A mechanism for zonal-asymmetric circulation and precipitation response to global warming in the subtropics

Zonally asymmetric circulations maintain an intense hydrologic contrast between monsoon regions and drylands in the present climate of Earth's subtropics. Simulations of 21st century global warming scenarios suggest that zonal contrasts in subtropical precipitation and surface evapotranspiration will increase during local summer, especially over land. Part of this projected change is associated with substantial variations in the zonally asymmetric vertical mass flux. Yet despite the clear importance of zonally asymmetric circulations in the hydrological cycle and its variations, existing theories for hydrological change are focused strongly on the zonal mean. A comprehensive understanding of zonally asymmetric tropical and subtropical circulations and their contribution to the hydrologic cycle remains elusive. Furthermore, simulations of 21st century climate exhibit little quantitative agreement on the magnitude of subtropical hydrological changes.

Here, we suggest a novel mechanism to explain projected changes in the strength of zonally asymmetric circulations with global warming. Using an analytical convective quasi-equilibrium, first-baroclinic mode framework, we relate changes in the vertical structure of the circulation to tropopause height and to the land-ocean contrast in near-surface moist entropy. In this framework, an increase in tropopause height strengthens zonally asymmetric circulations as climate warms, while the land-ocean contrast in entropy may weaken or strengthen those circulations depending on surface properties. We first demonstrate the relevance of this mechanism over a wide range of climates using an idealized moist GCM. We then use this mechanism to quantify the influence of changes in tropopause height and near-surface thermal properties on changes in zonally asymmetric circulations in CMIP5 simulations of next-century climate.