Reconciling long-term stratospheric circulation changes from ERA5 and CCMI2

Abstract

Climate models predict a global acceleration of the stratospheric Brewer-Dobson circulation (BDC) induced by rising greenhouse gas (GHG) levels. However, this predicted strengthening of the BDC has not yet been compared with long-term observations, due to the scarcity of long-term observation records.  The recent release of ERA5 long-term reanalysis data covering the period from 1950 to the present day offers new opportunities to assess the robustness of the projected BDC acceleration. These observation-based data make it possible to assess the ability of models to capture the impact of natural variability such as the Quasi- Biennial Oscillation (QBO), the El Nino Southern Oscillation (ENSO), the solar cycle, stratospheric volcanic aerosols and the Pacific Decadal Oscillation (PDO), on circulation changes and their interaction. In this presentation, I will review our knowledge of the dynamical mechanisms explaining changes in the BDC over the period 1960-2018 using ERA5 and CCMI-2. I will then highlight the impact of climate variability modes (e.i. QBO, ENSO, Solar, volcanoes and PDO) and their interaction on the BDC. Finally, I will discuss the robustness and main reasons for differences between modern reanalyses and climate models in the BDC changes.