How are natural variability and anthropogenic forcings affecting the variability and trends in the Brewer-Dobson circulation and downward ozone flux?

• Applicant: Mohamadou Diallo, Ph.D.
• Subject Area: Atmospheric Science
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429838442

The Brewer-Dobson circulation (BDC) is a key element of climate as it determines the transport and lifetime of ozone (O3), water vapor and aerosol above the tropopause, which significantly affect the Earth’s radiation budget. A strengthening BDC will impact the trace gas budgets in the upper troposphere and lower stratosphere (UTLS) and, thus, may have crucial consequences for climate. Recently, a strengthening BDC has been shown to modulate the downward O3 flux which, in turn, impacts on both climate and human health. Thus, understanding the BDC variability on seasonal to decadal time scales is a prerequisite for a reliable detection and attribution of the natural variability and anthropogenically-forced trends. However, BDC variability is often not reliably represented in current climate simulations, casting comparisons with measurements into doubt.We aim to assess the impact of natural variability and long-term anthropogenically-forced trends in the BDC on the UTLS trace gas distribution and on climate, and to analyze the dynamical mechanisms, leading to model-observation differences. The project combines established diagnostic tools, simulations with a reanalysis-driven Lagrangian transport model and a coupled chemistry-climate model together with available observations for investigating BDC changes and the related impacts on UTLS O3. To achieve these major goals, the work plan has three work-packages: (1) Investigations of the natural variability and anthropogenically-induced trends in the BDC, (2) Understanding dynamical mechanisms involved, and (3) Evaluation of the impact of BDC changes on the downward O3 flux.Available multi-year time series of observations (incl. O3 and mean age of air) will be used for investigating the variability and long-term BDC changes in the simulations with the Lagrangian transport model (CLaMS) and chemistry-climate model (EMAC). This comparison of observations with simulations is a prerequisite for disclosing the models’ ability to accurately capture the BDC variability. The use of regression analysis will then enable an attribution of variability and long-term trends in the BDC and in UTLS O3 distributions to different modes of climate variability.To investigate the dynamical mechanisms, we propose three types of sensitivity experiments in addition to the available simulations. These experiments will be conducted in a way that they will enable to disclose the dynamical mechanisms involved in the BDC changes induced by different forcings, and model-observation discrepancies.Finally, the impact of BDC changes on downward O3 flux and related effects on climate will be assessed using the long-term simulation with CLaMS driven by the EMAC output data. The downward O3 flux will be quantified using a budget approach to model O3 in the lowermost stratosphere. Regression analysis will attribute the variability in O3 flux to different modes of climate variability that critically impact the climate and air quality.

• DFG Programme: Research Grants