Trends in the strength of the Brewer-Dobson circulation and the wintertime Arctic ozone build-up

Recently a number of middle atmosphere climate models have predicted that the tropical upwelling associated with the Brewer-Dobson circulation (BDC) will intensify in response to greenhouse gas increases and/or ozone depletion. However, a strengthened BDC does not appear to be consistent with a reduction of the wintertime Arctic ozone build-up as reported by observational and modeling studies. In this paper, the tropical upwelling and extratropical downwelling in the stratosphere from coupled chemistry climate model results are used to investigate this “inconsistency”. It is found that the opposing trends between the tropical upwelling (or the total Northern Hemisphere downwelling) and the winter Arctic ozone build-up are directly related to the spatial structure of the trend in the strength of the BDC. The long-term changes in the Northern Hemisphere downwelling have different signs between the midlaitudes and high latitudes. The increase in the tropical upwelling is primarily balanced by enhanced downwelling at the midlatitudes (30^oN-50^oN). The downwelling over the Northern Hemisphere high latitudes (50^oN-90^oN) is reduced. The decrease in the strength of the BDC in the sub-polar middle stratosphere is strongly correlated with the reduction of the Arctic ozone build-up, consistent with the poleward and downward transport of ozone by the BDC. The mechanisms governing the spatial structure of the trend in the strength of the BDC are investigated. The relationship between the planetary wave activity entering the lower stratosphere (as represented by the meridional heat flux at 100 hPa) and the Arctic ozone is also clarified.