LOCALIZATION OF DEEP-OCEAN CONVECTION BY A MESOSCALE EDDY

Observations of open-ocean deep convection indicate that it is a highl y localized phenomenon, occurring over areas of tens of kilometers in diameter. The cause of this localization has been ascribed to ''precon ditioning''- the local weakening of the stable density stratification associated with upwardly domed isopycnal surfaces in a surface-intensi fied cyclonic circulation. However, most numerical and laboratory stud ies of localized convection have prescribed the localization artificia lly, by confining the surface buoyancy loss to a circular disk. In con trast. in the numerical simulations described here, deep convection fo rced by horizontally uniform buoyancy loss is localized within a regio n of initially weaker stratification than its surroundings. The precon ditioned region is associated with a cold-core cyclonic eddy in geostr ophic and cyclostrophic balance. As in previous studies of disk-shaped cooling, the localized convection region undergoes baroclinic instabi lity at late limes, causing the breakup of the convection region into multiple eddies. However, since the stratification structure determine s the extent of vertical buoyancy fluxes, the region of deepest heat l oss migrates with the eddy structures. Furthermore, the localized conv ection generates a secondary circulation in the plane perpendicular to the original eddy circulation. This circulation initiates the restrat ification of the eddy core even before the onset of baroclinic instabi lity. Eventually the original preconditioned eddy is destroyed by the baroclinic eddy Auras. The net effect of the eddy on the mean temperat ure held is to ensure a warmer upper layer and colder pycnocline than would be achieved with horizontally uniform convection. The authors in vestigate the dependence of this how evolution on the size and strengt h of the preconditioned eddy.