Mechanisms for Extreme Hadley Cell Expansion

Abstract:

We examine mechanisms associated with extreme expansions of the tropical belt. Using long equilibrium climate simulations, we diagnose the physics of large amplitude tropical widening events, exploring especially the role of decadal ocean variability. Our focus is on Northern Hemisphere wintertime conditions. We characterize large shifts in the tropical belt using various indices of the Hadley cell, subtropical dry zones, and relate these to regional patterns of drought over Asia, North America, and Europe. It is shown that a distinctive pattern of global SST variation, resembling the tropical-cold phase of the Pacific-Decadal Oscillation (PDO) is associated with and is responsible for the expanded phase of the Hadley cell. We hypothesize the observational indications for a rapid Hadley Cell expansion since 1979 can be explained in part by such a mechanism involving a transition to the PDO cold phase during 1979-2013. This mechanism for Hadley cell widening is compared to other factors, in particular the widening expected from anthropogenic climate change. This comparison of different plausible mechanisms provides a framework for understanding why observed tropical widening since 1979 has apparently been greater than expectations from global climate model simulations.