Sensitivity of Stationary Circulations to Global Warming: A First-Baroclinic Mode Theory in a Model Hierarchy

Zonally asymmetric circulations maintain an intense hydrologic contrast between monsoon regions and subtropical drylands in Earth’s present climate. Simulations of 21st century global warming scenarios suggest that zonal contrasts in subtropical precipitation and surface evapotranspiration will increase during local summer, with part of this projected change associated with variations in the zonally asymmetric component of the vertical mass flux. Yet despite the importance of zonally asymmetric circulations in the hydrological cycle and its variations, existing theories for hydrological change are focused on the zonal mean. Here, we propose a novel mechanism to explain projected changes in the strength of zonally asymmetric circulations with global warming.

Using an analytical convective quasi-equilibrium (QE), first-baroclinic mode framework, we relate changes in the circulation to changes in tropopause height and in the zonal tropospheric temperature gradient (ZTG). In this framework, a robust increase in tropopause height strengthens zonally asymmetric circulations as climate warms. Circulation strength also increases with the ZTG, but the ZTG can either weaken or strengthen with a global warming, depending on surface properties. A simple QE closure that relates the ZTG to near-surface thermal properties is used to express stationary circulation changes with global warming in terms of surface boundary conditions and radiative-convective properties of the tropical atmosphere.

We demonstrate the relevance of this mechanism in an idealized moist GCM integrated over a wide range of climates, as well as in a large ensemble of comprehensive climate simulations of a 21st century global warming scenario. Our first-baroclinic mode theory provides a novel and dynamically based understanding of stationary circulation sensitivity to climate change, complementing existing energetic constraints of the tropical circulation.