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, and with the goal of understanding the mechanisms and timescales governing the extratropical-to-tropical connection. Two distinct stages are observed in the teleconnection: (1) The 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 decreases and reduces evaporation, and the resulting change to sea surface temperature leads to the development of a cross-equatorial Hadley Cell and the Intertropical Convergence Zone shift to the warmer hemisphere. The Hadley Cell weakening in the first stage is related to decreased eddy momentum flux divergence, and with a response time scale independent of the mixed layer depth. In contrast, the time scales of the development of the cross equatorial cell in the latter stage increase 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. Our analysis combines the momentum and energetic perspectives on the extratropical-to-tropical teleconnection, and moreover shows that the subtropical circulation changes associated with the momentum budget occur with a timescale that is distinct from the deep tropical response determined by the thermal inertia of the tropical ocean.