The Brewer-Dobson Circulation during the Last Glacial Maximum

Abstract

The Brewer-Dobson circulation (BDC) during the last glacial maximum (LGM) is investigated in simulations using the Whole Atmosphere Community Climate Model version 6 (WACCM6). We examine vertical mass fluxes, age of stratospheric air, and the transformed Eulerian mean stream function and find that the modeled annual-mean BDC during the LGM is almost everywhere slower than that in the modern climate (with or without anthropogenic ozone depleting substances). Compared to the modern climate, the annual-mean tropical upwelling in the LGM is 11.3-16.9%, 11.2-15.8%, and 4.4-10.2% weaker, respectively, at 100, 70, and 30 hPa. It is shown that the decrease in the annual-mean tropical upwelling at 70 hPa is caused by a weaker orographic gravity wave drag.