Decomposing the Eddy-Mean Flow Response of the Hadley Circulation to Greenhouse Gas Forcings

Unlike the global thermodynamic response to greenhouse gas forcings, the response of the Hadley circulation remains poorly understood. Some of the uncertainty originates from the circulation's coupling to zonal-mean eddy heat and momentum forcings. At equilibrium, the total response of the circulation includes the non-linear interaction between changes in the mean flow and changes in the eddies.This study employs a new modeling framework within an idealized GCM that cleanly separates the eddy forcings on the mean flow from the mean flow itself. In one version of the model, both the eddies and the mean flow freely evolve in response to a greenhouse-gas-like forcing. In another, only the mean flow is allowed to respond.With eddy forcings on the mean flow fixed to their control state, zonal-mean temperature displays the canonical greenhouse gas response: amplified warming in the upper-troposphere and a resulting increase in static stability, stratospheric cooling, polar amplification, and accompanying changes to meridional temperature gradients. Despite these changes, the Hadley circulation does not expand. Further experiments indicate that nearly all of Hadley circulation expansion is driven by changes in the eddies, with half of this response purely due to eddy changes in the interior of the Hadley circulation. Hence, thermodynamic changes do not appear to be direct drivers of expansion. Expansion also appears to be driven by more than just the momentum balance at the edge of the circulation.