Mechanisms of an Intensified Hadley Circulation in Response to Solar Forcing in the Twentieth Century

Abstract

Ensemble experiments with a global coupled climate model for thetwentieth century with time-evolving solar, greenhouse gas (GHG),sulfate aerosol (direct effect), and ozone (tropospheric andstratospheric) forcing are analyzed to show that solar forcing producescoupled dynamical interactions in the tropics that strengthen regionalHadley and Walker circulation regimes over the first half of thecentury. Solar forcing produces feedbacks involving temperaturegradient-driven atmospheric circulations that can alter clouds. Overrelatively cloud-free oceanic regions in the subtropics, greater solarforcing in mid-century compared to the early century produces greaterevaporation, more moisture transport into the precipitation convergencezones, intensified regional Hadley and Walker circulations, less cloudsover the subtropical ocean regions, and even more solar input. Coupleddynamical interactions produce upper-ocean heat content anomalies inconcert with positive sea surface temperature (SST) anomalies thatintensify precipitation over the South Indian, South Pacific, and SouthAtlantic Convergence Zones, as well as the South Asian and West Africanmonsoons. Coupled regional responses are most evident when the solarforcing occurs in concert with increased greenhouse gas forcing of aboutthe same magnitude over the first half of the century. The latter isalso altered by interaction with solar forcing, and the base-statetropical SSTs are increased in the relatively cloud-free subtropicalregions of low-level moisture divergence to fuel the regional feedbacksinduced by the spatially differentiated solar forcing. Consequently, thegreater solar forcing acting in concert with increased greenhouse gasesduring the early twentieth century produces larger increases of tropicalprecipitation, calculated as a residual for the solar forcing, than forearly-century solar-only forcing, even though the size of the solarforcing is the same.