On the timescales of the response of the Brewer‐Dobson Circulation to an abrupt quadrupling of CO2

Changes in the Brewer‐Dobson circulation (BDC) in response to increasing CO 2 concentrations can arise from the direct effect of radiative cooling in the stratosphere or the indirect effects induced by warmer sea surface temperatures (SSTs). This study aims to disentangle these two contributions in the Whole Atmosphere Community Climate Model (WACCM) by analyzing the timescales of the tropical upwelling response to an abrupt quadrupling of CO 2 . Transient atmosphere‐ocean climate model simulations of 100 years under 4xCO 2 conditions are compared to preindustrial control simulations and to simulations with an atmosphere‐only version of WACCM that uses preindustrial SSTs. We find that most of the response in both shallow and deep branches of the BDC occurs on fast timescales (first 2‐3 decades). In the shallow branch of the BDC, the response is mainly driven by changes in SSTs in the well‐mixed shallow ocean, which cause tropospheric warming and an intensification and upward displacement of the subtropical jets, and alter wave forcing in their vicinity. The contribution from stratospheric radiative cooling is almost negligible. In the upper stratosphere, the response of tropical upwelling begins earlier and develops faster than in the shallow branch, owing to the larger contribution from the rapid adjustments. At 1 hPa, 70% of the fast response relates to stratospheric radiative cooling and 30% to warmer SSTs. The modulation of the filtering of non‐orographic gravity waves (mainly of frontal origin) in the subtropics explain most of the response in tropical upwelling in the deep branch, on both fast and slow timescales.