Revisiting the role of eddies in determining the strength of the Hadley circulation

Large-scale eddies originating in midlatitudes have been shown to play an important role in determining the strength of the Hadley circulation. Previous work has focused primarily on the atmospheric angular-momentum budget; in a flow regime where non-linear advection of angular momentum by the mean circulation is weak, eddy momentum fluxes strongly constrain the circulation strength. However, depending on the time of year, Earth's Hadley circulation may be relatively far from the linear limit. Here, we use idealized simulations on an equatorial beta-plane to explore the response of the Hadley circulation to large-scale eddies in a regime where non-linear momentum advection is important. In simulations run with a fixed distribution of sea-surface temperature (SST), the Hadley circulation strength is relatively insensitive to the presence of large-scale eddies despite their strong influence on the angular-momentum budget. In slab-ocean simulations, on the other hand, the eddies act to amplify the Hadley circulation substantially. The importance of the SST distribution for the Hadley circulation strength in our simulations suggests that one should consider the energy budget as well as the angular-momentum budget in order to fully understand the influence of large-scale eddies on the tropical-mean circulation.