Sensitivity of tropical responses to the latitudinal position of zonally asymmetric thermal forcing in an aqua-planet setting

Abstract:

We investigate the sensitivity of tropical response to the latitudinal position of zonally asymmetric thermal forcing. A series of aqua-planet experiments is used where ocean heat flux, either heating or cooling, with finite longitudinal width is prescribed at different latitudes in the Northern Hemisphere. The forcing located poleward of 30deg produces homogenized tropical precipitation response, whereas the forcing equatorward of 30deg leads to a localized tropical response. This implies that zonally asymmetric forcing can produce homogenized tropical response when its effect is effectively advected by midlatitude jet.

The magnitude of tropical precipitation response is proportional to the cross-equatorial atmospheric energy fluxes, which is determined by the competition between the negative feedback from clear-sky fluxes and the positive feedback from cloud effects. Both the negative and positive feedbacks respond stronger to prescribed cooling than to heating. However, the positive feedback is strengthened more than the negative feedback, so that the tropical precipitation responds more effectively to cooling. The result suggests that remote tropical impacts of aerosols emitted in the mid-latitudes will depend on the sign of aerosol radiative forcing.