Sensitivity of Basinwide Meridional Overturning to Diapycnal Diffusion and Remote Wind Forcing in an Idealized Atlantic–Southern Ocean Geometry

Abstract

Recent numerical experiments indicate that the rate of meridional overturning associated with North Atlantic Deep Water is partially controlled by wind stress in the Southern Ocean, where the zonal periodicity of the domain alters the nature of the flow. Here, the authors solve the cubic scale relationship of Gnanadesikan to find a simple expression for meridional overturning that is used to clarify the relative strength of the wind-forced component. The predicted overturning is compared with 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 confidence in the numerics of both models. Level model experiments with horizontal diffusivity have similar qualitative behavior but somewhat different sensitivity to forcing. The paper highlights the difference in meridional overturning induced by changes in wind stress or vertical diffusivity. Strengthening the Southern Ocean wind stress induces a circulation anomaly in which most of the water is subducted in the Ekman layer of the wind perturbation region, follows isopycnals down into the thermocline, and changes density again when the isopycnals near the surface in the Northern Hemisphere. Approximating the circulation anomaly by this subduction route allows for a surprisingly accurate prediction of the resulting heat transport anomaly, based on the surface temperature distribution. Some of the induced flow follows a second, near-surface northward route through low-latitude water that is lighter than the subducted flow. Overturning anomalies far from the wind stress perturbations are not completely determined by wind stress in the zonally periodic Southern Ocean: wind stress outside the periodic region strongly influences the transport of heat across the equator primarily by changing the temperature of the flow across the equator.