The morphology of the Brewer-Dobson circulation and its response to climate change in CMIP5 simulations

This article analyses the annual mean vertical and latitudinal structure of the Brewer-Dobson circulation in the CMIP5 models. The strength of the tropical mass upwelling is found to increase at all altitudes throughout the stratosphere due to climate change. However, the width of the tropical upwelling region narrows below about 20 hPa, and widens above 20 hPa, suggesting different physical mechanisms may play a role in this change above and below 20 hPa. In the lower stratosphere, an equatorward shift in the stationary wave critical line allows waves to propagate further into the Tropics. However, in the upper stratosphere, where the behaviour is dominated by what happens during the winter, an increase in the extratropical zonal mean westerly jet leads to a reduced equatorward refraction of planetary waves. The seasonal cycle of the change in the Brewer-Dobson circulation is also considered, and differences are found in the latitudinal structure of the increased extratropical downwelling between the Northern and Southern Hemispheres in winter.