Distinguishing the Direct Radiative, Surface Warming, and Ozone Mediated Contributions to the Acceleration of the Brewer-Dobson circulation under Abrupt CO2 Forcing (in preparation)"

As CO2 increases, stratospheric ozone is projected to decrease in the tropics and increase over high latitudes. While most climate models do not represent this so-called ``ozone feedback,’’ recent modeling studies show that these ozone changes modulate not only the response of stratospheric temperatures, but changes in the Brewer-Dobson Circulation (BDC). It is unclear, however, how important ozone’s impact is relative to other more explored components of the CO2 response–namely, the direct radiative response to CO2 and surface warming through changes in sea surface temperatures. To address this gap, we use the NASA GISS Model E2.2 to decompose the BDC’s response to 4xCO2 into contributions from direct radiative CO2 forcing, surface warming, and interactive ozone changes. First, we confirm that the fully coupled atmosphere-ocean response can be represented as the linear sum of contributions from direct radiative CO2 forcing, changing SSTs, and ozone adjustment. Second, we show that while surface warming induces about 80% of the BDC acceleration, its impact is limited to the lower stratosphere. By comparison, in the upper-and-middle stratosphere, the BDC response is dominated by changes due to direct radiative forcing from CO2 (80% of the acceleration at 10 hPa). Interestingly, the ozone adjustment opposes this acceleration nearly entirely, resulting in only a weak net acceleration of the deep branch of the BDC. Our results show that ozone’s adjustment to the stratospheric circulation response to CO2 is of similar magnitude to the direct response due to stratospheric cooling.

Journal of Climate