Response of the Brewer Dobson circulation to an abrupt CO2 increase

The Brewer-Dobson circulation is the mean meridional circulation in the stratosphere. It is important for the chemical distribution of trace gases in the stratosphere and its thermal structure. Chemistry climate models consistently project an acceleration of its shallow branch in response to increasing greenhouse gas concentrations, while changes in the deep branch have been much less explored. Most models agree that enhanced resolved wave forcing is the main driver of the trend in tropical upwelling in the lower stratosphere although the ultimate mechanism is not well understood. Both changes in wave generation and wave dissipation can lead to increased wave driving and modeling results are not conclusive. Some studies have shown changes in wave propagation through the strengthening and upward displacement of the subtropical jets in response to upper troposphere warming and lower stratospheric cooling under climate change. In contrast, other studies reported changes in wave generation through the modulation of deep convection and latent heat release associated to warmer SSTs under increasing GHGs.

Here, we revisit this issue from a different perspective based on the timescales of the BDC response to an abrupt quadrupling of CO2 concentrations. To do so, we use CMIP5 and CMIP6 preindustrial, 4xCO2 and AMIP simulations of the Whole Atmosphere Community Climate Model (WACCM) to compare the fast and slow responses of the BDC to the increase in CO2. While the fast response is associated with the direct radiative forcing of increasing CO2, the slower response of the BDC is related to warmer sea surface temperatures. The responses of both the shallow and deep branches of the BDC are analyzed to understand the mechanisms that operate in each case.