, Lesley Gray, and Scott Osprey

Sudden Stratospheric Warmings (SSWs) are major disruptions of the Northern Hemisphere (NH) stratospheric polar vortex and occur on average approximately 6 times per decade in observation based records. However, within these records, intervals of significantly higher and lower SSW rates are observed suggesting the possibility of low frequency variations in event occurrence. A better understanding of factors that influence this decadal variability may help to improve predictability of NH mid-latitude surface climate, through stratosphere-troposphere coupling. In this work, multi-decadal variability of SSW events is examined in a 1000-yr pre-industrial simulation of a coupled Atmosphere-Ocean-Land-Sea ice model. Using a wavelet spectral decomposition method, we show that hiatus events (intervals of a decade or more with no SSWs) and consecutive SSW events (extended intervals with at least one SSW in each year) vary on multi-decadal timescales of period between 60 and 90 years. Signals on these timescales are present for approximately 450 years of the simulation. We investigate the possible source of these long-term signals and find that the direct impact of variability in tropical sea surface temperatures, as well as the associated Aleutian Low, can account for only a small portion of the SSW variability. Instead, the major influence on long-term SSW variability is associated with long-term variability in amplitude of the stratospheric quasi biennial oscillation (QBO). The QBO influence is consistent with the well known Holton-Tan relationship, with SSW hiatus intervals associated with extended periods of particularly strong, deep QBO westerly phases. The results support recent studies that have highlighted the role of vertical coherence in the QBO when considering coupling between the QBO, the polar vortex and tropospheric circulation.

How to cite: Dimdore-Miles, O., Gray, L., and Osprey, S.: Origins of Multi-decadal Variability in Sudden Stratospheric Warmings, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1509, https://doi.org/10.5194/egusphere-egu21-1509, 2021.

13:46–13:48
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EGU21-1995
Vivien Matthias, Gunter Stober, Alexander Kozlovsky, Mark Lester, Evgenia Belova, and Johan Kero

In the middle atmosphere, spanning the stratosphere and mesosphere, spring transition is the time period where the zonal circulation reverses from winter westerly to summer easterly which has a strong impact on the vertical wave propagation influencing the tropospheric and ionospheric variability. The spring transition can be rapid in form of a final sudden stratospheric warming (SSW, mainly dynamically driven) or slow (mainly radiatively driven) but also intermediate stages can occur. In most studies spring transitions are classified either by their timing of occurrence or by their vertical structure. However, all these studies focus exclusively on the stratosphere and can give only tendencies under which pre-winter conditions an early or late spring transition takes place and how it takes place. Here we classify the spring transitions regarding their vertical-temporal development beginning in January and spanning the whole middle atmosphere in the core region of the polar vortex. This leads to five classes where the timing of the SSW in the preceding winter and a downward propagating Northern Annular Mode (NAM) plays a crucial role. The results show distinctive differences between the five classes in the months before the spring transition especially in the mesosphere allowing a certain prediction for some of the five spring transition classes which would not be possible considering the stratosphere only.

How to cite: Matthias, V., Stober, G., Kozlovsky, A., Lester, M., Belova, E., and Kero, J.: A new classification of the Arctic spring transition in the middle atmosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1995, https://doi.org/10.5194/egusphere-egu21-1995, 2021.

13:48–13:50
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EGU21-9318
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ECS
Viktoria Nordström and Annika Seppälä

During September 2019 there was a sudden stratospheric warming over Antarctica, which brought disruption to the usually stable winter vortex. The mesospheric winds reversed and temperatures in the stratosphere rose by over 50~K. Whilst this was only the second SSW in the Southern Hemisphere (SH), the other having occurred in 2002, its Northern counterpart experiences about six per decade. Currently, an amplification of atmospheric waves during winter is thought to trigger SSWs. Our understanding, however, remains incomplete, especially with regards to its occurrence in the SH. Here, we investigate the interaction of two equatorial atmospheric modes, the Quasi Biennial Oscillation (QBO) and the Semiannual Oscillation (SAO) during the SH winters of 2019 and 2002. Using MERRA-2 reanalysis data we find that the two modes interact at low latitudes during their easterly phases in the early winter, forming a zero wind line that stretches from the lower stratosphere into the mesosphere. This influences the meridional wave guide, resulting in easterly momentum being deposited in the mesosphere throughout the polar winter, reducing the magnitude of the westerly winds. As the winter progresses these features descend into the stratosphere, until SSW conditions are reached. We find similar behaviour in two other years leading to delayed dynamical disruptions later in the spring. The timing and magnitude of the SAO and the extent of the upper stratospheric easterly QBO signal, that results in the SAO-QBO interaction, was found to be unique in these years, when compared to the years with a similar QBO phase. We propose that this early winter behaviour may be a key physical process in decelerating the mesospheric winds which may precondition the Southern atmosphere for a SSW. Thus the early winter equatorial upper stratosphere-mesosphere together with the polar mesosphere may provide critical early clues to an imminent SH SSW.

How to cite: Nordström, V. and Seppälä, A.: SAO-QBO Coupling before the 2019 and 2002 Southern Sudden Stratospheric Warmings, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9318, https://doi.org/10.5194/egusphere-egu21-9318, 2021.

Strat-Trop Coupling
13:50–13:52
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EGU21-2767
|
ECS
Ian White and Chaim Garfinkel

An idealised model is used to examine the tropospheric response to stratospheric momentum torques with an emphasis on the response to high-latitude sudden stratospheric warmings (SSWs). Previous related studies have generally imposed such torques in models that lack a key element of realism; for instance, models that do not have a realistic stratosphere, models without stationary planetary waves (i.e., without topography), and models that do not have a troposphere and so precludes any investigation of a downward impact. The idealised moist model of an atmosphere (MiMA) used here overcomes these three shortcomings and is hence well-suited to study the downward impact of extreme events in the stratosphere in a more realistic setup. In particular, we impose transient zonally-symmetric momentum forcing to various latitude bands in the stratosphere, spun-off from a free-running control run (CTRL). In addition to varying the latitudinal location of the forcing, we vary the depth, duration and magnitude to examine the sensitivity of the tropospheric response. Preliminary results show that in contrast to thermally-forced SSWs for which the initial 'Eliassen adjustment' (i.e., the meridional circulation response during the forcing period) is opposite to that found during free-running SSWs, the momentum-forced events here, produce a meridional circulation that mimics that found in the free-running events. This meridional circulation immediately transfers the imposed momentum forcing to the surface, projecting onto the tropospheric Northern Annular Mode (NAM) and initiating a synoptic-wave feedback, a process that takes much longer to develop in the thermally-forced SSWs. Hence, a sudden and strong enough wave forcing (approximated here by an imposed momentum torque) can induce a meridional circulation that penetrates deep into the troposphere and immediately initiate a tropospheric NAM response. The applicability of these experiments to the real atmosphere will be discussed via comparing the evolution of the forced events to free-running SSWs identified in CTRL.

How to cite: White, I. and Garfinkel, C.: Tropospheric response to stratospheric momentum torques, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2767, https://doi.org/10.5194/egusphere-egu21-2767, 2021.

13:52–13:54
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EGU21-3113
Jonas Nycander, Paolo Ruggieri, and Maarten Ambaum

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.

13:54–13:56
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EGU21-5309
|
ECS
Israel Weinberger and Chaim Garfinkel

Extreme states of the polar stratospheric vortex are typically followed by anomalous surface circulation. These extreme stratospheric vortex states are in turn often associated with extreme heat flux between the tropopause and 100 hPa. 

The goal of this work is to better understand upward wave propagation between the tropopause and the bottom of the vortex near 100 hPa using both theory and reanalysis data.

Following Charney and Drazin (1961) we analytically solve a quasi-geostrophic planetary-scale model with three different layers: troposphere, tropopause inversion layer (TIL) and stratosphere. We allow for different buoyancy frequencies in each layer and show the dependence of transmission and reflection coefficients on the buoyancy frequencies, TIL depth and mean-state zonal wind. The dependence of heat flux in the TIL and stratosphere, as well as phase-lines for the wave solution, are presented. This analysis highlights the key role that the TIL and jumps in buoyancy frequency play for upward wave propagation.

We then use reanalysis data to consider the importance of this effect in observations. Four different specifications of the index of refraction are compared: that derived by Charney and Drazin in 1961, that derived by Matsuno in 1970, and two that relax some of the assumptions used in the derivations of the first two. The Charney and Drazin index of refraction includes terms ignored by Matsuno that are critical for understanding upward wave propagation just above the tropopause in both the climatology and associated with extreme heat flux events. By adding these ignored terms to the Matsuno index of refraction, it is possible to construct a useful tool that describes wave flux immediately above the tropopause and at the same time also describes the role of meridional gradients within the stratosphere. Specifically, a stronger tropopause inversion layer (TIL) tends to restrict upward wave propagation. It is also shown that while only 38% of extreme wave-1 Eliassen-Palm flux vertical component (Fz) at 100hPa events are preceded by extreme Fz at 300hPa, there are almost no extreme events at 100hPa in which the anomaly at 300hPa is of opposite sign or very weak.  

How to cite: Weinberger, I. and Garfinkel, C.: The Efficiency of Upward Wave Propagation Near the Tropopause, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5309, https://doi.org/10.5194/egusphere-egu21-5309, 2021.

13:56–13:58
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EGU21-6867
Peter Hitchcock

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.

Oceanic Impacts
13:58–14:00
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EGU21-13063
Blanca Ayarzagüena, Elisa Manzini, Natalia Calvo, and Daniela Matei

Major sudden stratospheric warmings (SSWs) are largest instances of the boreal polar stratospheric variability. Their effects extend farther from the polar stratosphere, affecting for example near-surface circulation. According to observations, SSWs are not equally distributed along time, with decades with almost no events and decades with SSWs happening almost every winter. This suggests the existence of multidecadal variability of SSWs. Some previous studies have pointed to phenomena in the ocean surface as the main precursors of this low-frequency variability. However, the relatively short observational record and the need of long model simulations with daily output have not enabled an analysis of the influences of these oceanic phenomena on SSWs

The goal of this study is to investigate the effects of Atlantic Multidecadal Variability (AMV) and Pacific Decadal Variability (PDV) on SSWs. To do so, we use for the first time a large ensemble of historical experiments (Max Planck Grand Ensemble) that allows us to examine the modulation of the frequency, precursors and surface impact of SSWs by both types of oceanic variability. Our results reveal that PDV has an impact on the frequency of SSWs, with a significant higher rate of SSWs for its positive than the negative phase. As for AMV, the main effect of AMV is centered on the tropospheric response to SSWs, with almost no modulation in the occurrence of the event. This last finding would be useful in order to predict the tropospheric fingerprint of SSWs.

How to cite: Ayarzagüena, B., Manzini, E., Calvo, N., and Matei, D.: Interactions between Decadal to Multidecadal Ocean Variability and Stratospheric Sudden Warmings, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13063, https://doi.org/10.5194/egusphere-egu21-13063, 2021.

14:00–14:02
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EGU21-14469
Nour-Eddine Omrani, Noel Keenlyside, Sandro Lubis, and Fumiaki Ogawa

The response of the Northern Hemisphere (NH) stratosphere to climate change has been usually studied within the classical Transformed Eulerian Mean framework, which focuses mainly on the impact of the resolved atmospheric waves. The role of the non-conservative (or wave-free) processes (like diabatic heating and diffusive potential vorticity mixing) in setting the stratospheric response to climate change remains poorly understood. Here we use different stand-alone atmospheric model experiments and the newly developed Finite Amplitude Local Wave Activity (FALWA) theory, in order to understand the role and the origins of the non-conservative processes in the NH stratospheric response to climate change.

Our model response can reproduce the well-known weakening of the NH polar stratospheric vortex and strengthening of mid-latitude and subtropical stratospheric westerlies.  It is shown that the overall structure of the wintertime response of the NH stratosphere to climate change is maintained mainly by the ocean-induced non-conservative processes with limited contribution of the wave-induced conservative dynamics. In particular, the tropical ocean warming due to climate change maintains the wave free component of the westerly wind, which setup the background wind for poleward wave propagation and the associated wave-induced weakening of the polar stratospheric vortex. The FALWA budget reveals that the weak response of the conservative (or wave induced) component of the stratospheric westerly is maintained mainly by the eddy meridional potential vorticity (PV) transport (or EP-flux divergence) against the non-conservative diffusive PV-mixing. Our work requires the consideration of the non-conservative processes for an accurate dynamical understanding of the stratospheric response to climate change.

How to cite: Omrani, N.-E., Keenlyside, N., Lubis, S., and Ogawa, F.: The key role of ocean-induced non-conservative processes in Northern Hemisphere stratospheric response to climate changes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14469, https://doi.org/10.5194/egusphere-egu21-14469, 2021.

14:02–15:00
Break
Chairpersons: Bo Christiansen, Blanca Ayarzagüena
Predictability
15:30–15:32
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EGU21-2774
Daniela I.V. Domeisen and Amy H. Butler

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.

15:32–15:34
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EGU21-2607
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ECS
Zheng Wu, Bernat Jiménez-Esteve, Raphael de Fondeville, Eniko Székely, Guillaume Obozinski, and Daniela Domeisen

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.

15:34–15:36
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EGU21-7212
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ECS
Rachel Wai-Ying Wu and Daniela I.V. Domeisen

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.

15:36–15:38
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EGU21-4629
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ECS
Hilla Afargan-Gerstman, Iuliia Polkova, Lukas Papritz, Paolo Ruggieri, Martin P. King, Panos Athanasiadis, Johanna Baehr, Ole Wulff, Michael Sprenger, and Daniela I.V. Domeisen

Variability of the stratospheric polar vortex has the potential to influence surface weather by imposing negative North Atlantic Oscillation (NAO) conditions, associated with cold air outbreaks in the Arctic and a southward shift of the extratropical storm track. In particular, the likelihood of cold temperature extremes over the ocean, known as marine cold air outbreaks (MCAOs), have been associated with a range of hazardous conditions, including strong surface winds and the occurrence of extreme cyclones known as Polar Lows (PLs), posing risks for Arctic marine activity and infrastructure. Likewise, winter storms can lead to high damage potential in the extratropics due to their associated extreme winds.

Skillful predictions of MCAOs and extratropical winter storms on subseasonal timescales have been linked to the strength of the stratospheric polar vortex. Using ERA-Interim reanalysis (1979-2019) and ECMWF forecasts from the S2S Prediction Project database we investigate the stratospheric influence on surface extremes such as MCAOs and high-impact winter storms. Following weak stratospheric vortex extremes, anomalous circulation patterns accompanied by increased storminess over the eastern North Atlantic are found to be strong indicators for enhanced MCAOs in high- and mid-latitudes. Understanding the role of the stratosphere in subseasonal variability and predictability of cold air outbreaks and storm tracks during winter can provide a key for a reliable forecast of severe impacts.

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.

15:38–15:40
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EGU21-5311
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ECS
Marisol Osman, Theodore Shepherd, and Carolina Vera

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.

15:40–15:42
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EGU21-8534
Hua Lu, Lesley Gray, Patrick Martineau, John King, and Thomas Bracegirlde

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.

Observations
15:42–15:44
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EGU21-6400
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ECS
Kamilya Yessimbet and Andrea Steiner

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.

15:44–15:46
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EGU21-9309
Bernard Legras, Hugo Lestrelin, Aurélien Podglajen, and Mikail Salihoglu

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.

QBO
15:46–15:48
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EGU21-3222
|
ECS
Kevin DallaSanta and Clara Orbe

The Quasi-Biennial Oscillation has exhibited remarkable stability over the observational record—until a well-documented 2015/16 disruption and an emerging disruption in 2020/21. The possibility that disruptions are more frequent in a changing climate is important to consider, as the QBO affects predictability, stratospheric composition, and surface weather. However, this possibility is challenging to assess for a variety of reasons. For instance, the 2015/16 disruption has been attributed to anomalous easterly momentum flux from extratropical waves. By comparison, the 2020/21 disruption involves anomalous westerly forcing, less likely to originate from the same mechanism.

We present a rich variety of QBO disruptions that spontaneously arise in integrations of the high-top NASA GISS Model E2.2. The disruptions loosely fall into several categories, some of which are analogous to the 2015/16 disruption and the 2020 disruption, as well as a previously undocumented possible disruption in 1988. Several factors appear to influence QBO disruptions in the model: natural variability, climate change, tropical SSTs, volcanic eruptions, and model physics/tuning. Although QBO representation is an ongoing challenge for models, the results point to a model-independent framework for assessment of disruptions.

 

How to cite: DallaSanta, K. and Orbe, C.: Simulated disruptions of the Quasi-Biennial Oscillation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3222, https://doi.org/10.5194/egusphere-egu21-3222, 2021.

15:48–15:50
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EGU21-7077
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ECS
Min-Jee Kang, Hye-Yeong Chun, and Rolando Garcia

In winter 2015/2016, the descent of the westerly phase of the quasi-biennial oscillation (QBO) was unprecedentedly disrupted by the development of easterly winds. Previous studies have shown that extratropical Rossby waves propagating into the tropics were the major cause of the 2015/16 QBO disruption. However, a large portion of the negative momentum forcing driving the disruption still stems from equatorial planetary and gravity waves, which calls for detailed analyses by separating each wave mode. In this study, the contributions of resolved equatorial planetary waves (Kelvin, Rossby, mixed Rossby–gravity (MRG), and inertia–gravity (IG) waves) and small-scale convective gravity waves (CGWs) obtained from an offline CGW parameterization to the 2015/16 QBO disruption are investigated using MERRA-2 global reanalysis data. In October and November 2015, anomalously strong negative forcing by MRG and IG waves weakened the QBO jet at 0–5°S near 40 hPa, possibly leading to Rossby wave breaking at the QBO jet core in the Southern Hemisphere. From December 2015 to January 2016, strong Rossby waves propagating horizontally (vertically) from the Northern Hemisphere (troposphere) decelerated the southern (northern) flank of the jet. In February 2016, when the westward CGW momentum flux at the source level was much stronger than the climatology, CGWs began to exert considerable negative forcing at 40–50 hPa near the Equator, in addition to the Rossby waves. The enhancement of the negative wave forcing in the tropics stems mostly from strong wave activity in the troposphere associated with increased convective activity and the westerly anomalies in the troposphere, except that the MRG wave forcing is more likely associated with increased barotropic instability in the lower stratosphere.

How to cite: Kang, M.-J., Chun, H.-Y., and Garcia, R.: Role of equatorial waves and convective gravity waves in the 2015/16 QBO disruption, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7077, https://doi.org/10.5194/egusphere-egu21-7077, 2021.

15:50–15:52
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EGU21-8173
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ECS
Henning Franke and Ulrike Niemeier

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.

Polar Vortex
15:52–15:54
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EGU21-7766
|
ECS
Antti Salminen, Timo Asikainen, Ville Maliniemi, and Kalevi Mursula

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.

15:54–15:56
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EGU21-9523
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ECS
Audrey Lecouffe, Sophie Godin-Beekmann, Andrea Pazmiño, and Alain Hauchecorne

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:

  • Akiyoshi, H., Zhou, L., Yamashita, Y., Sakamoto, K., Yoshiki, M., Nagashima, T., Takahashi, M., Kurokawa, J., Takigawa, M., and Imamura, T. A CCM simulation of the breakup of the Antarctic polar vortex in the years 1980–2004 under the CCMVal scenarios, Journal ofGeophysical Research: Atmospheres, 114, 2009.
  • Godin S., V. Bergeret, S. Bekki, C. David, G. Mégie, Study of the interannual ozone loss and the permeability of the Antarctic Polar Vortex from long-term aerosol and ozone lidar measurements in Dumont d’Urville (66.4◦S, 140◦E), J. Geophys. Res., 106, 1311-1330, 2001.
  • Hauchecorne, A., S. Godin, M. Marchand, B. Hesse, and C. Souprayen, Quantification of the transport of chemical constituents from the polar vortex to midlatitudes in the lower stratosphere using the high-resolution advection model MIMOSA and effective diffusivity, J. Geophys. Res., 107 (D20), 8289, doi:10.1029/2001JD000491, 2002.
  • Nash, E. R., Newman, P. A., Rosenfield, J. E., and Schoeberl, M. R. (1996), An objective determination of the polar vortex using Ertel’s potential vorticity, Journal of geophysical research, VOL.101(D5), 9471- 9478
  • World Meteorological Organization, Global Ozone Research and Monitoring Project – Report No. 58, 2018.

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.

Brewer-Dobson Circulation
15:56–15:58
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EGU21-11920
Mohamadou Diallo, Manfred Ern, and Felix Ploeger

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.

15:58–17:00