Displays

Sudden stratospheric warming (SSW) events are often followed by a surface impact, most commonly by a negative phase of the North Atlantic Oscillation (NAO). Recent work has emphasized the large variability among the tropospheric response after these events, showing that only about two thirds of the SSWs are dominated by this canonical negative NAO response. In this study, we use an idealized atmospheric model forced with seasonally varying sea surface temperatures to examine the influence of the pre-existing tropospheric conditions on the North Atlantic response to stratospheric forcing. In the model, the negative phase of the NAO is found to be the most common response to SSWs, occurring after ~85% of the SSWs (under climatological SST forcing).  For the remaining ~15% of the SSW events, the response is associated with a positive phase of the NAO. In the search for the origin of the different tropospheric response in the North Atlantic, the role of synoptic wave propagation from the eastern Pacific on the downward response to SSWs is investigated. By systematically varying the strength of the North Pacific circulation, we are able to assess the sensitivity of the downward response to tropospheric variability in the Pacific, and shed light on its contribution to the persistence of the downward impact of SSWs in the idealized model.

How to cite: Afargan-Gerstman, H., Jiménez-Esteve, B., and Domeisen, D. I. V.: Variability of the North Atlantic response to sudden stratospheric warming events in a simplified atmospheric model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-866, https://doi.org/10.5194/egusphere-egu2020-866, 2020.

Sudden stratospheric warming (SSW) characterized by a rapid increase in polar stratospheric temperature and an abrupt decrease in circumpolar westerly wind is accompanied by deformation in the shape of the polar vortex. The SSW type can be distinguished depending on the vortex shape. SSW events preceded by a displacement in polar vortex center are characterized by whether they retain their displaced form (displacement-displacement type) or split into two vortices (displacement-split type) after onset. Here, we show that existence of a polar vortex shape-transition during the course of the SSW life cycle can be attributable to the condition of North Atlantic Oscillation (NAO) preceding before onset: Positive NAO favors SSW of displacement-displacement type with no transition while negative NAO favors the displacement-split type. We show that, in positive NAO precondition, vertical flux of wave activity immediately before onset is mostly contributed only by wavenumber 1 component, which contrasts with the relatively stronger contribution of wavenumber 2 in negative NAO pre-condition. This study provides probability that the North Atlantic anomaly can induce a favorable condition for the development of small scale waves and lead to the occurrence of SSW type-transition. Whole Atmosphere Community Climate Model (WACCM) simulation results reproduce well the observational findings. Therefore, NAO can be regarded as a useful predictor for distinguishing the type of forthcoming SSW events.

How to cite: Choi, H., Choi, W., Kim, S.-J., and Kim, B.-M.: Polar vortex shape-transition during SSW depending on preceding NAO conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1925, https://doi.org/10.5194/egusphere-egu2020-1925, 2020.

Sudden Stratospheric Warming events (SSWs) are rapid disruptions of the Northern Hemisphere (NH) winter stratospheric polar vortex and represent the largest source of inter-annual variability in the NH winter stratosphere. They have been linked to winter surface climate anomalies such as cold snaps over North America and Eurasia. Representing these events accurately in large scale GCMs as well as developing a greater understanding of them is key to improving predictability of winter surface climate. A key component of a GCM is its representation of atmospheric chemistry. Chemical distributions are either prescribed or calculated interactively by coupling an atmospheric chemistry model to radiation and dynamical components, thus capturing any chemical dynamical feedback mechanisms but incurring significant running cost.

This work evaluates the impact of interactive chemistry when modelling SSW events and explores the feedback mechanisms between chemical distributions and stratospheric dynamical variability. Pre-industrial control runs from the MetOffice HadGEMGC3.1 model which prescribes chemical fields and UKESM1 which calculates trace gas concentration interactively are utilised. Over the whole season - The Earth System Model appears to suppress warmings while the model with prescribed physics overestimates their occurrence compared to reanalysis. The differing representation of the equatorial stratosphere appears to be partially responsible for this difference. Additionally we find that middle stratosphere equatorial ozone concentration in late NH summer is closely associated with SSW probability in the ensuing winter in UKESM1. Anomalously low ozone is generally associated with an elevated SSW rate. This implies a chemical-dynamical coupling between the equator and the vortex in this model which preliminary results suggest could be driven by chemical feedbacks influencing the state of the early winter Quasi Biennial Oscillation (QBO) and Semi-Annual Oscillation (SAO) in zonal winds which can alter the distribution of planetary wave propagation and breaking (the primary cause of SSWs). Further work will assess whether this phenomenon is observed in other GCMs and further explore the physical mechanisms responsible.

How to cite: Dimdore-Miles, O., Gray, L., and Osprey, S.: Dynamical-Chemical Feedbacks in General Circulation Models and Their Influence on Sudden Stratospheric Warming Events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3471, https://doi.org/10.5194/egusphere-egu2020-3471, 2020.

Sudden stratospheric warmings (SSWs) are impressive events that occur in the winter hemisphere's polar stratosphere and are capable of producing temperature anomalies upwards of +50 degrees within a matter of days. While much work has been dedicated towards determining how SSWs occur and their ability to interact with the underlying troposphere, one under-explored aspect of SSWs is the role of radiation. Using a radiative transfer model and an energy budget analysis for distinct layers of the stratosphere, this work accounts for the radiative contribution to the removal of the anomalous energy associated with SSWs. In total, 19 events are investigated over the 1979-2016 period. This work reveals that in the absence of dynamical heating following major SSWs, longwave radiative cooling dominates and often results in a strong negative temperature anomaly. The stratospheric temperature change driven by the radiative cooling is characterized by an exponential decay of the temperature anomaly with an increasing e-folding time of 6.3 ± 2.6 to 21.6 ± 8.3 days from the upper to lower stratosphere. This work also demonstrates a negligible impact that water vapour and ozone have on the longwave and shortwave radiative heating rates during SSWs when the concentrations of these gases are perturbed from their climatological state.

How to cite: Bloxam, K.: Sudden Stratospheric Warmings - The Role of Radiation Revealed Through an Energy Budget Analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5895, https://doi.org/10.5194/egusphere-egu2020-5895, 2020.

Sudden stratospheric warmings (SSWs) have a significant downward influence on the tropospheric circulation below, although the mechanisms governing this downward impact are not well understood. It is also not known if the type of SSW event – be them splits or displacements – play a role in determining the magnitude of the tropospheric response. We here examine the impacts of split- and displacement-type SSWs on the troposphere.

To do this, we use the recently developed model of an idealised moist atmosphere to impose zonally-asymmetric warming perturbations to the extratropical stratosphere, extending the work of a recent study by the authors in which a zonally-symmetric heating perturbation was imposed. This model of ‘intermediate complexity’ is particularly suited to this study as it incorporates the radiation scheme that is utilised by operational forecast systems, including both the ECMWF and NCEP. The radiation scheme also allows us to force the model with a realistic ozone profile, and thus to simulate realistic radiative timescales in the stratosphere. From a control run with a realistic climatology, we perform an ensemble of spin-off runs every January 1^st with imposed high-latitude stratospheric heating perturbations of varying degrees of magnitude. The heating perturbation is switched on for a limited period of time to mimic the sudden nature of a SSW event and the troposphere is allowed to evolve freely. We compare the evolution of the tropospheric response to the forced split and displacement-type SSWs with free-running SSWs of the same type in the control run.

By modifying only the temperature tendency equation as opposed to the momentum budget, our experiments allow us to isolate the tropospheric response associated with changes in the polar-vortex strength (e.g., a direct or indirect modulation of planetary waves and synoptic waves), rather than due to any planetary-wave momentum torques that initially drive the SSW. Nevertheless, the imposition of wave-1 and wave-2 heating perturbations provide a more realistic post-onset SSW state than that which occurs in response to zonal-mean heating perturbations as performed in our previous study.

How to cite: White, I., Garfinkel, C., Gerber, E., and Jucker, M.: The downward propagation of split- and displacement-type SSWs in an idealised model , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8339, https://doi.org/10.5194/egusphere-egu2020-8339, 2020.

This study uses the stratosphere-resolved Whole Atmosphere Community Climate Model to demonstrate the “independent” and “dependent” topographic forcing from the topography of East Asia (EA) and North American (NA), and their “joint” forcing in the northern winter stratosphere. The mutual interference between the EA and NA forcing is also demonstrated. Specifically, without EA, an independent NA can also, like EA, induce a severe polar warming and weakening of the stratospheric polar vortex. While EA favors a displacement of the polar vortex toward Eurasia, NA favors a displacement toward the North America–Atlantic region. However, the independent-EA-forced weakening effect on the polar vortex can be largely decreased and changes to a location displacement when NA exists, and the interference the other way around is even more critical, being able to completely offset the independent-NA-forced effect, because EA can substantively obstruct NA’s effect on the tropospheric wave pattern over the Eurasia–Pacific region. The much stronger/weaker interference of EA/NA is associated with its stronger/weaker downstream weakening effect on the zonal flow that impinges on NA/EA. The mutual interference always tends to further destruct the upward wave fluxes over the eastern North Pacific and enhance the downward wave fluxes over NA. The overall changes in upward wave fluxes, as well as that in the Rossby stationary wavenumber responsible for the stratospheric changes, are related to changes in the zonal-mean flow pattern. The joint effects of EA and NA, rather than being a linear superimposition of their independent effects, are largely dominated by the effects of EA.

How to cite: Ren, R., Xia, X., and Rao, J.: Topographic forcing from East Asia and North America in the northern winter stratosphere and their mutual interference , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1392, https://doi.org/10.5194/egusphere-egu2020-1392, 2020.

The importance of understanding the dynamical coupling of troposphere and stratosphere to make accurate weather and climate predictions is well-known. Over the past years and decades various signatures of such a
coupling have been discovered. A very robust result, for example, seems to be an equatorward shift of the tropospheric eddy driven jet following sudden stratospheric warming events, where the westerly winds of the stratospheric polar vortex weaken or even reverse. However, many aspects of this fundamental coupling are still not fully understood and research on how the state of the stratosphere can influence the tropospheric circulation and what dynamical processes are involved is still ongoing.

An important such process arises due to the interaction of a sharp, localised maximum in potential vorticity gradient near the tropopause with baroclinic eddies in the troposphere. Here, we analyse the sensitivity of baroclinic wave development and evolution to changes of various basic state characteristics, by performing a series of idealised baroclinic eddy life cycle experiments. Special attention is paid to sensitivities associated with the dynamical state of the stratosphere. We find that the final (steady) state of the life cycle simulations corresponds to an equatorward shift of the tropospheric jet in cases where the initial conditions do not include a stratospheric polar vortex (such as following sudden warming events) compared to those that do. These results further support the idea that the stratospheric state can strongly influence tropospheric dynamics and, in particular, highlight the robustness of the jet shift response following sudden warmings, that can be seen in a range of observations and numerical model experiments.

How to cite: Rupp, P. and Birner, T.: Stratospheric influence on idealised baroclinic life cycles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13680, https://doi.org/10.5194/egusphere-egu2020-13680, 2020.

We present evidence that stratospheric sudden warmings (SSWs) are, on average, a threshold behavior of finite-amplitude Rossby waves arising from wave-mean flow interaction. Competition between an increasing wave activity and a decreasing zonal-mean zonal wind sets a limit to the upward wave activity flux of a stationary Rossby wave.  A rapid, spontaneous vortex breakdown occurs once the upwelling wave activity flux reaches the limit, or equivalently, once the zonal-mean zonal wind drops below a certain fraction of the wave-free, reference-state wind obtained from the zonalized quasigeostrophic potential vorticity.  This threshold faction is 0.5 in theory and about 0.3 in reanalyses.  We use the ratio of the zonal-mean zonal wind to the reference-state wind as a local, instantaneous measure of the proximity to vortex breakdown, i.e. preconditioning.  The ratio generally stays above the threshold during strong-vortex winters until a pronounced final warming, whereas during weak-vortex winters it approaches the threshold early in the season, culminating in a precipitous drop in midwinter as SSWs form. The essence of the threshold behavior is captured by a semiempirical 1D model of SSWs, analogous to the “traffic jam” model of Nakamura and Huang for atmospheric blocking. This model predicts salient features of SSWs including rapid vortex breakdown and downward migration of the wave activity/zonal wind anomalies, with analytical expressions for the respective timescales. Model’s response to a variety of transient wave forcing and damping is discussed.

How to cite: Nakamura, N.: Stratospheric sudden warming as a threshold behavior of Rossby waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3165, https://doi.org/10.5194/egusphere-egu2020-3165, 2020.

We analyze the stratospheric waves in models participating in phase 1 of the Stratosphere–troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi). All models have robust Kelvin and mixed Rossby-gravity wave modes in winds and temperatures at and represent them better than most of the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. There is still some spread among the models, especially concerning the mixed Rossby-gravity waves. We attribute the variability in equatorial waves among the QBOi models in part to the varying horizontal and vertical resolutions, to systematic biases in zonal winds, and to the considerable variability in convectively coupled waves in the troposphere among the models: only roughly half of the QBOi models have realistic convectively coupled Kelvin waves and only a few models have convectively coupled mixed Rossby-gravity waves. The models with stronger convectively coupled waves produce larger zonal mean forcing due to resolved waves in the QBO region. Finally we evaluate the Eliassen-Palm (EP) flux and EP flux divergence of the resolved waves in the QBOi models. We find that there is a large spread in the forcing from resolved waves in the QBO region, and the resolved wave forcing has a robust correlation with model vertical resolution

How to cite: Holt, L. and Lott, F. and the QBOi Contributors: An evaluation of tropical waves and wave forcing of the QBO in the QBOi models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5635, https://doi.org/10.5194/egusphere-egu2020-5635, 2020.

Using 17 CMIP5/6 models with a spontaneously-generated quasi-biennial oscillation (QBO)-like phenomenon, this study explores and evaluates three dynamical pathways for impacts of the QBO on the troposphere: (i) the Holtan-Tan (HT) effect on the stratospheric polar vortex and the northern annular mode (NAM), (ii) the subtropical zonal wind downward arching over the Pacific, and (iii) changes in local convection over the Maritime Continent and Indo-Pacific Ocean. More than half of the models can reproduce at least one of the three pathways, but few models can reproduce all of the three routes. Firstly, most models are able to simulate a weakened polar vortex during easterly QBO (EQBO) winters, in agreement with the observed HT effect. However, the weakened polar vortex response during EQBO winters is underestimated or not present at all in other models, and hence the QBO → vortex → tropospheric NAM/AO chain is not simulated. For the second pathway associated with the downward arching of the QBO winds, seven models incorrectly or poorly simulate the extratropical easterly anomaly center over 20–40°N in the Pacific sector during EQBO, and hence the negative relative vorticity anomalies poleward of the easterly center is not resolved in those models, leading to an underestimated or incorrectly modelled height response over North Pacific. However the other ten do capture this effect. The third pathway is only observed in the Indo-Pacific Ocean, where the strong climatological deep convection and the warm pool are situated. Nine models can simulate the convection anomalies associated with the QBO over the Maritime Continent, which is likely caused by the near-tropopause low buoyancy frequency anomalies. No robust relationship between the QBO and El Niño–Southern Oscillation (ENSO) events can be established using the ERA-Interim reanalysis, and nine models consistently confirm little modulation of the ocean basin-wide Walker circulation and ENSO events by the QBO.

How to cite: Rao, J., Garfinkel, C., White, I., and Schwartz, C.: Impact of the Quasi-Biennial Oscillation on the boreal winter tropospheric circulation in CMIP5/6 models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14259, https://doi.org/10.5194/egusphere-egu2020-14259, 2020.

There is robust observational evidence that the troposphere is warming and the stratosphere is cooling in response to the radiative forcing of anthropogenic greenhouse gas (GHG) emissions. Temperature changes directly influence the vertical structure of the atmopshere. Numerous studies have analysed the thermal expansion of the troposphere, in particular the tropopause rise and its interaction with the Brewer-Dobson circulation (BDC). Stratospheric cooling, however, reduces the upward shift of pressure levels with increasing altitude so that it reverses sign at some height, leading to a downward shift of the middle to upper stratosphere. This "stratospheric shrinkage“ effect is a strong and robust feature of climate change and it is well documented through observations. Still, literature on this effect is relatively sparse and its impact on stratospheric dynamics is generally neglected.

In this study, we report and quantify the uncertainty in residual upward velocity (w*) trends that arises from the implicit neglection of stratospheric shrinkage in the data model request for the Chemistry-Climate Model Initiative part 1 (CCMI-1). Tropical w* is often taken as a proxy for diagnosing the BDC strength. In the data request, a constant scale height is assumed for conversion of w* from Pa/s to m/s . However, the scale height significantly decreases over time in the climate simulations as a result of stratospheric shrinkage.

We show that stratospheric cooling enhances the w* trends if the unit conversion is made with constant scale height, which can be misinterpreted as BDC acceleration. We quantify this effect to account for around 20% of the w* trend across the 21st century, consistently among the CCMI-1 climate projection simulations. Past studies that based w* trend analyses on these data therefore made a 20% error. Moreover, we call attention that other dynamical diagnostics are affected by the neglection of stratospheric shrinkage too and also the data requests of other multi-model assessments use the constant scale height assumtion for unit conversion in climate change simulations.

How to cite: Eichinger, R. and Sacha, P.: Artificial acceleration of the Brewer-Dobson circulation due to stratospheric cooling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12880, https://doi.org/10.5194/egusphere-egu2020-12880, 2020.

The stratospheric Brewer--Dobson circulation (BDC) is an important element of climate system as it determines the concentration of radiatively active trace gases like water vapor, ozone and aerosol above the tropopause. Climate models predict that increasing greenhouse gas levels speed up the stratospheric circulation. BDC changes is substantially modulated by different modes of climate variability (QBO, ENSO, solar cycle), including the volcanic aerosols. However, such variability is often not reliably included or represented in current climate model simulations, challenging the evaluation of models’ behavior against observations and constituting a major uncertainty in current climate simulations. 

Here, we investigate the main differences between the reanalysis and the CCMI/CMIP6 climate models’ response to stratospheric volcanic forcings regarding the depth/strength of the stratospheric BDC, with a focus on potential changes in the deep and shallow circulation branches. We also discuss the key reasons of the discrepancies (incl. uncertainties associated with volcanological forcing datasets and missing direct aerosol heating in the reanalysis) in the BDC response between reanalysis-driven and climate model simulations in the lower, mid and upper stratosphere. Finally, we assess the dynamical mechanisms involved in the volcanically-induced BDC changes to understand the opposite regime between lower, middle and upper stratosphere after the Mt Pinatubo eruption.

How to cite: Diallo, M., Garny, H., Eichinger, R., Aquila, V., Ern, M., and Ploeger, F.: Reconciling the BDC response in climate models to the volcanic forcings with reanalyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17508, https://doi.org/10.5194/egusphere-egu2020-17508, 2020.

Solar cycle (SC) induces variations in the UV radiation. The UV variations change the ozone production rate in the middle atmosphere. Responses to the SC-induced variations occur mainly in the equatorial upper stratosphere and the lower mesosphere. It has been reported that zonal mean temperature difference is 1--2 K between solar maximum and minimum. The temperature variation in the equatorial upper stratosphere modifies the meridional temperature gradient between the equatorial region and winter polar region. Change in the temperature gradient induces difference in the strength of the stratospheric polar vortex, which accompanies change in poleward meridional mass circulations and as a result change in the horizontal distribution of the sea-level pressure (SLP) in the winter polar region. In the present study, this mechanism of SC-induced SLP variations in the Northern Hemisphere (NH) winter polar regions is examined using an idealized whole-atmosphere general circulation model. This global model covers from the ground to the lower thermosphere and includes gravity wave drag parameterization and realistic topography. This idealized model is driven by the zonally-averaged radiative equilibrium temperature, but it nevertheless simulates quite realistically atmospheric variabilities such as sudden stratospheric warmings and quasi-biennial oscillations. Perpetual January simulations for solar maximum and minimum show that this idealized model can reproduce the negative SLP anomaly in the NH polar regions in solar maximum, but the magnitude of the anomaly is weak compared with reanalysis studies. Mechanisms of this SLP anomaly are examined through planetary wave dynamics and gravity-wave processes.

How to cite: Song, I.-S., Kim, J.-H., and Jee, G.: Impact of solar cycle variation of UV radiation in the Northern Hemisphere winter polar troposphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2516, https://doi.org/10.5194/egusphere-egu2020-2516, 2020.

We investigate the effect of systematic model biases on teleconnections influencing the Northern Hemisphere wintertime circulation. We perform a two-step nudging and bias-correcting scheme for the dynamic variables of the ECHAM6 atmospheric model to reduce errors in the model climatology relative to ERA-Interim. The developed scheme is efficient in removing errors in model’s climatology. In particular, large negative bias in December-February mean zonal stratospheric winds is reduced by up to 75%, significantly increasing the strength of the Northern Hemisphere wintertime stratospheric polar vortex. The bias-corrections are applied to the full atmosphere or stratosphere only.

We compare the response of bias-corrected and control runs to internal stratospheric variability and surface forcings that are important on seasonal timescales: Siberian snow cover in October; the Quasi-Biennial Oscillation (QBO); and ENSO. We find the bias-corrected model has the potential for a strengthened and more realistic response to the teleconnections, either in the stratospheric or surface response. In particular, the bias-corrected model has a strong QBO teleconnection which modulates the extratropical polar vortex and sea level pressure variability in a manner similar to that seen in observations. The Siberian snow forcing with the stratosphere-only bias-corrections also leads to an enhanced surface response relative to the control. The mechanism behind the sensitivity of the teleconnections to model biases is discussed.

How to cite: Tyrrell, N., Karpechko, A., and Rast, S.: The stratospheric response and surface influence in a bias-corrected model., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12888, https://doi.org/10.5194/egusphere-egu2020-12888, 2020.

There is a need for inferring mid-stratosphere to mid-mesosphere wind profiles between approximately 25 and 75 km altitude as a consequence of the relatively poor characterization of the complex atmospheric dynamics at these altitudes by the existing monitoring techniques. A better knowledge of the wind field could help improve the quality of global circulation models for this region of the atmosphere. Our goal is to measure strato-mesospheric horizontal wind profiles over the Arctic region. To this objective, we are using data from continuous measurements recorded for ozone chemistry monitoring purposes that have been ongoing since 2002 and from dedicated wind measurements recorded since 2014. An accurate interpretation of the wind patterns could give us an insight into past and present wind dynamics in the Arctic. For the measurements, we are using the ground-based millimeter-wave radiometer KIMRA [Raffalski et al., 2002] with a spectral range between 195-233GHz, situated at the Swedish Institute of Space Physics, in Kiruna, Sweden. Within KIMRA’s spectral range, the thermal emission spectrum of a strong ozone line at 231.3GHz has been used for ozone monitoring, and it might also be the most suitable to use for inferring wind speeds. It has both a strong contribution compared to other, secondary gases identified in this frequency region, and it has an enhanced spectral signature compared to the baseline effects induced by the radiometer itself. By determining the difference between the observed (wind-affected) and the simulated reference spectra (without wind), we can characterize the Doppler shift of the line with the help of our retrieval system and subsequently infer the speeds of the winds that induced the shift. The wind profile retrievals are performed with the Qpack2 package [Eriksson et al., 2005] for inverting the set of observations using an optimal estimation retrieval approach (OEM). The OEM provides the best solution given the measurements and their errors and the a priori knowledge and its errors. The a priori knowledge that we consider has been estimated from ERA5 re-analysis data downloaded from the Copernicus Climate Data Store. In connection with Qpack2, we use the Atmospheric Radiative Transfer Simulator (ARTS-2) [Bühler et al., 2018] as a forward model. We test the retrieval capabilities of strato-mesospheric horizontal wind profiles and assess if the retrieval can be performed with sufficient accuracy. In addition, we intend to evaluate the technical capabilities of KIMRA regarding possible improvements to the instrument’s performance. Besides the implementation of new hardware, we will analyze how adjusting certain parameters that currently limit its spectral resolution affects the sensitivity of the measurements. Furthermore, we will aim to decrease the intrinsic noise of the radiometer and increase its stability over time.

How to cite: Kajtár, R. E., Milz, M., Raffalski, U., and Mendrok, J.: Measurement capabilities of strato-mesospheric winds in the Arctic region , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-187, https://doi.org/10.5194/egusphere-egu2020-187, 2020.

The model of the middle and upper atmosphere circulation (MUAM) and the data of the ERA-interim archive are used to study the mean zonal variations of wind velocity and temperature in the middle atmosphere. Comparison of model calculations and data of ERA-interim archives showed that the model adequately reproduces the main features of circulation processes in the winter stratosphere. The analysis of variations in the mean zonal characteristics of the atmosphere is show that synchronous variations there are in wind velocity and temperature in the stratosphere and mesosphere in the range of 10-30 days . These synchronous variations occupy long latitudinal zones horizontally (tens degree) and have a significant length vertically (tens km). The sign of the variations change horizontally in the region of jet streams (and does not change at the equator), and the vertical change of sign occurs in areas of the stratopause and the mesopause. The nature of  the variations practically does not depend on the phase of the quasi-biennial cycle in the Equatorial stratosphere. The variations are global scale and are reminiscent of the fluctuations in the meridional circulation cells. The dynamic processes of destruction of the polar vortex during sudden stratospheric warming are coordinated with these synchronous variations.

Acknowledgements. This work was supported by the Russian Science Foundation, project No. 19-77-00009.  The authors gratefully acknowledge the access to the ECMWF ERA-Interim.

How to cite: Zorkaltseva, O., Mordvinov, V., Dombrovskaya, N., and Pogoreltsev, A.: Dynamics of zonal mean characteristics of circulation in stratosphere and mesosphere in winter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-295, https://doi.org/10.5194/egusphere-egu2020-295, 2020.

In this study, we investigate the Sudden Stratospheric Warming that took place on 12 February 2018 (SSW2018), its predictability and teleconnection with the Madden-Julian Oscillation (MJO) by analysing ECMWF ensemble forecast initialised on 1 February 2018. Several days prior to that date MJO was in Phase 6 and had a strong amplitude potentially contributing to triggering the SSW. Two wave trains can be identified in the upper troposphere over the northern Atlantic and Pacific regions. Starting from the 3 February, the amplitude of planetary wave with wavenumber 2 (PW2) started to increase and reached record high values, while the PW1 amplitude decreased.

In order to better understand the sources of uncertainties, we divided the forecast ensemble members into two groups. The first group predicted the SSW onset in time while the second group of ensemble members did not capture the wind reversal at 60°N 10 hPa. The results obtained with the ensemble forecast data were compared with the ECMWF’s reanalysis ERA-Interim (ERA-I). The analysis of the two groups of ensemble forecasts shows that in the first group of forecasts PW2 prevailed with ridges over the Ural and Alaska and troughs over the west Siberia and Canada, as observed. Instead, PW1 is seen in the second group of ensemble members with a broad ridge over Eurasia. Calculations of wave activity fluxes show that there is less zonal wave energy propagation in the second group compared to the first group and ERA-I over Eurasia, which can be associated with the errors in the forecasted location of the Ural high. There is also wave energy propagation towards an area of high pressure over Alaska, as seen in ERA-I. Here, wave energy propagation is similarly underestimated by both groups. Overall, the structure of the geopotential anomalies averaged for 5-7 February for the first group and ERA-I is more consistent with the climatological response from MJO phase 6 taken with lag 5-9 days than that in the second group.

How to cite: Erner, I., Karpechko, A., and Järvinen, H.: Mechanisms and predictability of Sudden Stratospheric Warming in winter 2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-705, https://doi.org/10.5194/egusphere-egu2020-705, 2020.

In this work we investigate interannual variations in lower stratospheric ozone from 1984 to 2016 based on a satellite-derived dataset and simulations from a chemical transport model. An empirical orthogonal function (EOF) analysis of ozone variations between 2000 and 2016 indicates that the first, second, and third EOF modes are related to the quasi-biennial oscillation (QBO), canonical El Niño–Southern Oscillation (ENSO), and ENSO Modoki events, respectively; these three leading EOFs capture nearly 80% of the variance. However, for the period 1984–2000, the first, second, and third modes are related to the QBO, ENSO Modoki, and canonical ENSO events, respectively. The explained variance of the second mode in relation to ENSO Modoki is nearly twice that of the third mode for canonical ENSO. Since the frequency of ENSO Modoki events was higher from 1984 to 2000 than after 2000, the Brewer–Dobson circulation anomalies related to ENSO Modoki were stronger during 1984–2000, which caused ENSO Modoki events to have a greater effect on lower stratospheric ozone before 2000 than after. Ozone anomalies associated with QBO, ENSO Modoki, and canonical ENSO events are largely caused by dynamic processes, and the effect of chemical processes on ozone anomalies is opposite to that of dynamic processes. Ozone anomalies related to dynamic processes are 3–4 times greater than those related to chemical processes.

How to cite: Lu, J., Xie, F., Tian, W., Li, J., Feng, W., Chipperfield, M., Zhang, J., and Ma, X.: Interannual variations in Lower Stratospheric Ozone during the period 1984-2016, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1390, https://doi.org/10.5194/egusphere-egu2020-1390, 2020.

                                                                                                                             Abstract

As the products of the complicated interactions and the kinematic, chemical and radiative balances between upper troposphere and lower stratosphere, dynamical tropopause is recognized as a key boundary of atmosphere closely related to weather and climate change. In this study, a high-spatio-temporal-resolution dynamical tropopause pressure estimation scheme based on the measurements from the Advanced Geostationary Radiation Imager boarded on the new generation geostationary satellite of China, FengYun-4A, is proposed. The implemented retrieval model is quantitative validated against ERA-Interim reanalysis dataset showing a high accuracy with the correlation coefficient of 0.9603 and root-mean-square-error of 42.96 hPa. The method is applied to a 1-year period starting in January 2018 over the east hemisphere. The geographic distributions and the seasonal cycle show that the dynamical tropopause height is varied with the latitudes and seasons which has the mean pressure of about 50 hPa over low latitudes and about 300 hPa over the high latitudes where the tropopause height reaches the minimum during March-May. Generally, the folds preferentially occur in the subtropics around 20-40º latitude where the upper fronts located. They are found to have potential connection with the rain band splitting during the Mei-Yu season in East Asia.

Keywords：Dynamic tropopause, FengYun-4 geostationary satellite, WV channels

How to cite: Shou, Y., Lu, F., and Shou, S.: Characteristics of the dynamical tropopause derived from satellite observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1852, https://doi.org/10.5194/egusphere-egu2020-1852, 2020.

We explored the gravity wave behavior and its role for the unusual QBO structure in 2015/2016 by analyzing the data of U.S. radiosonde with high vertical resolution over four equatorial stations from 1998 to 2017. The result implies that the gravity wave behavior should play an important role during the QBOW phase interrupted around 22 km in 2015/2016 winter. While the role of gravity wave was not as important as Kelvin waves during the prolonged and upward propagating westerly zonal wind around 27 km. The enhanced gravity wave may be generated by the instability of the stratospheric atmosphere rather than the tropospheric convection because the convection is weak during the unusual QBO structure over the four equatorial stations.

How to cite: Li, H. and Li, Q.: The behavior of Graviy wave during the unusual QBO structure in 2015/2016, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2560, https://doi.org/10.5194/egusphere-egu2020-2560, 2020.

The impact of the solar cycle on the NH winter stratospheric circulation is analyzed using
simulations of a Model of an idealized Moist Atmosphere (MiMA). By comparing solar minimum
periods to solar maximum periods, the solar impact on the stratosphere is evaluated: Solar
maximum periods are accompanied by warming of the tropics that extends into the midlatitudes
due to an altered Brewer Dobson Circulation. This warming of the subtropics and the altered
Brewer Dobson Circulation leads to an increase in zonal wind in midlatitudes, which is then
followed by a decrease in E-P flux convergence near the winter pole which extends the enhanced
westerlies to subpolar latitudes.
We use the transformed Eulerian mean framework to reveal the processes that lead to the
formation of this sub-polar zonal wind anomaly and its downward propagation from the top of the
stratosphere to the tropopause.

How to cite: Givon, Y. and Garfinkel, C.: Solar Influences on Stratospheric Circulation Patterns, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5287, https://doi.org/10.5194/egusphere-egu2020-5287, 2020.

How to cite: Hamouda, M., Pasquero, C., and Tziperman, E.: A breakdown of the link between the Arctic and North Atlantic Oscillations in warm climate projections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5528, https://doi.org/10.5194/egusphere-egu2020-5528, 2020.

Recent work has highlighted that not all periods with anomalous heat flux at 100hPa were preceded by anomalous heat flux in the troposphere (Birner and Alberts 2017; White et al 2019; Camara et al 2019), and the goal of this work is to understand the factors that govern the efficiency of upward wave propagation near the tropopause. The index of refraction of Matsuno (1970) has been used to offer guidance on the direction of wave propagation within the stratosphere. Specifically, waves are preferentially refracted towards regions with a more positive index of refraction and ducted away from regions in which the index of refraction is more negative. However, the index of refraction was derived under the assumption that buoyancy frequency is constant at all height levels, which is clearly not true near the tropopause. This assumption allowed Matsuno to ignore certain height dependent buoyancy frequency terms, and here we explore the impact of these terms near the tropopause.

Using the dataset of the European Center for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5) we defined 'transmitting' composites consisting of more efficient upward propagation events between 300hPa and 100hPa. Similarly, periods of less efficient upward propagation events between 300hPa and 100hPa are composited as 'decaying' events. We computed the index of refraction profile using a median, percentage of negative days and the trimmed mean (Wilks 2011), and also consider the terms neglected by Matsuno. We find that  the index of refraction can account for the difference between the decaying and transmitting composite.

How to cite: Weinberger, I., Garfinkel, C., and Birner, T.: The Efficiency of Upward Wave Propagation Near the Tropopause, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6676, https://doi.org/10.5194/egusphere-egu2020-6676, 2020.

Extreme events in the stratosphere, so-called sudden stratospheric warming (SSW) events, can have a significant impact on surface weather. However, only about two thirds of SSW events have a surface impact, and the expected response is not always observed at the same time lag after the stratospheric event. In order to achieve skillful long-range predictions it will be necessary to understand the reasons for the presence or absence of a response, and to successfully predict the timing of the surface impact.
This contribution investigates several potential long-range predictors for the tropospheric response: the persistence of the lower stratospheric temperature signal, anomalous eddy driving in the east Pacific, as well as the North Atlantic weather regime present at the onset of the stratospheric event. All of these are found to help determine the type and timing of the tropospheric surface impact, and a strong potential for long-range predictability of several weeks based on these predictors is found over Europe.

How to cite: Domeisen, D., Afargan-Gerstman, H., Baehr, J., Dobrynin, M., Grams, C., Hitchcock, P., and Papritz, L.: Prospects for predicting the type and timing of the surface response after stratospheric events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7862, https://doi.org/10.5194/egusphere-egu2020-7862, 2020.

Both theory and climate model results suggest that the Brewer-Dobson circulation should strengthen in the stratosphere with increasing greenhouse gas concentrations. Directly measuring the circulation strength is not possible, so verification of this sensitivity has been limited to indirect inferences from observed tracer fields of long-lived species. These methods, however, are complex and accumulation of the data required for them is difficult. When limiting discussion to the tropical lower stratosphere, ozone concentrations have shown to be consistent with an accelerating circulation. These measurements are particularly useful because of the long timeseries available from multiple datasets, but they have only been used for indirect investigations of the circulation strength, up until now.

In this work, we invert the ozone balance equation to solve for upwelling. By limiting the investigation to 70 hPa in the southern tropics and estimating upwelling anomalies from the long-term mean (and not the absolute value of upwelling) most chemical terms and both horizontal and vertical mixing can be neglected, and calculation of the remaining terms is straight-forward. To verify the validity of the method, a calculation of upwelling is performed using climate model data, from which a comparison of actual upwelling and upwelling from the inverse method can be made. The seasonal cycle of upwelling anomalies is compared to upwelling anomalies from reanalyses and model results, and trends and variability are discussed.

How to cite: Charlesworth, E., Ploeger, F., Diallo, M., Birner, T., and Joeckel, P.: A Method for Estimating the Evolution of Brewer-Dobson Circulation Upwelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8905, https://doi.org/10.5194/egusphere-egu2020-8905, 2020.

The northern polar vortex experiences considerable inter-annual variability, which is also reflected to tropospheric weather. Recent research has established a link between polar vortex variations and energetic electron precipitation (EEP) from the near-Earth space into the polar atmosphere, which is mediated by EEP-induced chemical changes causing ozone loss in the mesosphere and stratosphere. However, the most dramatic changes in the polar vortex are due to sudden stratospheric warmings (SSW), a momentary breakdown of the polar vortex associated to enhanced planetary wave convergence and meridional circulation. Here we consider the influence of SSWs on the atmospheric response to EEP in 1957-2017 using combined ERA-40 and ERA-Interim re-analysis data and geomagnetic activity as a proxy of EEP. We find that the EEP-related enhancement of the polar vortex and other associated dynamical responses are seen only during winters when a SSW occurs, and that the EEP-related changes take place slightly before the SSW onset. We show that the atmospheric conditions preceding SSWs favor enhanced wave-mean-flow interaction, which can dynamically amplify the initial polar vortex enhancement caused by ozone loss. These results highlight the importance of considering SSWs and sufficient level of planetary wave activity as a necessary condition for observing the effects of EEP on the polar vortex dynamics.

How to cite: Asikainen, T., Salminen, A., Maliniemi, V., and Mursula, K.: Influence of sudden stratospheric warmings on the polar vortex enhancement related to energetic electron precipitation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9102, https://doi.org/10.5194/egusphere-egu2020-9102, 2020.

Arctic stratospheric ozone has been shown to exert a statistically significant influence on Northern Hemispheric surface climate. This suggests that Arctic ozone is not only passively responding to dynamical variability in the stratosphere, but actively feeds back into the circulation through chemical and radiative processes. However, the extent and causality of the chemistry-dynamics coupling is still unknown. Since many state-of-the-art climate models lack a sufficient representation of ozone-dynamic feedbacks, a quantification of this coupling can be used to improve intra-seasonal weather and long-term climate forecasts.

We assess the importance of the ozone-dynamics coupling by performing simulations with and without interactive chemistry in two Chemistry Climate Models. The chemistry-dynamics coupling was examined in two different sets of time-slice simulations: one using pre-industrial, and one using year-2000 boundary conditions. We focus on the impact of sudden stratospheric warmings (SSW) and strong vortex events on stratosphere-troposphere coupling, since these go along with strong ozone anomalies and therefore an intensified ozone feedback.  We compare the runs with and without interactive chemistry.

For pre-industrial conditions, simulations without interactive ozone show a more intense and longer lasting surface signature of SSWs compared to simulations with interactive chemistry. Conversely, for year-2000 conditions, the opposite effect is found: interactive chemistry amplifies the surface signature of SSWs. Following these results, atmospheric CFC concentrations, which differ greatly in the pre-industrial and year-2000 runs, determine the sign of the ozone-circulation feedback, and thus have a strong impact on chemistry-climate coupling. Implications for modeling of stratosphere-troposphere coupling and future projections are discussed.

How to cite: Friedel, M., Chiodo, G., Muthers, S., Anet, J., Stenke, A., and Peter, T.: Coupling of Arctic ozone and stratospheric dynamics and its influence on surface climate: the role of CFC concentrations., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9118, https://doi.org/10.5194/egusphere-egu2020-9118, 2020.

During the winter, a polar vortex, a strong westerly thermal wind, forms in the polar stratosphere. In the northern hemisphere the polar vortex varies significantly during and between winters. The Sun and the solar wind affect the polar vortex via two separate factors: electromagnetic radiation and energetic particle precipitation. Earlier studies have shown that increased energetic electron precipitation (EEP) decreases ozone in the polar upper atmosphere and strengthens the northern polar vortex, while solar irradiance affects temperature and ozone in the stratosphere directly at low latitudes and indirectly at high latitudes. In addition to the solar-related drivers, the northern polar vortex is also affected by different atmospheric internal factors such as Quasi-Biennial Oscillation (QBO), El-Nino Southern Oscillation (ENSO) and volcanic aerosols. Several studies have shown that the QBO modulates the effects that the solar-related drivers and ENSO cause to the polar vortex. In this study we examine and compare effects of the two solar-related drivers (solar radiation and EEP) and three atmospheric internal factors (QBO, ENSO and volcanic aerosols) on the polar vortex. We use multiple linear regression analysis to estimate the effects of each factor on temperature and zonal wind. We concentrate on the northern wintertime stratosphere and troposphere and examine the period of 1957-2017 using a combination of ERA-40 and ERA-Interim re-analysis data. We also study these effects separately in the two QBO phases. While we confirm that increased EEP is associated with a strengthened polar vortex, in accordance with the earlier studies, we further show that EEP is the largest and most significant factor among those studied affecting  the northern polar vortex variability. We also find that the EEP effect on polar vortex is particularly strong in the easterly phase of QBO while in the westerly phase the EEP effect is weakened and does not stand out from other effects.

How to cite: Salminen, A., Asikainen, T., Maliniemi, V., and Mursula, K.: Solar-related and internal drivers of the northern polar vortex, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9475, https://doi.org/10.5194/egusphere-egu2020-9475, 2020.

The frequency of sudden stratospheric warming events (SSWs) is an essential characteristic of the coupled stratosphere-troposphere system. This study is motivated by the fact that many of the CMIP5 and CMIP6 climate models considerably over- or underestimate the observed SSW frequency. The goal is to understand the causes for the large intermodel spread in the number of SSWs and relate it to specific model configurations. To this end, various dynamical quantities associated with the simulation of SSWs are investigated. It is found that variations in the SSW frequency are closely related to the strength of the polar vortex and the stratospheric wave activity. While it is difficult to explain the variations in the strength of the polar vortex, the stratospheric wave activity is strongly influenced by the background state (i.e., zonal wind and index of refraction) of the lower stratosphere. An important regulator for the background is the extratropical tropopause temperature, which in turn is associated with the vertical model resolution. Low-resolution models tend to have large biases in simulating the location and temperature of the extratropical tropopause. The results indicate that the simulated SSW frequency is a useful metric for model performance, as the frequency is highly sensitive to a number of stratospheric and tropospheric factors.

How to cite: Wu, Z. and Reichler, T.: Variations in The Frequency of Sudden Stratospheric Warmings in CMIP5 and CMIP6, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10591, https://doi.org/10.5194/egusphere-egu2020-10591, 2020.

The implementation of the Turbulent Mountain Stress (TMS) parametrization in the Whole Atmospheric Community Climate Model (WACCM) is found to be critical to obtain a realistic Sudden Stratospheric Warming (SSW) frequency in the Northern Hemisphere. Comparing two 50-year simluations, one with TMS (TMS-on) and one without (TMS-off) reveals lower than observed SSW frequency in TMS-off from December to February, while in March both simulations show SSW frequencies comparable to reanalysis. Meridional eddy heat fluxes in the lower stratosphere are stronger in TMS-on than in TMS-off, except in March. These differences are accompanied by increased orographic gravity wave drag (OGWD) in TMS-off that comes mainly from the Himalayas and the Rocky Mountains in response to stronger surface winds. Two different mechanisms of how planetary and GWs interact are identified in the simulations. In the lower stratosphere, enhanced dissipation of GWs in TMS-off modifies the subtropical jet and thus the conditions for refraction of planetary waves. In early winter, wave geometry diagnostics shows waveguides formation from 55N to 75N in TMS-on, enhancing wave propagation to the polar vortex. On the contrary, vertical propagation in TMS-off is in inhibited above the lower stratosphere and confined to latitudes south of 50N. Compensation between resolved and parametrized GWs is also observed, leading to weaker Eliassen-Palm flux divergence in response to stronger OGWD in TMS-off. In late winter, conditions for propagation are similar in both simulations by late winter, which explains the reduced TMS-off bias in the frequency of March SSWs.

How to cite: Palmeiro, F. M., Garcia, R. R., Calvo, N., Barriopedro, D., and Jiménez-Esteve, B.: How turbulent mountain stress influences sudden stratospheric warming ocurrence in WACCM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10766, https://doi.org/10.5194/egusphere-egu2020-10766, 2020.

Major sudden stratospheric warmings (SSWs), vortex formation and final breakdown dates are key highlight points of the stratospheric polar vortex. These phenomena are relevant for stratosphere-troposphere coupling, which explains the interest in understanding their future changes. However, up to now, there is not a clear consensus on which projected changes to the polar vortex are robust, particularly in the Northern Hemisphere, possibly due to short data record or relatively moderate CO₂ forcing. The new simulations performed under the Coupled Model Intercomparison Project, Phase 6, together with the long daily data requirements of the DynVarMIP project in preindustrial and quadrupled CO₂ (4xCO₂ ) forcing simulations provide a new opportunity to revisit this topic by overcoming the limitations mentioned above.

In this study, we analyze this new model output to document the change, if any, in the frequency of SSWs under 4xCO₂ forcing. Our analysis reveals a large disagreement across the models as to the sign of this change, even though most models show a statistically significant change. The models, however, are in good agreement as to the impact of SSWs over the North Atlantic: there is no indication of a change under 4xCO₂ forcing. Over the Pacific, however, the change is more uncertain. Finally, the models show robust changes to the seasonal cycle in the stratosphere. Specifically, we find a longer duration of the stratospheric polar vortex, and thus a longer season of stratosphere-troposphere coupling.

How to cite: Ayarzagüena, B., Charlton-Pérez, A. J., Butler, A. H., Hitchcock, P., Simpson, I. R., Polvani, L. M., Butchart, N., Gerber, E. P., Gray, L., Hassler, B., Lin, P., Lott, F., Manzini, E., Mizuta, R., Orbe, C., Osprey, S., Saint-Martin, D., Sigmond, M., Taguchi, M., and Volodin, E. and the DynVarMIP-SSW: Uncertainty in the response of sudden stratospheric warmings and stratosphere- troposphere coupling to quadrupled CO2 concentrations in CMIP6 models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11839, https://doi.org/10.5194/egusphere-egu2020-11839, 2020.

The Antarctic polar vortex and the associated ozone change have been recognized as a key factor that influences both local and large-scale circulations in the Southern Hemisphere (SH) extratropics. Their downward impacts are also evident in the subseasonal-to-seasonal (S2S) and long-term climate predictions especially in austral spring and summer. However, most operational S2S models, including the Global Seasonal Forecasting System version 5 (GloSea5), use climatological ozone and ignore time-varying ozone associated with polar vortex variability. This study explores the possible impact of stratospheric ozone on SH S2S prediction skill by conducting the two sets of reforecast experiments with the GloSea5. The reforecasts are initialized on 1st September of every year for the period of 2004-2018 with either climatological or observed ozone from the Stratospheric Water and OzOne Satellite Homogenized (SWOOSH) data. It turns out that the reforecasts with observed ozone have an improved prediction skill at 5- and 6-week lead forecasts than those with climatological ozone. The surface prediction skills also increase over the southern Australia and New Zealand. These results suggest that more realistic stratospheric ozone forcing could improve the SH prediction skill on subseasonal-to-seasonal timescale.

How to cite: Oh, J., Son, S.-W., and Han, B.-R.: Impact of stratospheric ozone on the subseasonal prediction skill in the Southern Hemisphere spring , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13358, https://doi.org/10.5194/egusphere-egu2020-13358, 2020.

Planetary waves (PW) dominate the meridional Brewer-Dobson circulation in the stratosphere and therewith, the large-scale mass transport of ozone. As PW break, ozone poor air masses are irreversibly mixed into mid-latitudes. Due to the disproportionate warming of the North Pole, an increase in PW activity (PWA) is expected. This should also have consequences for ozone streamer events.

We derived the PWA of ERA 5 and Interim Reanalysis temperature from ground level up the mesosphere. We identify Ozone-streamer events with a statistical based approach on the basis of total column concentration measured by GOME-2. We deconvoluted the time series of the PWA and the ozone-streamer events with the empirical mode decomposition method (EMD). Moreover, we developed a simple spectral model of the meridional wind shear on the basis of PW. This model serves as a measure of the atmospheric instability in the stratosphere.

As we deconvolute the PWA with the EMD we find signatures of QBO, ENSO and solar cycles and quantify their contributions. As PW dominate the circulation in the stratosphere, it appears to be a coherent consequence that ozone streamers are modulated on the same time scales as the PWA.With the spectral model of the meridional wind shear we find regions in the atmosphere, where PW are most likely to break. As a result there is an increased meridional transport of air masses, in particular of ozone. This is why ozone streamers occur most frequently at the transition zones from ocean to continent; strongest from North Atlantic to Europe. Moreover, we find significant long-term trends of the PWA in the stratosphere. Due to the increase of the PWA in the stratosphere, ozone streamer events are likely to occur more often in the future.

How to cite: Lisa, K., Dominik, L., and Bittner, M.: Variability and changes of the stratospheric large scale circulation and possible consequences for ozone streamer events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13818, https://doi.org/10.5194/egusphere-egu2020-13818, 2020.

How to cite: Serva, F., Cagnazzo, C., Christiansen, B., and Yang, S.: The influence of the tropical troposphere on the QBO in model simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15897, https://doi.org/10.5194/egusphere-egu2020-15897, 2020.

The stratospheric polar vortex is highly variable in winter and thus, models often struggle to capture its variability and strength. Yet, the influence of the stratosphere on the tropospheric circulation becomes highly important in Northern Hemisphere winter and is one of the main potential sources for subseasonal to seasonal prediction skill in mid latitudes. Mid-latitude extreme weather patterns in winter are often preceded by sudden stratospheric warmings (SSWs), which are the strongest manifestation of the coupling between stratosphere and troposphere. Misrepresentation of the SSW-frequency and stratospheric biases in models can therefore also cause biases in the troposphere.

In this context this work comprises the analysis of four seasonal ensemble experiments with a high-resolution, nonhydrostatic global atmospheric general circulation model in numerical weather prediction mode (ICON-NWP). The main focus thereby lies on the variability and strength of the stratospheric polar vortex. We identified the gravity wave drag parametrisations as one important factor influencing stratospheric dynamics. As the control experiment with default gravity wave drag settings exhibits an overestimated amount of SSWs and a weak stratospheric polar vortex, three sensitivity experiments with adjusted drag parametrisations were generated. Hence, the parametrisations for the non-orographic gravity wave drag and the subgrid‐scale orographic (SSO) drag were chosen with the goal of strengthening the stratospheric polar vortex. Biases to ERA-Interim are reduced with both adjustments, especially in high latitudes. Whereas the positive effect of the reduced non-orographic gravity wave drag is strongest in the mid-stratosphere in winter, the adjusted SSO-scheme primarily affects the troposphere by reducing mean sea level pressure biases in all months. A fourth experiment using both adjustments exhibits improvements in the troposphere and stratosphere. Although the stratospheric polar vortex in winter is strengthened in all sensitivity experiments, it is still simulated too weak compared to ERA-Interim. Further mechanisms causing this weakness are also investigated in this study.

How to cite: Köhler, R., Handorf, D., Jaiser, R., Dethloff, K., Zängl, G., Majewski, D., and Rex, M.: Influence of gravity wave drag parametrisations on the stratospheric circulation of seasonal ICON-NWP experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17139, https://doi.org/10.5194/egusphere-egu2020-17139, 2020.

The Stratosphere-troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) seeks to improve confidence in general circulation and earth system model (GCM and ESM) simulations of the QBO, a prominent feature of middle atmosphere tropical variability first identified nearly sixty years ago. Although only five out of 47 models contributing to the Coupled Model Intercomparison Project Phase 5 (CMIP5) had spontaneous QBOs, simulated QBOs are anticipated to be more common among CMIP6 models as more atmospheric GCMs are able to reproduce the phenomenon, both by ensuring adequate vertical resolution in the stratosphere and by parametrizing accelerations due to subgrid nonorographic gravity waves (NOGWs). The complexity of CMIP6 models and their forcing scenarios, however, is an obstacle to using the CMIP6 multimodel ensemble for analysis of modelling uncertainties that are specific to the QBO and its impacts. The QBOi multimodel ensemble represents an alternative approach in which modelling uncertainties related to the QBO are assessed by performing coordinated experiments with atmospheric GCMs that have simplified external forcings and boundary conditions, designed to characterize QBO representation and its response to idealised future climate scenarios. 

Results are presented from an analysis of QBOs in thirteen atmospheric GCMs forced with both observed and annually repeating sea surface temperatures (SSTs). Mean QBO periods in most of these models are close to, though shorter than, the period of 28 months observed in ERA-Interim. Amplitudes are within ±20% of the observed QBO amplitude at 10hPa, but typically about half of that observed at lower altitudes (50 and 70hPa). For almost all models the oscillation's amplitude profile shows an overall upward shift compared to reanalysis and its meridional extent is too narrow. Asymmetry in the duration of eastward and westward phases is reasonably well captured though not all models replicate the observed slowing as the westward shear descends. Westward phases are generally too weak, and most models have an eastward time mean wind bias throughout the depth of the QBO. Intercycle period variability is realistic and in some models is enhanced in the experiment with observed SSTs compared to the experiment with repeated annual cycle SSTs. Mean periods are also sensitive to this difference between SSTs but only when parametrized NOGW sources are coupled to tropospheric parameters and not prescribed with a fixed value. But, overall, modelled QBOs are very similar whether or not the prescribed SSTs vary interannually. A portrait of the overall ensemble performance is provided by a normalised grading of QBO metrics. To simulate a QBO all but one model used parametrized NOGWs, which provided the majority of the total wave forcing at altitudes above 70hPa in most models. Thus the representation of NOGWs either explicitly or through parametrization is still a major uncertainty underlying QBO simulation in these present-day experiments.

How to cite: Bushell, A. and Lott, F. and the QBOi Exp1+2 Paper Contributors: Evaluation of the Quasi-Biennial Oscillation in global climate models for the SPARC QBO-initiative, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17338, https://doi.org/10.5194/egusphere-egu2020-17338, 2020.

We compare the response of the quasi-biennial oscillation (QBO) to a warming climate in eleven atmosphere general circulation models that performed time-slice simulations for present-day, doubled,  and  quadrupled CO₂ climates.  No consistency was found among the models for the QBO period response, with the period decreasing by eight months in some models and lengthening by up to thirteen months in others in the doubled CO₂  simulations.  In the quadruped CO₂ simulations  a reduction in QBO period of 14 months was found in some models, whereas in several others the tropical oscillation no longer resembled the present day QBO, although could still be identified in the deseasonalized zonal mean zonal wind timeseries.  In contrast, all the models projected a decrease in the  QBO amplitude in a warmer climate with the largest relative decrease  near 60 hPa. In simulations with doubled and quadrupled CO₂ the multi-model mean QBO amplitudes decreased by 36\% and 51\%, respectively. Across the  models the differences in the QBO period response were most strongly related to how the gravity wave momentum flux entering the stratosphere and tropical vertical residual velocity responded to the increases in CO₂ amounts. Likewise it was found that the robust decrease in QBO amplitudes was correlated across the models to changes in vertical residual velocity, parameterized gravity wave momentum fluxes, and to some degree the resolved upward wave flux.  We argue that uncertainty in the representation of the parameterized gravity waves is the most likely cause of the spread among the eleven models in the QBO's response to climate change.

How to cite: Richter, J. and Lott, F. and the QBOi contributors: Response of the quasi-biennial oscillation to a warming climate in global climate models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18795, https://doi.org/10.5194/egusphere-egu2020-18795, 2020.

Tropopause folds are known as regions of intense trace gas exchange between the troposphere and the stratosphere. They occur in upper-level fronts and it is known since the 1970s that turbulence plays a major role in their formation. However, only a limited number of turbulence measurements under these conditions exist. In this study, we present a turbulence sounding in an upper-level front measured with the balloon-borne instrument LITOS (Leibniz-Institute Turbulence Observations in the Stratosphere). This instrument infers turbulent kinetic energy dissipation rates from velocity fluctuations at the Taylor microscale. By using a radiosonde on board of the same balloon, we can observe wind fluctuations across multiple spatial scales.

In the classical picture of a tropopause fold from the 1970s, we expect turbulence to occur in both shear zones above and below the tropopause jet. For the time of our measurement on 06 August 2016, a similar turbulence distribution is expected due to low Richardson numbers in the respective areas shown by the ECMWF-IFS. Our in-situ turbulence measurement with LITOS, however, shows a different picture: we find turbulence to occur in the upper shear zone above the jet but not in the lower one located in the stratospheric intrusion. In our contribution, we will examine potential reasons for this difference between theoretical expectations and the observation. Furthermore, we will discuss possible implications of the lack of turbulence in the stratospheric intrusion on the exchange of trace gases across the tropopause.

How to cite: Faber, J., Gerding, M., and Lübken, F.-J.: Measurement Study of Turbulence in a Tropopause Fold, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18870, https://doi.org/10.5194/egusphere-egu2020-18870, 2020.