Directly Quantifying the Role of Eddies in Driving Hadley Circulation Expansion in an Idealized GCM

Anticipating the response of the Hadley circulation to anthropogenic forcings is critical to understanding the future evolution of earth’s climate. The Hadley circulation brings warm moisture-laden air to the equator, sustaining the tropical rains. As the air rises at the equator, flows poleward, and sinks in the subtropics, it dries the atmosphere and contributes to the formation of deserts on land and near-permanent cloudy skies over the oceans. An expansion of the Hadley circulation could shift and expand this regime poleward, significantly impacting the climate for a large fraction of earth’s population.

The processes driving the expansion of the Hadley circulation in response to greenhouse gas forcings are still poorly understood. In climate models and reanalyses, variability and change in the eddy-driven jet and Hadley circulation edge latitudes are shown to be strongly correlated. On the other hand, the subtropical jet latitude is uncorrelated with either. This suggests that wave-mean flow processes, rather than the thermally-direct circulation response, may drive a large fraction of Hadley circulation expansion. Assessing the importance of these two processes is essential as a majority of Hadley circulation scaling theories neglect the role of waves.

Here, the Hadley circulation is examined in an idealized gray radiation general circulation model in both a “full” (eddy-permitting) and axisymmetric configuration. The time-mean, zonal-mean eddy momentum, heat, and moisture flux convergences derived from the full control simulation are applied as forcing terms to the axisymmetric simulations, creating a climate that is essentially identical to the full control simulation and in which the wave-mean flow processes are permanently fixed to their unperturbed state. By applying the same greenhouse gas-like forcings to both simulations, the changes to the Hadley circulation driven by the thermally-direct circulation response are easily separated from those driven by changes to the eddies. The fraction of these changes driven by latent heating increases and the direct radiative response to greenhouse gas-like forcings is also quantified.