Sensitivity of subtropical stationary circulations to global warming in climate models: a baroclinic Rossby gyre theory

Abstract

Time-mean, zonally asymmetric circulations maintain an intense hydrologic contrast between monsoon regions and subtropical drylands in Earth’s present climate. Climate model simulations suggest that this hydrologic contrast will increase in twenty-first-century global warming scenarios, but the response of the zonally asymmetric circulations to global mean temperature is poorly understood. Here we adapt a simple theory for the strength of time-mean, subtropical, zonally asymmetric circulations (hereafter called stationary circulations) and demonstrate its relevance to summer stationary circulation changes in the Northern Hemisphere in an ensemble of comprehensive climate model simulations of global warming. The theory, which is based on the dynamics of a subtropical Rossby gyre that is in Sverdrup balance and has the vertical structure of a first-baroclinic mode, shows that the weakening of stationary ascent with global warming in the multi-model mean can be represented as a compensation between two processes: a lifting of the tropical tropopause and a decrease of the tropospheric zonal temperature gradient, which respectively require strengthening and weakening of vertical mass flux in the Rossby gyre. A large fraction of the intermodel variance in global warming-induced changes in stationary ascent is associated with intermodel variance in zonal tropospheric temperature gradient changes, which we in turn attribute to intermodel variance in zonal sea surface temperature gradient changes. These results show that much of the sensitivity of subtropical hydrologic contrasts to global mean temperature can be understood in terms of a linear vorticity balance and properties of moist adiabats.