A HETON MODEL OF THE SPREADING PHASE OF OPEN-OCEAN DEEP CONVECTION

A point-vortex heton model of the lateral dispersion of cold water formed in open-ocean deep convection is developed and studied as an idealized representation of the sinking and spreading phase of open-ocean deep convection. The overturning and geostrophic adjustment of dense fluid on and below the radius of deformation scale, formed by cooling on the large-scale, are parameterized in the model by introducing paired, discrete point vortices (hetons) of cyclonic sense in the surface layer, anticyclonic below, driving a cold baroclinic vortex. The convection site is imagined to be made up of many such baroclinic vortices, each with a vertically homogeneous core carrying cold, convectively tainted waters. The point vortices are introduced at a rate that depends on the large-scale cooling and the intensity assumed for each vortex. The interaction of many cold baroclinic vortices, making up a cloud, is studied using point-vortex Green's function techniques. The current solenoids of the individual elements sum together to drive a large-scale rim current around the convection site, cyclonic above, anticyclonic below, which is associated with a baroclinic zone on a scale of the order of the ambient radius of deformation. For parameters typical of open-ocean deep convection, the cloud of point vortices breaks down baroclinically on a time scale of a few days, into Rossby radius-scale ''clumps.'' These extended hetons efficiently flux the cold water away laterally from the convection site and affect an inward transfer of heat sufficient to offset loss to the atmosphere.