AS3.6

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.

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.

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.

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.