The impact of changes in the strength of the Brewer-Dobson circulation on water entering the stratosphere

General circulation models suggest generally an increase in the strength of the stratospheric residual circulation in response to increasing greenhouse gases. This increase in cross-isentropic mass flux across the tropical tropopause will impact the amount of water entering the stratosphere, which in turn will affect the planetary radiative balance. Here, we analyse the change in stratospheric water vapor to processes that are directly attributable to the change in the residual circulation, deliberately excluding other effects in a changing climate on water vapour from processes not directly related to the strengthening of the residual circulation. We quantify the temperature change required to balance a change in the diabatic residual circulation in the TTL using radiative transfer calculations, including the temperature adjustment arising from changes in ozone due to changes in the timescale of transport across the tropical tropopause layer. We show that the change in transport timescale also impacts water entering the stratosphere, counteracting partially the impact of the temperature change. This effect is particularly pronounced during boreal summer, when we estimate this effect to offset about 15% of the temperature change. In the annual mean, the effect implies a scaling between temperature change and water vapour change that lies about 10% below that expected from the temperature dependence of the water vapour pressure.