Sensitivity of the meridional overturning circulation to the pattern of the surface density flux

Abstract

The dynamic response of the thermocline to the thermodynamic forcing at the sea surface is analyzed in terms of its ability to influence the pattern of the oceanic meridional overturning. The technique utilized in this study expands upon an improved version of Walin's (1982) water-mass transformation theory, developed by Radko et al. (2008). Utilizing a general circulation model (GCM), two series of experiments are performed where surface air-sea density flux distributions are systematically perturbed in different locations to test the Meridional Overturning Circulation (MOC) response. The experiments are diagnosed using the water-mass transformation theory, which makes it possible to quantify the role of adiabatic processes in the global overturning circulation. Through these tests, this study finds that adiabatic processes dominate the MOC in the upper ocean and that the general circulation structure can be described using the improved water-mass transformation model. However, strength of flow in the Western Boundary regions is not well described by the theoretical model. The highest sensitivity of the MOC to thermodynamic forcing is found in the eastern Atlantic circumpolar regions while western boundary regions are characterized by low sensitivity. Further, the MOC does not show a high sensitivity to heating perturbations in the context of a larger zonal temperature structure. The finding is significant in its potential application to U.S. Navy's ocean modeling efforts and long-term strategy related to climate change.