Localization of deep ocean convection by a mesoscale eddy

Abstract

Observations of open-ocean deep convection indicate that it is a highly localized phenomenon, occurring over areas of tens of kilometers in diameter. The cause of this localization has been ascribed to "preconditioning" - the local weakening of the stable density stratification associated with upwardly domed isopycnal surfaces in a surface-intensified cyclonic circulation. However, most numerical and laboratory studies of localized convection have prescribed the localization artificially, by confining the surface buoyancy loss to a circular disk. In contrast, in the numerical simulations described here, deep convection forced by horizontally uniform buoyancy loss is localized within a region of initially weaker stratification than its surroundings. The preconditioned region is associated with a cold-core cyclonic eddy in geostrophic and cyclostrophic balance. As in previous studies of disk-shaped cooling, the localized convection region undergoes baroclinic instability at late times, causing the breakup of the convection region into multiple eddies. However, since the stratification structure determines the extent of vertical buoyancy fluxes, the region of deepest heat loss migrates with the eddy structures. Furthermore, the localized convection generates a secondary circulation in the plane perpendicular to the original eddy circulation. This circulation initiates the restratification of the eddy core even before the onset of baroclinic instability. Eventually the original preconditioned eddy is destroyed by the baroclinic eddy fluxes. The net effect of the eddy on the mean temperature field is to ensure a warmer upper layer and colder pycnocline than would be achieved with horizontally uniform convection. The authors investigate the dependence of this flow evolution on the size and strength of the preconditioned eddy.