A mechanism for the response of the zonally asymmetric subtropical hydrologic cycle to global warming


Time-mean, zonally asymmetric circulations (hereafter referred to as stationary circulations) maintain intense hydrologic contrasts in Earth’s subtropics in the present climate, especially between monsoon regions and deserts during local summer. Such zonal contrasts in hydrology generally increase in comprehensive GCM simulations of a warming climate, yet a full understanding of stationary circulations and their contribution to the hydrologic cycle in present and future climates is lacking. This study uses an idealized moist GCM to investigate the response of subtropical stationary circulations to global warming. Stationary circulations are forced by a prescribed subtropical surface heat source, and atmospheric infrared opacity is varied to produce a wide range of climates with global-mean surface temperatures between 267 and 319 K. The strength of stationary circulations varies nonmonotonically with global mean temperature in these simulations. Zonal asymmetries in precipitation increase with temperature in climates colder than or comparable to that of Earth but remain steady or weaken in warmer climates. A novel mechanism is proposed in which this behavior is caused by the changes in tropopause height and zonal SST gradients expected to occur with global warming. Casting this mechanism in terms of the first-baroclinic mode of the tropical troposphere produces a theory that quantitatively captures the nonmonotonic dependence of stationary circulation strength on global mean temperature. Zonally asymmetric changes in precipitation minus surface evaporation (P - E) are predicted by combining this dynamical theory with the tropospheric moisture changes expected if relative humidity remains constant.

Journal of Climate