Stratospheric ozone depletion, along with increases in long-lived greenhouse gases, is well known to cause a strengthening of the Southern Annular Mode (SAM), the leading mode of variability in the Southern Hemisphere.  I here analyze simulations contributed to CMIP6 for signatures of these two leading drivers of climate change. For the period 1957-2014, seasonally large disagreements are found between four observational references; CMIP6-derived trends are in agreement with two out of four commonly used references. Using a regression analysis applied to model simulations with and without interactive ozone chemistry, a strengthening of the SAM in summer is attributed nearly completely to ozone depletion because a further strengthening influence due to long-lived greenhouse gases is almost fully counterbalanced by a weakening influence due to stratospheric ozone increases associated with these greenhouse gas increases. Ignoring such ozone feedbacks (an approach commonly used with no-chemistry climate models) would yield comparable contributions from these two influences, an incorrect result. In winter, trends are smaller but an influence of greenhouse gas-mediated ozone feedbacks is also identified. The regression analysis furthermore yields significant differences in the attribution of SAM changes to the two influences between models with and without interactive ozone chemistry, with ozone depletion and GHG increases playing seasonally a stronger and weaker, respectively, role in the chemistry models versus the no-chemistry ones. The results suggest that adequately representing stratospheric ozone feedbacks in climate models is critical for a correct attribution of trends in the SAM.

How to cite: Morgenstern, O.: The Southern Annular Mode in CMIP6 simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6755, https://doi.org/10.5194/egusphere-egu21-6755, 2021.

Stratospheric ozone depletion in the Antarctic is well known to cause changes in Southern Hemisphere tropospheric climate; however, because of 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 ozone-depleting substances (ODS) emissions are higher, are related to a decrease in upper tropospheric stability and an increase in high cloud fraction, which may contribute to enhanced Arctic surface warming in spring through a positive longwave cloud radiative effect. 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 2021, online, 19–30 Apr 2021, EGU21-3239, https://doi.org/10.5194/egusphere-egu21-3239, 2021.

Recent studies have noted that tropical mid-stratospheric ozone decreased in the 1990s and has remained persistently low since. Current analyses suggest that these observations are linked to dynamical processes rather than being chemically-driven, although this has not been fully explored. Using measurements and chemistry-climate model simulations, we show that 50 ± 10% of these observed trends can be accounted for through multi-decadal variability in the Brewer-Dobson circulation (BDC) tied to the Pacific Ocean sea surface temperatures (the Interdecadal Pacific Oscillation, or IPO), via dynamical and chemical couplings. Moreover, accounting for this low frequency variability in the BDC can also help interpret previous observationally-derived changes in that circulation since year 1979. Overall, these findings demonstrate strong links between stratosphere-troposphere variability at decadal time scales and their potential importance for future ozone recovery detection.

How to cite: Iglesias-Suarez, F., Wild, O., Kinnison, D. E., Garcia, R. R., Marsh, D. R., Lamarque, J.-F., Ryan, E. M., Davis, S. M., Eichinger, R., Saiz-Lopez, A., and Young, P. J.: Multi-decadal climate variability in the tropical stratosphere coupled to the Pacific, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3953, https://doi.org/10.5194/egusphere-egu21-3953, 2021.

Besides chlorine, bromine is the major halogen species affecting stratospheric ozone with both anthropogenic and natural sources. Despite the significantly lower concentrations of bromine in the atmosphere, its potential for ozone depletion is similar to that of chlorine. An important prerequisite for the effectiveness of bromine ozone destruction cycles versus those of chlorine is the larger instability of bromine reservoir gases, especially the faster photolysis of bromine nitrate (BrONO₂) compared to chlorine nitrate (ClONO₂). With BrONO₂ abundances in the stratosphere available from observations, (1) homogeneous, heterogeneous as well as photochemical processes involving bromine as implemented in atmospheric models can be assessed, and (2) independent information on the total stratospheric bromine content can be gained which is important, e.g. to analyse the amount of short-lived bromocarbons entering the stratosphere.

The first detection of BrONO₂ in the atmosphere had been achieved by analysis of infrared limb-emission spectra from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the Envisat satellite (doi: 10.5194/acp-9-1735-2009). On availability of improved infrared cross-sections, this was followed by the analysis of the behaviour of BrONO₂ during sunrise and sunset through MIPAS balloon observations (doi: 10.5194/acp-17-14631-2017). Here we present a novel dataset of global stratospheric BrONO₂ distributions based on the recently available MIPAS version 8 dataset of calibrated level-1b spectra. The altitude profiles of BrONO₂ volume mixing ratios are zonally averaged in 10° latitude and 3-day bins, separated between day- and night-time observations, with a vertical resolution of 3-8 km between 15 and 35 km altitude for the whole MIPAS period from July 2002 until April 2012. The typical characteristics of this new dataset will be discussed. Furthermore, we will compare it to a multi-annual simulation of the chemistry climate model EMAC. Specific differences between observation and model simulation of BrONO₂ will be highlighted and discussed by means of sensitivity 1-d model runs. Finally, a time series of the derived stratospheric Br_y content normalized to the time of the entry into the stratosphere on basis of MIPAS age-of-air information will be discussed with regard to estimated uncertainties as well as independent observations.

How to cite: Höpfner, M., Kirner, O., Wetzel, G., Sinnhuber, B.-M., Haenel, F., Orphal, J., Ruhnke, R., Stiller, G., and von Clarmann, T.: The MIPAS climatology of BrONO2: a test for stratospheric bromine chemistry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6322, https://doi.org/10.5194/egusphere-egu21-6322, 2021.

High quality global ozone profile datasets are necessary to monitor changes in stratospheric ozone. Hence, various methodologies have been used to merge and homogenise different satellite datasets in order to create long-term observation-based ozone profile datasets with minimal data gaps. However, individual satellite instruments use different measurement methods and retrieval algorithms that complicate the merging of these different datasets. Furthermore, although atmospheric chemical models are able to simulate chemically consistent long-term datasets, they are prone to the deficiencies associated with the computationally expensive processes that are generally represented by simplified parameterisations or uncertain parameters.

Here, we use chemically consistent output from a 3-D Chemical Transport Model (CTM, TOMCAT) and an ensemble of three machine learning (ML) algorithms (Adaboost, GradBoost, Random Forest), to create a 42-year (1979-2020) stratospheric ozone profile dataset. The ML algorithms are primarily trained using the Stratospheric Water and OzOne Satellite Homogenized (SWOOSH) dataset by selecting the UARS-MLS (1992-1998) and AURA-MLS (2005-2019) time periods. This ML-corrected version of monthly mean zonal mean TOMCAT (hereafter ML-TOMCAT) ozone profile data is available at both pressure (1000 hPa - 1 hPa) and geometric height (surface to 50 km) levels at about 2.5 degree horizontal resolution.

We will present a detailed evaluation of ML-TOMCAT profiles against range of merged satellite datasets (e.g. GOZCARDS, SAGE-CCI-OMPS, and BVertOzone) as well high quality solar occultation observations (e.g. SAGE-II v7.0 (1984-2005), HALOE v19 (1991-2005) and ACE v4.1 (2004-2020). Overall, ML-TOMCAT shows good agreement with the evaluation datasets but with poorer agreement at low latitudes. We also show that, as in different merged satellite data sets, ML-algorithms show larger spread in the tropical middle stratosphere. Finally, we will present a trend analysis from TOMCAT and ML-TOMCAT profiles for the post-1998 ozone recovery phase.

How to cite: Dhomse, S. and Chipperfield, M.: Machine-Learning-Based Satellite-Corrected Global Stratospheric Ozone Profile Dataset from a Chemical Transport Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9613, https://doi.org/10.5194/egusphere-egu21-9613, 2021.

Laube et al. (2020) investigated stratospheric changes between 2009 and 2018 with halogenated trace gas data (CFC-11, CFC-12, H-1211, H-1301, HCFC-22, and SF₆) from air samples collected via aircraft and AirCores, and compared the mixing ratios and average stratospheric transit times derived from these observations with those from a global model. We here expand this analysis in three ways: firstly, by adding data from further traces gases such as CFC-115, C₂F₆, and HCFC-142b to broaden the range of tropospheric trends and stratospheric lifetimes, both of which help to assess the robustness of inferred long-term trends in the stratosphere; secondly, by increasing the temporal span of the observations to nearly three decades using new AirCore observations as well as reanalysed archived air samples collected on board high altitude aircraft and large balloons in the 1990s and 2000s; and thirdly, by investigating the fractional release factors and mean ages of air derived from the aforementioned species as measures of their stratospheric chemistry and the strength of the Brewer-Dobson circulation. In combination with model data from the Chemical Langrangian Model of the Stratosphere (CLaMS) this unique data set allows for an unprecedented evaluation of stratospheric chemistry and dynamics in the mid-latitudes of the Northern Hemisphere.

References

Laube, et al., Atmos. Chem. Phys., 20, 9771–9782, 2020, https://doi.org/10.5194/acp-20-9771-2020

How to cite: Laube, J., Atlas, E., Adcock, K., Droste, E., Heikkinen, P., Kaiser, J., Kivi, R., Leedham Elvidge, E., Hind, A., Röckmann, T., Sturges, W., Thomas, M., Tuffnell, E., and Plöger, F.: Investigating stratospheric circulation and chemistry changes over three decades with trace gas data from aircraft, large balloons, and AirCores, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10711, https://doi.org/10.5194/egusphere-egu21-10711, 2021.

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 (CCM). While not only the first, to our knowledge, model study addressing this problem using a CCM, it is also the first such study employing a whole atmosphere model, thereby simulating the tropospheric Cl-VSLSs emissions and the resulting stratospheric impacts in a fully consistent manner. 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.

We examine the impacts of rising Cl-VSLSs emissions on atmospheric chlorine tracers and ozone, including their long-term trends. We pay particular attention to the role of ‘nudging’, as opposed to the free-running model set up, for the simulated Cl-VSLSs impacts, thereby demostrating the role of atmospheric dynamics in modulating the atmospheric responses to Cl-VSLSs. In addition, we employ novel estimates of Cl-VSLS emissions over the recent past and compare the results with the simulations that prescribe Cl-VSLSs using simple lower boundary conditions. This allows us to demonstrate the impact such choice has on the dominant location and seasonality of the Cl-VSLSs transport into the stratosphere.

How to cite: Bednarz, E., Hossaini, R., Abraham, L., Braesicke, P., and Chipperfield, M.: Atmospheric impacts of short-lived chlorinated species over the recent past: a chemistry-climate perspective, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12824, https://doi.org/10.5194/egusphere-egu21-12824, 2021.

Ozone is expected to fully recover from the CFC-era by the end of the 21st century. Furthermore, because of the anthropogenic climate change, cooler stratosphere accelerates the ozone production and is projected to lead to a super recovery. We investigate the ozone distribution over the 21st century with four different future scenarios using simulations of the Whole Atmosphere Community Climate Model (WACCM). At the end of the 21st century, higher polar ozone levels than pre CFC-era are obtained in scenarios that have highest atmospheric radiative forcing. This is true in the Arctic stratosphere and the Antarctic lower stratosphere. The Antarctic upper stratosphere forms an exception, where different scenarios have similar level of ozone during winter. This results from excess nitrogen oxides (NOx) descending from above in stronger future scenarios. NOx is formed by energetic electron precipitation (EEP) in the thermosphere and the upper mesosphere, and descends faster through the mesosphere in stronger scenarios. This indicates that the EEP indirect effect will be important factor for the future Antarctic ozone evolution, and is potentially able to prevent the super recovery in the upper stratosphere.

How to cite: Maliniemi, V., Arsenovic, P., Nesse Tyssøy, H., Smith-Johnsen, C., and Marsh, D. R.: Ozone super recovery cancelled in the Antarctic upper stratosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5376, https://doi.org/10.5194/egusphere-egu21-5376, 2021.

Stratospheric ozone and water vapor are key components of the Earth system, and past and future changes to both have important impacts on global and regional climate. Here we evaluate long-term changes in these species from the pre-industrial (1850) to the end of the 21^st century in CMIP6 models under a range of future emissions scenarios. There is good agreement between the CMIP multi-model mean and observations for total column ozone (TCO), although there is substantial variation between the individual CMIP6 models. For the CMIP6 multi-model mean, global mean TCO has increased from ~300 DU in 1850 to ~305 DU in 1960, before rapidly declining in the 1970s and 1980s following the use and emission of halogenated ozone depleting substances (ODSs). TCO is projected to return to 1960’s values by the middle of the 21^st century under the SSP2-4.5, SSP3-7.0, SSP4-3.4, SSP4-6.0 and SSP5-8.5 scenarios, and under the SSP3-7.0 and SSP5-8.5 scenarios TCO values are projected to be ~10 DU higher than the 1960’s values by 2100. However, under the SSP1-1.9 and SSP1-1.6 scenarios, TCO is not projected to return to the 1960’s values despite reductions in halogenated ODSs due to decreases in tropospheric ozone mixing ratios. This global pattern is similar to regional patterns, except in the tropics where TCO under most scenarios is not projected to return to 1960’s values, either through reductions in tropospheric ozone under SSP1-1.9 and SSP1-2.6, or through reductions in lower stratospheric ozone resulting from an acceleration of the Brewer-Dobson Circulation under other SSPs. In contrast to TCO, there is poorer agreement between the CMIP6 multi-model mean and observed lower stratospheric water vapour mixing ratios, with the CMIP6 multi-model mean underestimating observed water vapour mixing ratios by ~0.5 ppmv at 70hPa. CMIP6 multi-model mean stratospheric water vapor mixing ratios in the tropical lower stratosphere have increased by ~0.5 ppmv from the pre-industrial to the present day and are projected to increase further by the end of the 21^st century. The largest increases (~2 ppmv) are simulated under the future scenarios with the highest assumed forcing pathway (e.g. SSP5-8.5). Tropical lower stratospheric water vapor, and to a lesser extent TCO, show large variations following explosive volcanic eruptions.

How to cite: Keeble, J., Hassler, B., Banerjee, A., Checa-Garcia, R., Chiodo, G., Davis, S., Eyring, V., Griffiths, P., Morgenstern, O., Nowack, P., Zeng, G., and Zhang, J.: Evaluating stratospheric ozone and water vapor changes in CMIP6 models from 1850-2100 , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15994, https://doi.org/10.5194/egusphere-egu21-15994, 2021.

Previous studies indicate a possible role of stratospheric ozone chemistry feedbacks in the climate response to 4xCO2, either via a reduction in equilibrium climate sensitivity (ECS) or via changes in the tropospheric circulation (Nowack et al., 2015; Chiodo and Polvani, 2017). However, these effects are subject to uncertainty. Part of the uncertainty may stem from the dependency of the feedback on the pattern of the ozone response, as the radiative efficiency of ozone largely depends on its vertical distribution (Lacis et al., 1990). Here, an analysis is presented of the ozone layer response to 4xCO2 in chemistry–climate models (CCMs) which participated to CMIP inter-comparisons. In a previous study using CMIP5 models, it has been shown that under 4xCO2, ozone decreases in the tropical lower stratosphere, and increases over the high latitudes and throughout the upper stratosphere (Chiodo et al., 2018). It was also found that a substantial portion of the spread in the tropical column ozone is tied to inter-model spread in tropical upwelling, which is in turn tied to ECS. Here, we revisit this connection using 4xCO2 data from CMIP6, thereby exploiting the larger number of CCMs available than in CMIP5. In addition, we explore the linearity of the ozone response, by complementing the analysis with simulations using lower CO2 forcing levels (2xCO2). We show that the pattern of the ozone response is similar to CMIP5. In some models (e.g. WACCM), we find larger ozone responses in CMIP6 than in CMIP5, partly because of the larger ECS and thus larger upwelling response in the tropical pipe. In this presentation, we will discuss the relationship between radiative forcing, transport and ozone, as well as further implications for CMIP6 models.

How to cite: Chiodo, G., Ball, W. T., Nowack, P., Orbe, C., Keeble, J., Diallo, M., and Hassler, B.: The response of the ozone layer under abrupt 4xCO2 in CMIP6, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15283, https://doi.org/10.5194/egusphere-egu21-15283, 2021.

Photolysis of molecular oxygen (O₂) maintains the stratospheric ozone layer, protecting living organisms on Earth by absorbing harmful ultraviolet radiation. The atmospheric oxygen level has not always been constant, and has been held responsible for species extinctions via a thinning of the ozone layer in the past. On paleo-climate timescales, it ranged between 10 and 35% depending on the level of photosynthetic activity of plants and oceans. Previous estimates, however, showed highly uncertain ozone (O₃) responses to atmospheric O₂ changes, including monotonic positive or negative correlations, or displaying a maximum in O₃ column around a certain oxygen level. Motivated by these discrepancies we reviewed how the ozone layer responds to atmospheric oxygen changes by means of a state-of-the-art chemistry-climate model (CCM). We used the CCM SOCOL-AERv2 to assess the ozone layer sensitivity to past and potential future concentrations of atmospheric oxygen varying from 5 to 40 %. Our findings are at odds with previous studies: we find that the current mixing ratio of O₂, 21 %, indeed maximizes the O₃ layer thickness and, thus, represents an optimal state for life on Earth. In the model, any alteration in atmospheric oxygen would result globally in less total column ozone and, therefore, more UV reaching the troposphere. Total ozone column in low-latitude regions is less sensitive to the changes, because of the “self-healing” effect, i.e. more UV entering lower levels, where O₂ photolyzes, can partly compensate the O₃ lack higher up. Mid- and high-latitudes, however, are characterized by ±20 DU ozone hemispheric redistributions even for small (±5 %) variations in O₂ content. Additional regional patterns result from the hemispheric asymmetry of meridional transport pathways via the Brewer-Dobson circulation (BDC). We will discuss the different ozone responses resulting from changes in the BDC. These effects are further modulated by the influence of ozone on stratospheric temperatures and thus on the BDC. Lower O₂ cases result in a deceleration of the BDC. This renders the relation between ozone and molecular oxygen changes non-linear on both global and regional scales.

How to cite: Józefiak, I., Sukhodolov, T., Egorova, T., Rozanov, E., Chiodo, G., Stenke, A., and Peter, T.: The response of stratospheric ozone and dynamics to changes in atmospheric oxygen, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12783, https://doi.org/10.5194/egusphere-egu21-12783, 2021.