Transient Diapycnal Mixing and the Meridional Overturning Circulation

Abstract

An idealized three-dimensional model of buoyancy-forced flow in a single hemisphere is used to investigate whether transient diapycnal mixing can sustain the meridional overturning circulation. In the annual mean, mixing transience had little effect on the meridional overturning. When mixing occurred on basin boundaries, the overturning strength was found to be insensitive to mixing transience. For mixing that was highly localized in space away from basin boundaries, oceanic meridional mass and heat transport decreased as mixing became more transient. The increased sensitivity in the highly localized case is likely due to the inhibition of surface heat flux into the thermocline. The dynamic response to transient mixing featured large-scale, internal oscillations that increased in amplitude with the transience of mixing but were confined to the Tropics and had little effect on the overturning cell through midlatitudes. These results indicate that transient diapycnal mixing, with a distribution suggestive of tropical cyclones, can effectively drive a meridional overturning.