Disentangling the Effects of Eddy Fluxes of Heat and Momentum on the Hadley Circulation

Abstract:

Large-scale eddies originating in midlatitudes have been shown to play an important role in determining the strength of Earth's Hadley circulation. Previous work has focused primarily on the atmospheric angular-momentum budget and the effect of eddies via their transport of momentum. But midlatitude eddies are also associated with a substantial fraction of the poleward heat transport in the subtropics and midlatitudes, and examining the energy budget provides a different perspective on the effects of eddies on the zonal-mean circulation. Here, we contrast these two perspectives using idealized simulations of a Hadley circulation with and without the effects of large-scale eddies. In simulations run with a fixed distribution of sea-surface temperature, the Hadley circulation strength is relatively insensitive to the presence of eddies despite their strong influence on the angular-momentum budget. In slab-ocean simulations, on the other hand, large-scale eddies can act to amplify the Hadley circulation substantially. These results point to the importance of considering the energetic perspective in order to understand eddy influences on Earth's Hadley circulation. We discuss the implications of this conclusion for theories of monsoon onset as well as predictions of the behavior of the Hadley circulation in future climates.