Eddy-mediated Hadley cell expansion caused by zonally-symmetric angular momentum adjustment

Abstract:

A poleward expansion of the Hadley cells is one of the most robust modeled responses to increasing greenhouse gas concentrations. There are many proposed mechanisms for expansion, and most are consistent with modeled changes in thermodynamics, dynamics, and clouds. The adjustment of the eddies and the mean flow to greenhouse gas forcings, and to each other, complicates any effort toward a deeper understanding of this and other circulation responses.

A simple modification to the Gray Radiation AND Moist Aquaplanet (GRANDMA) model that decouples the zonal mean eddy forcings from the zonally-symmetric circulation reveals the direct responses of the zonally-symmetric circulation to greenhouse-gas-like forcings and to changes in the eddies.

We will show evidence that the zonally-symmetric response to a greenhouse-gas-like forcing - an increase in subtropical angular momentum - drives the poleward shift of the eddy momentum stresses and the resulting poleward expansion of the Hadley cells. We will also show evidence that increases in static stability and changes in baroclinic instability do not drive Hadley cell expansion.

Extending this decoupling approach to other aspects of the atmosphere, and to more comprehensive earth system models, could provide more clarity on mechanisms for variability and future change.