Restratification after Deep Convection

Abstract

An important, yet poorly understood, aspect of the water mass transformation process in the ocean is the manner in which the convected fluid, once formed, is accommodated and drawn into the general circulation. Following ?violent mixing? in the open ocean that creates a deep homogenous body of fluid, restratification of the surface (?500 m) layer is observed to occur rapidly, sealing over the convection patch. Recent hydrographic casts and tomography inversions in the Gulf of Lions by Send et al., for example, show that very quickly, within a week or so of the cessation of cooling, a stratified near-surface layer develops on top of the mixed patch. This restratification occurs much more rapidly than can be accounted for by air?sea fluxes. By analytical and numerical study the authors argue that advection by geostrophic eddies spawned by the baroclinic instability of the mixed patch is likely to be a principal mechanism by which restratification occurs. A restratification timescale, τrestrat ≈ 56r/(Nh), where r is the radius of the patch of mixed water, h its depth, and N the ambient stratification, can be deduced from the magnitude of the lateral buoyancy flux associated with the geostrophic eddy field. This formula finds support from numerical results and is in broad agreement with the observations. Finally the results of the study are used to interpret recent field observations in the Labrador and Mediterranean Seas.