Sensitivity of basin-wide meridional overturning to diapycnal diffusion and remote wind forcing in an idealized Atlantic-Southern Ocean geometry

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Description/Abstract

Recent numerical experiments indicate that the rate of meridional overturning associated with North Atlantic Deep Water is controlled both by mixing and by windstress in the Southern Ocean, where the zonal periodicity of the domain alters the nature of the flow. We find a simple approximate expression for meridional overturning as a solution to Gnanadesikan's (1999) cubic scale relation. We compare the predicted overturning to coarse-resolution numerical experiments with an idealized Atlantic-Ocean/Southern Ocean geometry. The scaling accurately predicts the sensitivity to forcing for experiments with a level model employing isopycnal diffusion of temperature, salinity, and “layer thickness". A layer model produces similar results, increasing our confidence in the numerics of both models. Level model experiments with horizontal diffusivity have similar qualitative behavior but somewhat different sensitivity to forcing.

We highlight the difference in meridional overturning induced by changes in windstress or vertical diffusivity. Wind-driven circulation anomalies outside the region of windstress perturbation include strongly cross-isopycnal flow near the surface and approximately along-isotherm flow in the thermocline. Overturning anomalies far from the windstress perturbations are not completely determined by windstress in the zonally-periodic Southern Ocean: windstress outside the periodic region strongly influences the transport of heat across the equator primarily by changing the temperature of the flow across the equator. Most of the total meridional heat transport across the basin can be decomposed into contributions due to the westerlies, easterlies, and vertical diffusivity; here we show how the westerlies contribution is related to the surface temperature profile.