Stratospheric Ozone Changes damp the CO2-induced Acceleration of the Brewer-Dobson Circulation

In this study, we perform dedicated model experiments with the ECHAM/MESSy (European Center Hamburg Model / Modular Earth Submodel System) Atmospheric Chemistry (EMAC, Jöckel et al. 2005, 2010, 2016) model version 2.54.0.3. to quantify the modification of the circulation response to CO₂ forcing by stratospheric ozone. Specifically, we compare simulations with pre-industrial (piCtrl) and with quadrupled CO₂ climate conditions, in which stratospheric ozone is either held fixed (4xCO2piO3) or is adapted to the new climate state (4xCO2Ctrl).

The diagnostic of the residual circulation and mean age of air shows that ozone changes dampen the CO₂-induced BDC acceleration by up to 20%. This damping of the BDC acceleration is caused by an enhancement of the meridional temperature gradient in the lower stratosphere in summer, thereby leading to stronger stratospheric easterlies that suppress wave propagation. Additionally, we find a systematic weakening of the polar vortices in winter and spring. In the SH, ozone reduces the CO₂-induced delay of the final warming date by 50 %.

The output data that are used for this study are located in the following files:

The zonal mean ozone fields (for the pre-industrial and 4xCO2 condition) are located in these files: abr4xCO2_O3_ml47_zm_1960-1989_clim.nc; piCtrl_O3_ml47_zm_1960-1989_clim.nc 

We use v* and w* from the TEM monthly mean climatology data (files with the ending ...TEM_ymm.nc) for calculating the mass streamfunction as well as for the tropical upwelling. In addition, the TEM climatology contains also the EP-flux and its divergence (EPFD).

The AoA climatology data (files with the ending ...AoA_1980-2029_ymm.nc) is also used for investigation of the mass transport represented by the BDC.

We apply the monthly mean climatology of the temperature, specific humidity, zonal wind and gravity wave drag fields  from the files with the ending...plev-ymm.nc.

The monthly mean climatology of the shortwave heating rates are in the files with the ending ...._heatsw_pl_ymm.nc. 

The interaction between the stratosphere and troposphere and the abruptness of the final warming are analyzed with daily climatological data from the files which end with ...._geopot_yd.nc.

The standard deviation of these climatologies that are located in the files with the ending ...ymstd.nc (for monthly data)  or ...ydstd.nc (for daily data) is used for the significance tests. 

The tropopause data (for the pre-industrial and 4xCO2 condition) can be found in these two files: C2-4xCO2ctrl_1980-2029_tropop-p-ymm.nc; C2-4xCO2ctrl_1980-2029_tropop-p-ymm.nc 

For the detection of the final warming date following by the method of Charlton and Polvani (2007), the daily mean climatology of the zonal mean zonal wind (file with the ending..10hPa_60degN/S.nc) is applied.