The chemical composition of the middle atmosphere is not only relevant for understanding radiative forcing or protection of the biosphere from harmful UV radiation, but it also has an influence on tropospheric circulation and dynamics that act as a feedback on climate. Increasing greenhouse gases are expected to modify the large-scale circulation of the stratosphere, termed Brewer-Dobson circulation (BDC), and the chemical compositions of radiatively active gases, notably ozone and water vapour, in the upper troposphere and lower stratosphere (UTLS) region. Such changes in the BDC and UTLS composition are expected to change levels of surface UV radiation, modify the radiative forcing of climate, and feedback on the dynamics both within the stratosphere and at the surface. This session is particularly interested in evidence of the direct influence of climate change upon stratospheric dynamics and chemistry, as well as indirect feedbacks from these changes back upon surface climate. We welcome abstracts focused on stratospheric composition changes on time-scales encompassing inter-annual to centennial timescales,on local to global spatial scales, future projections from chemistry climate models, and discussing changes induced by both natural and anthropogenic factors, observations, as well as theoretical studies.

Co-organized by CL3
Convener: Gabriel ChiodoECSECS | Co-conveners: William Ball, Mohamadou Diallo
| Attendance Thu, 07 May, 14:00–15:45 (CEST)

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Chat time: Thursday, 7 May 2020, 14:00–15:45

D3224 |
| solicited
Michaela I. Hegglin

Water vapour is the most important natural greenhouse gas in the atmosphere and provides a positive feedback to the climate forcing from carbon dioxide. Water vapour is also the source of hydroxyl (OH) which controls the lifetime of shorter-lived pollutants and long-lived greenhouse gases. Despite the importance of water vapour to chemistry and the radiative balance of the atmosphere, its observed long-term changes in the stratosphere are not well understood, and may even conflict with the theoretical understanding of its drivers. 

I here present a new climate data record of stratospheric water vapour developed within the ESA Water Vapour Climate Change Initiative and discuss recent changes in stratospheric water vapour concentrations in the light of earlier observational studies, modelling results from the SPARC Chemistry-Climate Model Initiative, and our theoretical understanding of its drivers. In addition, the radiative forcing of surface climate and inferred changes in the Brewer-Dobson Circulation will be highlighted. 

How to cite: Hegglin, M. I.: Long-term changes in stratospheric water vapour and its implications for climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8874, https://doi.org/10.5194/egusphere-egu2020-8874, 2020.

D3225 |
Yi Huang and Yuwei Wang

Global warming is amplified by radiative feedbacks. Compared to the feedback in the troposphere, the feedback in the stratosphere is less understood. The stratospheric water vapor (SWV), one of the primary feedbacks in the stratosphere, is argued to be an important contributor to global warming. This, however, is at odds with the finding that the overall stratospheric feedback does not amount to a significant value in global climate models (GCMs). The key to reconciling these seemingly contradictory arguments is to understand the stratospheric temperature (ST) change since the impact of SWV on the top-of-atmosphere (TOA) radiation budget results more from its cooling of the stratosphere than its direct radiative impact on the TOA radiation. Here, we develop a method to decompose the ST change and to quantify the effects of different climate responses associated with SWV on the TOA radiation budget. We find that although the SWV feedback by itself would lead to strong stratospheric cooling, this cooling is strongly offset by the radiative coupling between the stratosphere and troposphere. Such compensation results in an insignificant overall stratospheric feedback. SWV-locking experiments verify that the SWV feedback does not significantly modify the overall climate sensitivity in the GCM global warming simulations.

How to cite: Huang, Y. and Wang, Y.: On the magnitude of the stratospheric radiative feedback in global warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2420, https://doi.org/10.5194/egusphere-egu2020-2420, 2020.

D3226 |
| solicited
Peer Nowack, Nathan Luke Abraham, and Peter Braesicke

There is a plethora of ways in which the representation of upper tropospheric and stratospheric ozone (‘ozone feedbacks’) can affect the outcome of climate change simulations. Prominent examples include modulations of the tropospheric and stratospheric circulation, climate sensitivity, cloud formation, and stratospheric water vapour (e.g. [1-8]). Here I first revisit some recent work providing evidence for such effects. I then provide an update on a recently developed machine learning parameterization for ozone using the UK Earth System Model (UKESM1, [9]). Such a parameterization could adequately represent ozone feedbacks without adding the high computational expense of a fully interactive atmospheric chemistry scheme. The parameterization could also provide several notable scientific advantages, for example concerning the treatment of important chemistry-climate model biases. Finally, I put my results into the context of several other methods suggested as potential means for addressing ozone-related effects in idealized climate sensitivity simulations, also considering the still substantial uncertainties related to modelling ozone [10,11] and associated climate feedbacks [5,12].


[1] Son et al. (2008), The impact of stratospheric ozone recovery on the Southern Hemisphere westerly jet. Science 320, 1486, doi:10.1126/science.1155939.

[2] Dietmüller et al. (2014), Interactive ozone induces a negative feedback in CO2-driven climate change simulations, Journal of Geophysical Research: Atmospheres 119, 1796-1805, doi:10.1002/2013JD020575.

[3] Chiodo & Polvani (2016), Reduction of climate sensitivity to solar forcing due to stratospheric ozone feedback, Journal of Climate 29, 4651-4663, doi:10.1175/JCLI-D-15-0721.1.

[4] Chiodo & Polvani (2017), Reduced Southern Hemispheric circulation response to quadrupled CO2 due to stratospheric ozone feedback, Geophysical Research Letters 43, 465-474, doi:10.1002/2016GL071011.

[5] Nowack et al. (2015), A large ozone-circulation feedback and its implications for global warming assessments. Nature Climate Change 5, 41-45, doi:10.1038/nclimate2451.

[6] Nowack et al. (2017), On the role of ozone feedback in the ENSO amplitude response under global warming, Geophysical Research Letters 44, doi:10.1002/2016GL072418.

[7] Nowack et al. (2018), The impact of stratospheric ozone feedbacks on climate sensitivity estimates. Journal of Geophysical Research: Atmospheres 123, 4630-4641, doi:10.1002/2017JD027943.

[8] Rieder et al. (2019), Is interactive ozone chemistry important to represent polar cap stratospheric temperature variability in Earth-System Models?, Environmental Research Letters 14, 044026, doi: 10.1088/1748-9326/ab07ff.

[9] Nowack et al. (2018), Using machine learning to build temperature-based ozone parameterizations for climate sensitivity simulations, Environmental Research Letters 13, 104016, doi:10.1088/1748-9326/aae2be.

[10] Chiodo & Polvani (2019), The response of the ozone layer to quadrupled CO2 concentrations: implications for climate, Journal of Climate 31, 3893-3907, doi:10.1175/JCLI-D-17-0492.1.

[11] Keeble et al. (2020), Evaluating stratospheric ozone and water vapour changes in CMIP6 models from 1850-2100, Atmospheric Chemistry and Physics Discussions.

[12] Dacie et al. (2019), A 1D RCE study of factors affecting the tropical tropopause layer and surface climate. Journal of Climate 32, 6769-6782, doi:10.1175/JCLI-D-18-0778.1.

How to cite: Nowack, P., Abraham, N. L., and Braesicke, P.: Towards fast machine learning parameterizations of stratospheric ozone feedbacks in climate change simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2436, https://doi.org/10.5194/egusphere-egu2020-2436, 2020.

D3227 |
Karen Smith, Sarah Maleska, and John Virgin

Stratospheric ozone depletion in the Antarctic is well known to cause changes in Southern Hemisphere tropospheric climate; however, due to its smaller magnitude in the Arctic, the effects of stratospheric ozone depletion on Northern Hemisphere tropospheric climate are not as obvious or well understood. Recent research using both global climate models and observational data has determined that the impact of ozone depletion on ozone extremes can affect interannual variability in tropospheric circulation in the Northern Hemisphere in spring. To further this work, we use a coupled chemistry-climate model to examine the difference in high cloud between years with anomalously low and high Arctic stratospheric ozone concentrations. We find that low ozone extremes during the late twentieth century, when ODS emissions are higher, are related to a decrease in upper tropospheric stability and an increase in high cloud fraction, which may have contributed to Arctic surface warming via a positive longwave cloud radiative effect in the past few decades compared to other regions. A better understanding of how Arctic climate is affected by ODS emissions, ozone depletion and ozone extremes will lead to improved predictions of Arctic climate and its associated feedbacks with atmospheric fields as ozone levels recover.

How to cite: Smith, K., Maleska, S., and Virgin, J.: Impacts of stratospheric ozone extremes on Arctic high cloud, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9282, https://doi.org/10.5194/egusphere-egu2020-9282, 2020.

D3228 |
Sabine Haase, Jaika Fricke, and Katja Matthes

Southern hemisphere lower stratospheric ozone depletion has been shown to lead to a poleward shift of the tropospheric jet stream during austral summer, influencing surface atmosphere and ocean conditions, such as surface temperatures and sea ice extend. The characteristics of stratospheric and tropospheric responses to ozone depletion, however, differ among climate models largely depending on the representation of ozone in the model.

The most accurate way to represent ozone in a model is to calculate it interactively. However, due to computational costs, in particular for long-term coupled ocean-atmosphere model integrations, the more common way is to prescribe ozone from observations or calculated model fields.

Here, we investigate the difference between an interactive chemistry and a specified chemistry version of the same atmospheric model in a fully-coupled setup using a large 9-member model ensemble. In contrast to most earlier studies, we use daily-resolved ozone fields in the specified chemistry simulations to achieve a better comparability between the ozone forcing with and without interactive chemistry. We find that although the short-wave heating rate trend in response to ozone depletion is the same in the different chemistry settings, the interactive chemistry ensemble shows a stronger trend in polar cap stratospheric temperatures and circumpolar stratospheric and tropospheric zonal mean zonal winds as compared to the specified chemistry ensemble. We attribute part of these differences to the missing representation of feedbacks between chemistry and dynamics in the specified chemistry ensemble and part of it to the lack of zonal asymmetries in the prescribed ozone fields.

This study emphasizes the value of interactive chemistry for the representation of the southern hemisphere tropospheric jet response to ozone depletion.

How to cite: Haase, S., Fricke, J., and Matthes, K.: Sensitivity of the southern hemisphere tropospheric jet response to ozone depletion: specified versus interactive chemistry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2831, https://doi.org/10.5194/egusphere-egu2020-2831, 2020.

D3229 |
Kevin DallaSanta, Clara Orbe, and Lorenzo Polvani

Long-term projections of the Quasi-Biennial Oscillation (QBO) remain highly uncertain. This is partly due to the paucity of models which are able to properly simulate that phenomenon. Only 5 of the 47 CMIP5 models are capable of spontaneously generating a realistic QBO (Butchart et al., 2018), and even those models exhibit large biases in key QBO characteristics (e.g. amplitude, period, vertical extent) when compared with observations. Furthermore, only 1 of these 5 employed interactive atmospheric chemistry, which is known to modulate QBO dynamics.

We here investigate the QBO response to increased greenhouse gases using the NASA Goddard Institute for Space Studies Middle Atmosphere Model E2.2. Compared to lower vertical resolution versions of Model E, version 2.2 has a higher model top (0.002 hPa), and additional interactive non-orographic gravity wave drag sources from convection and shear, which produce a sufficiently realistic QBO, thus rendering it suitable for use in climate change studies. Steady-state responses to doubled and quadrupled CO2 from a pre-industrial control are analyzed, as well as the transient response to a 1% per year CO2 increase. In addition, we systematically explore the impact of interactive chemistry in modulating the QBO response to increased CO2 by contrasting interactive, prescribed, and linearized ozone chemistry configurations of the model. Overall, in response to increase CO2 concentrations the QBO is seen to increase in frequency and weaken in amplitude, consistent with previous results, but the memory of the tropical stratosphere may complicate assessments of trends in chemistry and surface impacts. We also discuss implications for the trade-off between ensemble size and the complexity of the chemistry scheme in the model.

How to cite: DallaSanta, K., Orbe, C., and Polvani, L.: The Response of the QBO to Increases in CO2 Using Three Atmospheric Chemistry Configurations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-283, https://doi.org/10.5194/egusphere-egu2020-283, 2020.

D3230 |
Laura Stecher, Franziska Winterstein, Martin Dameris, Patrick Jöckel, and Michael Ponater

Methane (CH4) is the second most important anthropogenic greenhouse gas and its atmospheric abundance is rising rapidly at the moment (e.g. Nisbet et al., 2019). 

We assess the effects of doubled and fivefold present-day (2010) CH4 lower boundary mixing ratios on the basis of sensitivity simulations with the
chemistry-climate model EMAC. As a follow-up on Winterstein et al. (2019) we investigate slow adjustments by applying a mixed layer ocean (MLO) model
instead of prescribed oceanic conditions. In the simulations with prescribed oceanic conditions, tropospheric temperature changes are largely suppressed,
while with MLO tropospheric temperatures adjust to the forcing. In the present study we compare the changes in the MLO sensitivity simulations to the
sensitivity simulations with prescribed oceanic conditions (Winterstein et al., 2019). Comparing the responses of these two sets of sensitivity simulations separates rapid adjustments and the effects of slow climate feedbacks associated with tropospheric warming.

The chemical interactions in the stratosphere in the MLO set-up (slow adjustments) compare in general well with the results of Winterstein et al. (2019) (rapid adjustments). The increase of stratospheric water vapor is albeit 5 % (15 %) points weaker in the MLO doubling (fivefolding) experiment compared to the doubling (fivefolding) experiment with prescribed oceanic conditions in line with a weaker increase of stratospheric OH. Stronger O3 decrease and CH4
increase in the lowermost tropical stratosphere in the MLO sensitivity simulations compared to the sensitivity simulations with prescribed oceanic conditions indicate a more distinct strengthening of tropical up-welling due to tropospheric warming in the MLO set-up. The MLO simulations also show evidence of a strengthening of the Brewer-Dobson Circulation. When separating the quasi-instantaneous chemically induced O3 response from the O3 response pattern in the MLO set-up, the O3 response to slow climate feedbacks remains. This pattern is consistent with the O3 response to slow climate feedbacks induced by increases of CO2.

This first of its kind study shows the climatic impact of strongly enhanced CH4 mixing ratios and how the slow climate response of tropospheric warming potentially damp instantaneous chemical feedbacks.

How to cite: Stecher, L., Winterstein, F., Dameris, M., Jöckel, P., and Ponater, M.: Investigation of strongly enhanced methane Part II: Slow climate feedbacks., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9852, https://doi.org/10.5194/egusphere-egu2020-9852, 2020.

D3231 |
| solicited
Brian Zambri, Susan Solomon, David Thompson, and Qiang Fu

Ozone depletion in the Southern Hemisphere (SH) stratosphere in the late 20th century cooled the air there, strengthening the SH stratospheric westerly winds near 60ºS and altering SH surface climate. Since ~1999, trends in Antarctic ozone have begun to recover, exhibiting a flattening followed by a sign reversal in response to decreases in stratospheric chlorine concentration due to the Montreal Protocol, an international treaty banning the production and consumption of ozone-depleting substances. Here we show that the post–1999 increase in ozone has resulted in thermal and circulation changes of opposite sign to those that resulted from stratospheric ozone losses, including a warming of the SH polar lower stratosphere and a weakening of the SH stratospheric polar vortex.  Further, these altered trends extend to the upper troposphere, albeit of smaller magnitudes.  Observed post–1999 trends of temperature and circulation in the stratosphere are about 20–25% the magnitude of those of the ozone depletion era, and are broadly consistent with expectations based on modeled depletion-era trends and variability of both ozone and reactive chlorine, thereby indicating the emergence of healing of dynamical impacts of the Antarctic ozone hole.

How to cite: Zambri, B., Solomon, S., Thompson, D., and Fu, Q.: Emergence of Southern Hemisphere circulation changes in response to ozone recovery, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22211, https://doi.org/10.5194/egusphere-egu2020-22211, 2020.

D3232 |
Bo-Reum Han, Jung Choi, and Seok-Woo Son

 The impacts of stratospheric ozone and greenhouse gas changes on the Southern Hemisphere (SH) climate are re-visited by examining the single forcing experiments from the Chemistry-Climate Model Initiative (CCMI) project. In particular, the fixed ozone-depleting substance (ODS) runs and the fixed greenhouse gas (GHG) concentration runs are directly compared with the reference runs for both the past and future. Consistent with the previous studies, the SH-summer general circulation changes, such as changes in the jet location, Hadley cell edge, and Southern Annular Mode (SAM), show the opposite trends from the past to the future in response to the Antarctic ozone depletion and recovery. The GHG-induced circulation changes largely enhance the ozone-induced circulation changes in the past, but partly cancel them in the future. The ozone recovery-related tropospheric circulation return dates are also estimated in this study. We will further discuss the inter-model diversity among the CCMI models.

How to cite: Han, B.-R., Choi, J., and Son, S.-W.: Impacts of stratospheric ozone and greenhouse gas changes on the Southern Hemisphere circulation in the CCMI models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6570, https://doi.org/10.5194/egusphere-egu2020-6570, 2020.

D3233 |
Petr Pisoft and Petr Šácha

There is a well-established observational evidence that the tropopause is shifting upward. More generally, warming of the troposphere is directly connected with a positive trend of geopotential of pressure levels in the troposphere, which reaches its maximum around the tropopause. In the stratosphere, the geopotential height trends are affected by the stratospheric cooling resulting in a gradual reduction of the upward shift and even its reversal in the upper stratosphere. That leads to a decreasing trend of the stratospheric thickness - a so-called stratospheric shrinkage. In GCMs, shrinkage is one of the strongest and most robust fingerprints of the changing climate. In this study, we investigate the question whether the shrinkage presents additional dynamical feedback influencing other detected trends in the middle atmosphere (besides the influence of vertical shift). Analyzing set of CCMI models, we compute inter-model correlations of shrinkage with trends of various variables to separate the possible shrinkage effect, which is otherwise a non-local function of the temperature.

How to cite: Pisoft, P. and Šácha, P.: Influence of the stratospheric shrinkage on the detected CCMI simulation trends , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13468, https://doi.org/10.5194/egusphere-egu2020-13468, 2020.

D3234 |
Sergei Smyshlyaev, Polina Blakitnaya, Maxim Motsakov, and Vener Galin

The INM RAS – RSHU chemistry-climate model of the lower and middle atmosphere is used to compare the role of natural and anthropogenic factors in the observed and expected variability of stratospheric ozone. Numerical experiments have been carried out on several scenarios of separate and combined effects of solar activity, stratospheric aerosol, sea surface temperature, greenhouse gases, and ozone-depleting substances emissions on ozone for the period from 1979 to 2050. Simulations for the past and present periods are compared to the results of ground-based and satellite observations, as well as MERRA and ERA-Interim re-analysis. Estimation of future ozone changes are based on different scenarios of changes in solar activity and emissions of ozone-depleting substances and greenhouse gases, as well as the possibility of large volcanic aerosol emissions at different periods of time.

How to cite: Smyshlyaev, S., Blakitnaya, P., Motsakov, M., and Galin, V.: Numerical modeling of the natural and anthropogenic factors influence on the past and future changes in polar, mid-latitude and tropical ozone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20440, https://doi.org/10.5194/egusphere-egu2020-20440, 2020.

D3235 |
| solicited
Franziska Winterstein, Patrick Jöckel, Martin Dameris, Michael Ponater, Fabian Tanalski, and Laura Stecher

Methane (CH4) is the second most important greenhouse gas, which atmospheric concentration is influenced by human activities and currently on a sharp rise. We present a study with numerical simulations using a Chemistry-Climate-Model (CCM), which are performed to assess possible consequences of strongly enhanced CH4 concentrations in the Earth's atmosphere for the climate.

Our analysis includes experiments with 2xCH4 and 5xCH4 present day (2010) lower boundary mixing ratios using the CCM EMAC. The simulations are conducted with prescribed oceanic conditions, mimicking present day tropospheric temperatures as its changes are largely suppressed. By doing so we are able to investigate the quasi-instantaneous chemical impact on the atmosphere. We find that the massive increase in CH4 strongly influences the tropospheric chemistry by reducing the OH abundance and thereby extending the tropospheric CH4 lifetime as well as the residence time of other chemical pollutants. The region above the tropopause is impacted by a substantial rise in stratospheric water vapor (SWV). The stratospheric ozone (O3) column increases overall, but SWV induced stratospheric cooling also leads to enhanced ozone depletion in the Antarctic lower stratosphere. Regional  patterns of ozone change are affected by modification of stratospheric dynamics, i.e. increased tropical up-welling and stronger meridional transport  towards the polar regions. We calculate the net radiative impact (RI) of the 2xCH4 experiment to be 0.69 W m-2 and for the 5xCH4 experiment to be 1.79 W m-2. A substantial part of the RI is contributed by chemically induced O3 and SWV changes, in line with previous radiative forcing estimates and is for the first time splitted and spatially asigned to its chemical contributors.

This numerical study using a CCM with prescibed oceanic conditions shows the rapid responses to significantly enhanced CH4 mixing ratios, which is the first step towards investigating the impact of possible strong future CH4 emissions on atmospheric chemistry and its feedback on climate.

How to cite: Winterstein, F., Jöckel, P., Dameris, M., Ponater, M., Tanalski, F., and Stecher, L.: Investigation of strongly enhanced methane Part I: Chemical feedbacks and rapid adjustments., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9786, https://doi.org/10.5194/egusphere-egu2020-9786, 2020.

D3236 |
Ewa Bednarz, Ryan Hossaini, Luke Abraham, and Martyn Chipperfield

The emissions of most long-lived halogenated ozone-depleting substances (ODSs) are now decreasing, owing to controls on their production introduced by Montreal Protocol and its amendments. However, short-lived halogenated compounds can also have substantial impact on atmospheric chemistry, including stratospheric ozone, particularly if emitted near climatological uplift regions. It has recently become evident that emissions of some chlorinated very short-lived species (VSLSs), such as chloroform (CHCl3) and dichloromethane (CH2Cl2), could be larger than previously believed and increasing, particularly in Asia. While these may exert a significant influence on atmospheric chemistry and climate, their impacts remain poorly characterised.


We address this issue using the UM-UKCA chemistry-climate model. We use a newly developed Double-Extended Stratospheric-Tropospheric (DEST) chemistry scheme, which includes emissions of all major chlorinated and brominated VSLSs alongside an extended treatment of long-lived ODSs. Employing novel estimates of Cl-VSLS emissions we show model results regarding the atmospheric impacts of chlorinated VSLSs over the recent past (2000-present), with a focus on stratospheric ozone and HCl trends. Finally, we introduce our plans regarding examining the impacts of chlorinated VSLSs under a range of potential future emissions scenarios; the results of which will be directly relevant for the next WMO/UNEP assessment.

How to cite: Bednarz, E., Hossaini, R., Abraham, L., and Chipperfield, M.: Improved characterisation of the impact of chlorinated VSLSs on atmospheric chemistry and climate: past, present and future, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10611, https://doi.org/10.5194/egusphere-egu2020-10611, 2020.

D3237 |
Sheena Loeffel, Roland Eichinger, Hella Garny, Thomas Reddmann, Stefan Versick, Frauke Fritsch, Gabriele Stiller, and Florian Haenel

Mean age of air (AoA) is a common diagnostic for the stratospheric overturning circulation in both climate models and observations. Observations of AoA mostly base on measurements of SF6, which is an almost ideal AoA tracer because its emssions across the recent decades increased nearly linearly and it is fairly stable in the troposphere and stratosphere. Over the last ten years, however, researchers were puzzled as to why AoA climatologies and trends of model simulations and observational data do not coincide. AoA in climate models is generally much lower than in observations and models show a clear decrease of AoA over time while measurements show a non-significant increase.

What is commonly not considered in the models is that SF6 has chemical sinks in the mesosphere, and these lead to apparently older air in the stratosphere. In our experiment, we explicitely calculate SF6 sinks based on physical processes in simulations with the global chemistry-climate model EMAC (ECHAM MESSy Atmospheric Chemistry). We show that considering the SF6 removal reactions strongly increases stratospheric AoA and leads to much better agreement between the climatologies of EMAC and MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) satellite observations. Moreover, the stratospheric AoA trend over the recent decades reverses sign when we derive it from SF6 with sinks. This means that the trend can such be reconciled with the trend that has been derived from long-term balloon-borne measurements. Our specifically designed sensitivity studies moreover reveal that this positive trend results neither from circulation changes, nor from variations of the reactive species involved in mesospheric SF6 depletion. Instead, it is generated through the temporally growing influence of the SF6 sinks themselves, an effect that overcompensates the negative trend resulting from the accelerating stratospheric overturning circulation.

How to cite: Loeffel, S., Eichinger, R., Garny, H., Reddmann, T., Versick, S., Fritsch, F., Stiller, G., and Haenel, F.: Reconciling modelled and observed age of air through SF6 sinks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3375, https://doi.org/10.5194/egusphere-egu2020-3375, 2020.

D3238 |
Jonas Debosscher, Quentin Errera, Simon Chabrillat, Daniele Minganti, Yves Christophe, and Didier Fussen

Historically, many photochemical models suffered from an underestimation of the ozone abundance in the upper stratosphere – lower mesosphere, known as the “Ozone deficit problem” (Prather, 1981; Eluszkiewicz et al., 1993, Siskind at al., 2013). Despite improvements in models and increased accuracy of observations, it seems this problem is still present, as evidenced by comparing models participating in the Chemistry-Climate Model Initiative (CCMI) with observations.

The Belgian Assimilation System for Chemical ObsErvations (BASCOE), developed at BIRA-IASB, is used to study and monitor the chemical composition of the stratosphere. It consists of a 3D chemical transport model (CTM) in combination with two data-assimilation methods (4D-Var and EnKF). BASCOE shows an ozone deficit of ~20 % against MLS observations around 1hPa. Since BASCOE will provide operational analysis of ozone based on the assimilation of the future ALTIUS satellite data, and is part of the Integrated Forecasting System of the ECMWF (C-IFS-CB05-BASCOE, Huijnen et al., 2016), the CTM needs to better model the ozone observations in this region of the stratosphere.

We present the results of a sensitivity study using the BASCOE CTM to identify factors that have the largest influence on the ozone budget in the upper stratosphere and can provide clues to solve the ozone deficit. We investigated the effects of solar spectral irradiance, surface albedo, photo-dissociation computation, reaction rate uncertainties and temperature.

Ozone concentrations in the upper stratosphere turned out to be very sensitive to temperatures and to a lesser degree to the solar spectral irradiances used to drive the model. The sensitivity to temperature is compatible with predictions made using a photochemical equilibrium approximation based on pure oxygen chemistry. Given the relatively large temperature uncertainties in the upper stratosphere, we believe temperature biases could substantially contribute to the ozone deficit.

How to cite: Debosscher, J., Errera, Q., Chabrillat, S., Minganti, D., Christophe, Y., and Fussen, D.: Why we believe there is still a model ozone deficit in the upper stratosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17593, https://doi.org/10.5194/egusphere-egu2020-17593, 2020.

D3239 |
Thierry Portafaix, Kevin Lamy, Jean-Baptiste Forestier, Solofo Rakotoniaina, and Vincent Amélie

Radiation (UV) is one of the main components of solar radiation transmitted by the Earth's atmosphere. Exposure to UV radiation can have both positive and negative effects on the biosphere and humans in particular. Overexposure significantly increases the risk of skin cancer and eye problems.

Ozone, cloud cover and zenithal solar angle are the main parameters affecting UV radiation levels at the surface. Stratospheric ozone in particular strongly absorbs UV radiation. A dense cloud cover absorbs UV radiation, while a split cloud cover may tend to amplify it.

Although the stratospheric ozone layer is showing signs of recovery from reduced ozone-depleting substances. The impact of greenhouse gases on the climate is still in increase and global climate models anticipate an acceleration in Brewer-Dobson Circulation, which would lead to lower ozone levels in the tropics. Butler et al. (2016) estimate a decrease in stratospheric ozone in the tropics of 5 to 10 DU for all climate scenarios. Some recent projections (Lamy et al., 2019) predict a 2-3% increase in UVR in the southern tropical band, a region where UV levels are already extreme.

The purpose of the UV-Indien network is to :

- Monitor UV levels at different sites in the Western Indian Ocean (WIO)

- Describe the annual and inter-annual variability of UV radiation in the WIO

- Perform regional climate projections of UV radiations, validated by quality ground measurements.

UV-Indien is split into three phases. The first phase began in 2016, with the deployment of the first measurement sites (Reunion Island, Madagascar, Seychelles, Rodrigues). These sites are equipped with a broadband radiometer measuring the UVI and a camera estimating the coverage and sometimes a spectrometer for the measurement of total ozone. The second phase from 2019, sees the extension of this network to 4 other sites (Juan de Nova, Diego Suarez, Fort Dauphin and Grande Comoros). The data validation phase began in 2019 (comparative study with satellite data) and will also propose the study of the variability of UV radiation on different sites. Finally, climate projections will be made from 2020 onwards and will use data from the network to validate the results.

The aim of this communication is to describe the entire network and its objectives. The first results, as well as the first climatologies will also be discussed.

How to cite: Portafaix, T., Lamy, K., Forestier, J.-B., Rakotoniaina, S., and Amélie, V.: UV-Indien Network- a network dedicated to the long-term monitoring of UV radiation in the Indian Ocean., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21692, https://doi.org/10.5194/egusphere-egu2020-21692, 2020.

D3240 |
Sigmund Guttu, Yvan Orsolini, Frode Stordal, Odd Helge Otterå, Thomas Toniazzo, and Pekka Verronen

There is an ongoing discussion whether the lagged surface impact of the 11-year solar cycle, which peaks 2-4 years after solar maximum, may be contributed by the North Atlantic Oscillation (NAO) coupling to the ocean. Several studies have suggested that this atmosphere-ocean feedback is involving annual re-emergence of anomalous ocean temperatures stored below the mixed layer. Energetic Electron precipitation effects also lag the solar maximum by a few years, peaking in the declining phase of the solar cycle. While recent studies have incorporated the stratospheric UV radiation component of the solar forcing, the importance of the effect from precipitating medium-to-high energy electrons (MEE), which are able to significantly disturb the stratospheric chemical composition, is not fully addressed, partly due to lack of realistic forcing in current Earth System Models. In this study, we use the high-top atmospheric model WACCM coupled to the MICOM ocean model and adopt a state-of-the-art MEE forcing data set. Results will be presented from two decadal ensemble experiments with solar cycle induced forcings, one with UV and one with UV and MEE. The anomalous forcing from MEE precipitation is studied in relation to patterns of Northern Hemispheric atmospheric variability modes.

How to cite: Guttu, S., Orsolini, Y., Stordal, F., Otterå, O. H., Toniazzo, T., and Verronen, P.: Effects of the 11-year Solar Cycle including Medium-Energy Electron Precipitation in WACCM decadal climate predictions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21369, https://doi.org/10.5194/egusphere-egu2020-21369, 2020.

D3241 |
Ville Maliniemi, Daniel R. Marsh, Hilde Nesse Tyssøy, and Christine Smith-Johnsen

Energetic electron precipitation (EEP) is an important source of polar nitrogen oxides (NOx) in the upper atmosphere. During winter, mesospheric NOx has a long chemical lifetime and is transported to the stratosphere by the mean meridional circulation. Climate change is expected to accelerate this circulation and therefore increase polar mesospheric descent rates. We investigate the southern hemispheric polar NOx distribution during the 21st century under a variety of future scenarios using simulations of the Whole Atmosphere Community Climate Model (WACCM). Each future scenario has the same moderate variable solar activity scenario, where EEP activity is lower than during the 20th century. We simulate stronger polar mesospheric descent in all future scenarios that increase the atmospheric radiative forcing. By the end of 21st century polar NOx in the upper stratosphere is significantly enhanced in two future scenarios with the largest increase in radiative forcing. This indicates that the ozone depleting NOx cycle will become more important in the future, especially if stratospheric chlorine species decline. Thus, EEP-related atmospheric effects may become more prominent in the future.

How to cite: Maliniemi, V., Marsh, D. R., Nesse Tyssøy, H., and Smith-Johnsen, C.: Will climate change impact polar NOx produced by energetic particle precipitation?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5120, https://doi.org/10.5194/egusphere-egu2020-5120, 2020.