Moist convection and the Hadley cell

Studies with general circulation models (GCMs) suggest that momentum transport by large-scale extratropical eddies plays a key role in determining of the strength of the equinoctial Hadley circulation. However, GCMs do not explicitly resolve moist convection, and recent work has found that the Hadley cell and monsoon circulations simulated by GCMs are also sensitive to the parameterization of convective momentum transport.

Here we use the diabatic acceleration and rescaling (DARE) technique to simulate idealized Hadley circulations on an equatorial beta-plane with an explicit representation of moist convection. We find that the Hadley circulation achieves a similar strength in simulations in which large-scale eddies are present and in simulations in which the domain is too narrow to allow extratropical eddies. This result is true for both equinoctial and solsticial conditions. In the narrow-domain simulations, eddy-momentum fluxes are weak, but streamlines of the descending branch of the Hadley cell are approximately aligned with angular momentum contours allowing for a strong circulation nonetheless. These results suggest that the Hadley circulation strength is not directly constrained by extratropical eddy-momentum fluxes, even under equinoctial conditions.