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Salt feedback in the adiabatic overturning circulation


The adiabatic overturning circulation is the part of the meridional overturning circulation that persists in the limit of vanishing diffusivity. Two conditions are required for the existence of the adiabatic overturning circulation: a high-latitude zonally reentrant channel subject to surface westerlies and a set of outcropping isopycnals shared between the channel and the opposite hemisphere. This paper examines how different buoyancy forcing regimes, particularly freshwater flux, affect the surface buoyancy distribution and the strength of the adiabatic overturning circulation. Without freshwater forcing, salinity is uniform and buoyancy is determined by temperature only. In this case, the size of the shared isopycnal window is effectively fixed by the coupling between atmospheric and sea surface temperatures. With freshwater forcing (applied as a surface flux), the salinity, and thus the sea surface buoyancy and the size of the shared isopycnal window, is not specified by the atmospheric state alone. It is found that a salt-advection feedback leads to surface buoyancy distributions that increase the size of the isopycnal window and strengthen the adiabatic overturning circulation. The strength of the feedback is controlled by processes in high latitudes-the southern channel, where the surface salinity is determined by a balance between freshwater input from the atmosphere, salt input from upwelling deep water, and freshwater export by Ekman transport; and the Northern Hemisphere, where the overturning and wind-driven transport in the thermocline advect salty water from the subtropics, mitigating the freshening effect of the surface freshwater flux. The freshwater budget in the channel region provides an estimate of the size of the isopycnal window. © 2014 American Meteorological Society.

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