Relative roles of energy and momentum fluxes in the tropical response to extratropical thermal forcing

Abstract

This study investigates the transient responses of atmospheric energy and momentum fluxes to a time-invariant extratropical thermal heating in an atmospheric model coupled to an aquaplanet mixed layer ocean with the goal of understanding the mechanisms and time scales governing the extratropical-to-tropical connection. Two distinct stages are observed in the teleconnection: 1) A decrease in the meridional temperature gradient in midlatitudes leads to a rapid weakening of the eddy momentum flux and a slight reduction of the Hadley cell strength in the forced hemisphere. 2) The subtropical trades in the forced hemisphere decrease and reduce evaporation. The resulting change to sea surface temperature leads to the development of a cross-equatorial Hadley cell, and the intertropical convergence zone shifts to the warmer hemisphere. The Hadley cell weakening in the first stage is related to decreased eddy momentum flux divergence, and the response time scale is independent of the mixed layer depth. In contrast, the time taken for the development of the cross-equatorial cell in the latter stage increases as the mixed layer depth increases. Once developed, the deep tropical cross-equatorial cell response is an order of magnitude stronger than the initial subtropical response and dominates the anomalous circulation. The analysis combines the momentum and energetic perspectives on this extratropical-to-tropical teleconnection and moreover shows that the subtropical circulation changes associated with the momentum budget occur with a time scale that is distinct from the deep tropical response determined by the thermal inertia of the tropical ocean.