AS3.6

EDI
The role of the middle atmosphere in a changing climate: circulation, composition changes and radiative feedbacks

Anthropogenic emissions of greenhouse gases and ozone depleting substances have caused substantial changes in the chemical composition of the middle atmosphere that, in turn, can influence tropospheric processes. Increasing greenhouse gas levels are expected to modify the stratospheric amount of key radiatively active gases, such as water vapor, ozone and stratospheric aerosols through changes in the stratospheric Brewer-Dobson circulation (BDC). Changes in stratospheric ozone can in turn affect the biosphere (via e.g. changes in UV exposure) and feed back on surface climate via their influence on Earth's radiative budget. In addition, long-term changes in the ozone layer (e.g. ozone hole and recovery) are known to influence the tropospheric circulation and may be further coupled to a variety of Earth system feedbacks, which are to date poorly understood.


We welcome abstracts which explore composition changes and resulting radiative impacts and feedbacks on the tropospheric and stratospheric circulation as well as on surface weather and climate. Abstracts may address these issues on time-scales encompassing inter-annual to centennial timescales as well as impacts ranging from the tropics to poles. In particular, new studies on the influence of stratospheric ozone and composition on weather and climate are of interest. Research might also concern long-term ozone trends (depletion and recovery), as well as water vapor changes and volcanic aerosol impact in the stratosphere. We welcome contributions using chemistry-climate and Earth system models such as the new Chemistry Climate Model Initiative (CCMI-2) and Coupled Model Intercomparison Project (CMIP6), observations, as well as contributions using novel statistical approaches (e.g. Machine, causal inference) to gain insights into composition changes, related feedbacks and theoretical studies.

Convener: Mohamadou DialloECSECS | Co-conveners: Gabriel ChiodoECSECS, William Ball, Birgit Hassler, James KeebleECSECS, Peer Johannes Nowack
Presentations
| Thu, 26 May, 15:10–15:55 (CEST)
 
Room M1

Presentations: Thu, 26 May | Room M1

Chairpersons: Mohamadou Diallo, Gabriel Chiodo, Peer Johannes Nowack
15:10–15:15
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EGU22-1200
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ECS
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Highlight
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On-site presentation
Diego Jiménez de la Cuesta Otero and Hauke Schmidt

Several modeling studies identified a climate-chemistry feedback mechanism that modulates the global equilibrium climate sensitivity (ECS) through changes in the tropical upper-tropospheric and lower-stratospheric (UTLS) water vapor. The main factors producing this feedback are the upward shift of the tropical tropopause layer (TTL) and the acceleration of the Brewer-Dobson circulation (BDC). These two processes change the ozone (O3) concentration, resulting in a drier UTLS region than without O3 changes. Thus, the planetary long-wave emissivity increases and the ECS decreases. However, the BDC alone provides a tropical dynamical cooling in the UTLS region. This cooling is modified by the carbon dioxide (CO2) diabatic effects. Thus, the magnitude of the BDC changes can directly impact the tropical, if not the global, ECS. We build upon the work of Dacie et al. (2019), who analyzed how O3 changes affected the tropical ECS and TTL temperature. We study how the changes in the tropical upwelling directly affect the tropical ECS using the forcing-feedback framework. We find that the tropical upwelling changes dampen the effective radiative forcing, thereby reducing the ECS. Adding O3 chemistry shows that the changes in upwelling greatly enhance the climate chemistry-feedback but, more importantly, enhance the dampening of the effective radiative forcing and the reduction in ECS. Nonetheless, we cannot answer if these tropical effects of upwelling affect the global ECS until we include the extratropical regions.

How to cite: Jiménez de la Cuesta Otero, D. and Schmidt, H.: The tropical stratospheric upwelling sets the tropical equilibrium climate sensitivity by reducing the effective forcing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1200, https://doi.org/10.5194/egusphere-egu22-1200, 2022.

15:15–15:20
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EGU22-1805
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Presentation form not yet defined
Mahesh Kovilakam, Larry Thomason, and Travis Knepp

The Stratospheric Aerosol and Gas Experiment (SAGE III/ISS) aboard ISS began its mission in June 2017. SAGEIII/ISS is an updated version of SAGEIII-Meteor instrument that makes observations of stratospheric aerosol extinction coefficient at wavelengths that range between 385 and 1550 nm with a near global coverage between 60S-60N. While SAGEIII/ISS makes reliable and robust solar occultation measurements in stratosphere—similar to its predecessors, interpreting aerosol extinction measurements in the vicinity of tropopause and in the troposphere have been a challenge for all SAGE measurements. Here, we study the challenges associated with the discrimination of aerosols and clouds from the extinction measurements. Additionally, recent volcanic/PyroCb events make it more challenging to separate aerosols from clouds. Here, we describe the methods implemented to categorize Clouds and aerosols using available SAGEIII/ISS aerosol measurements. Cloud categorization is developed based on a method proposed by Thomason and Vernier (2013) with some modifications that now incorporates the influence of recent volcanic/PyroCb events and a new method of locating aerosol centroid based on k-medoid clustering. We use version 5.2 of SAGE III/ISS extinction coefficients for the analysis. The current algorithm now classifies standard (background) and non-standard (enhanced) aerosols in the stratosphere and identify enhanced aerosols and aerosol/cloud mixture in the tropopause region. Extinction coefficient measurements from SAGE series of observations make an important contribution in the GloSSAC data base and therefore, the impact of cloud-filtered aerosol extinction coefficient measurements on the latest version of GloSSAC (version 2.1) is also discussed.

How to cite: Kovilakam, M., Thomason, L., and Knepp, T.: SAGE III/ISS aerosol/cloud categorization and its impact on GloSSAC, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1805, https://doi.org/10.5194/egusphere-egu22-1805, 2022.

15:20–15:25
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EGU22-1928
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On-site presentation
Felix Ploeger and Hella Garny

Despite the expected opposite effects of ozone recovery, the stratospheric Brewer-Dobson circulation (BDC) has been found to weaken in the Northern hemisphere (NH) relative to the Southern hemisphere (SH) in recent decades, inducing substantial effects on chemical composition. We investigate hemispheric asymmetries in BDC changes since about 2000 in simulations with the transport model CLaMS driven with different reanalyses (ERA5, ERA-Interim, JRA-55, MERRA-2) and contrast those to a suite of free-running climate model simulations. We find that age of air increases robustly in the NH stratosphere relative to the SH in all reanalyses considered. Related nitrous oxide changes agree well between reanalysis-driven simulations and satellite measurements, providing observational evidence for the hemispheric asymmetry in BDC changes. Residual circulation metrics further show that the composition changes are caused by structural BDC changes related to an upward shift and strengthening of the deep BDC branch, resulting in longer transit times, and a downward shift and weakening shallow branch in the NH relative to the SH. All reanalyses agree on this mechanism. Although climate model simulations show that ozone recovery will lead to overall reduced circulation and age of air trends, the hemispherically asymmetric signal in circulation trends is small compared to internal variability. Therefore, the observed circulation trends over the recent past are not in contradiction to expectations from climate models. Furthermore, the hemispheric asymmetry in BDC trends imprints on the composition of the lower stratosphere and the signal might propagate into the troposphere, potentially affecting composition down to the surface.

How to cite: Ploeger, F. and Garny, H.: Hemispheric asymmetries in recent changes of the stratospheric circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1928, https://doi.org/10.5194/egusphere-egu22-1928, 2022.

15:25–15:30
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EGU22-2102
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ECS
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On-site presentation
Brett McKim, Nadir Jeevanjee, and Geoffrey Vallis

The longwave clear-sky feedback (the dependence of outgoing longwave radiation on surface temperature) is a major determinant of the climate's stability. Various studies have suggested that the feedback is largely independent of both surface temperature and relative humidity, which implies that the climate stability is also independent of surface temperature and relative humidity. However, this uniformity seems to contradict other work which shows that the subtropics are relatively stable and the deep tropics are relatively unstable, implying the feedback must vary between the two regions. We resolve this apparent contradiction by systematically computing the feedback as a function of both surface temperature and relative humidity. Above 275 K, the feedback depends significantly on relative humidity. We then show the feedback does indeed vary in the tropics and that this difference arises from regional differences in relative humidity. Finally, we estimate the effects of clouds on the feedback with a simple model and find that although clouds have a destabilizing influence, the significant dependence on relative humidity persists. Our work gives a renewed appreciation for how the feedback can vary significantly with both surface temperature and relative humidity.

How to cite: McKim, B., Jeevanjee, N., and Vallis, G.: The joint dependence of longwave feedback on surface temperature and relative humidity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2102, https://doi.org/10.5194/egusphere-egu22-2102, 2022.

15:30–15:35
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EGU22-7232
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On-site presentation
Ville Maliniemi, Pavle Arsenovic, Annika Seppälä, and Hilde Nesse Tyssøy

Chlorofluorocarbon (CFC) emissions in the latter part of the 20th century reduced the stratospheric ozone abundance substantially, especially in the Antarctic region. Simultaneously, polar stratospheric ozone is also depleted catalytically by reactive nitrogen (NOx) gasses. Energetic particle precipitation linked to solar activity and space weather produces NOx in the polar mesosphere/lower thermosphere, which during winter descend to stratospheric altitudes via mean meridional residual circulation. NOx can also limit the CFC ozone destruction, e.g., by transforming active chlorine and nitrogen into a reservoir of chlorine nitrate. We study the interaction between EPP produced NOx, ClO and ozone over the 20th century by using free running climate simulations of the chemistry-climate model SOCOL3-MPIOM. Substantial increase of NOx descending to polar stratosphere is found during winter, which causes ozone depletion in the upper and mid-stratosphere. However, the EPP-NOx induced ozone depletion becomes less efficient in the Antarctic mid-stratosphere after 1960s, especially during springtime. At the same time, significant decrease in Antarctic stratospheric ClO between 1-30 hPa over winter and spring can be ascribed to the EPP-NOx. This is true even during the CFC era. Hence, chlorine gasses contributed to reducing the efficiency of the EPP-NOx ozone depletion at these altitudes and vice versa. Our results show that EPP has been a significant modulator of reactive chlorine in the Antarctic stratosphere during the CFC era. With the implementation of the Montreal Protocol, stratospheric chlorine is estimated to return to pre-CFC era levels after 2050. We can thus expect increased efficiency of chemical ozone destruction by EPP-NOx in the future Antarctic upper and mid-stratosphere.

How to cite: Maliniemi, V., Arsenovic, P., Seppälä, A., and Nesse Tyssøy, H.: Influence of energetic particle precipitation on Antarctic stratospheric chlorine and ozone over the 20th century, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7232, https://doi.org/10.5194/egusphere-egu22-7232, 2022.

15:35–15:40
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EGU22-10706
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ECS
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Presentation form not yet defined
Mohamadou Diallo, Roland Eichinger, Fernando Iglesias-Suarez, and Felix Ploeger

The recent release of the long-term ERA5 reanalysis data spanning from 1950 to present offers new opportunities for analysing trends and variability of stratospheric dynamics. For the first time, a 60 year period (1960-2020) can be analysed in reanalysis data and compared with chemistry-climate model simulations. The analyses of stratospheric circulation trends and seasonalities over this long time period can help us to better understand the long-term evolution of the Brewer-Dobson circulation (BDC), and the related inter-model differences and model dependencies. Therefore, this way an improved credibility in future projections of the BDC can be obtained.
We find that the global trend patterns of the temperature, zonal wind and residual vertical velocity agrees well between ERA5 and the multi model mean. However, differences occur in the width and altitude of the maximum trend. The tropical upwelling mass flux time series in the lower stratosphere of models and reanalysis disagrees at the beginning of the period, but they converge after around 1980. The agreement of the time series increases with altitude, where the QBO dominates the signal. Moreover, we find a generally good agreement in the zonal wind trends, although some differences are detected in the subtropical jet strength and upward shift, as well as in the polar vortex region where the models exhibit larger changes than ERA5. Another striking difference is the temperature trend in the tropical upper troposphere/lower stratosphere, where models show a more extended warming trend into the lower stratosphere. In this presentation, we show these results, put them in relation to what had been shown in previous studies for other time periods and discuss possible explanations for the differences as well as implications for the further evolution of the BDC.

How to cite: Diallo, M., Eichinger, R., Iglesias-Suarez, F., and Ploeger, F.: Evidence for the long-term climate model predicted-stratospheric circulation changes in the ERA5 reanalysis over 1960-2020, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10706, https://doi.org/10.5194/egusphere-egu22-10706, 2022.

15:40–15:45
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EGU22-12267
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ECS
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Highlight
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Virtual presentation
Jessica Kult-Herdin, Timofei Sukhodolov, Gabriel Chiodo, and Harald Rieder

Increasing computational resources have led to the advent of Earth System Models. However, to date many models still do not incorporate interactive chemistry due to its high computational costs. Previous work has shown the importance of interactive ozone in enhancing extreme springtime variability under present-day ozone depleting substance (ODS) levels. Here, we aim to understand the role of different greenhouse gas and ODS levels on this result. In the present study we aim to answer this question for the Arctic springtime stratosphere, contrasting a suite of multi-decadal simulations performed with the ESMs WACCM4 and SOCOLv3-MPIOM with interactive and prescribed ozone chemistry. Our analysis focuses on the contribution of carbon dioxide and ozone for temperature and temperature variability at lower, middle and upper stratospheric levels. The ensembles comprise simulations with 1xCO2 or 4xCO2  (without anthropogenic ODS) and year 2000 (peak ODS) forcings, allowing us to investigate the relative importance of interactive chemistry vs. prescribed ozone under different "climate states and ODS levels". Our results show 1) that CO2 is the primary driver of the mean temperature response in the upper stratosphere while ozone largely contributes to the mean change in the lower stratosphere; 2) the importance of interactive chemistry for a coherence (coupling) between temperature, zonal wind (used as proxy for the polar vortex strength) and ozone in the lower stratosphere; and 3) an important contribution of interactive chemistry to temperature variability under year 2000 but also 1xCO2 forcing in contrast to simulations forced with 4xCO2.

How to cite: Kult-Herdin, J., Sukhodolov, T., Chiodo, G., and Rieder, H.: Exploring the importance of interactive ozone chemistry under different GHG and ODS levels, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12267, https://doi.org/10.5194/egusphere-egu22-12267, 2022.

15:45–15:55
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EGU22-13471
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solicited
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Highlight
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Virtual presentation
Amanda C. Maycock

Over the last two decades a growing body of literature has shown that stratospheric processes play a key role for near- and long-term projections of surface climate under different scenarios for anthropogenic and natural forcings. The effect of the stratosphere on surface climate change occurs through two main interconnected pathways: 1) stratosphere-troposphere dynamical coupling; 2) radiative feedbacks predominantly through changes to stratospheric composition. This talk will give some examples of both of these pathways relevant to climate change in the northern and southern hemispheres. Topics will include the role of the strength of the polar vortices for midlatitude circulation change, the stratospheric water vapour feedback and ozone-climate coupling. Two further factors will be discussed that affect how stratospheric processes contribute to surface climate projections: 1) the role of the annual cycle and 2) model biases. In conclusion some thoughts on key future research questions will be offered.

How to cite: Maycock, A. C.: The role of the stratosphere in understanding future climate change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13471, https://doi.org/10.5194/egusphere-egu22-13471, 2022.