Convective modifications of a geostrophic eddy field

Ocean convection often occurs in regions of mesoscale eddy activity, where convective mixing and geostrophic eddy dynamics interact. The authors exami ne the interactions between a group of geostrophic eddies and convective mi xing induced by surface buoyancy loss through a series of numerical simulat ions using a nonhydrostatic Boussinesq model. The eddies are initially baro clinic, with a surface-intensified density anomaly and sheared flow, but th ey are stable to baroclinic instability because of their small size. In the absence of buoyancy loss, eddy mergers occur much as in previous studies o f geostrophic turbulence. With the addition of surface buoyancy loss, the s urface stratification is eroded by small-scale convection. The convective m ixing is highly heterogeneous, being deeper in regions of weaker initial st ratification and shallower in more strongly stratified regions. The deforma tion radius is reduced in mixing regions and the weakly stratified eddies b ecome baroclinically unstable. The barotropic component of kinetic energy i ncreases as convection proceeds, largely due to the conversion of the avail able potential energy of the eddies in the baroclinic instability process. The convective forcing therefore provides a means of increasing the barotro pic component of the eddy kinetic energy, by enabling the baroclinic instab ility. The fluid is efficiently homogenized by the energetic eddy field, le ading to a few isolated eddies separated by a well-mixed fluid. These simul ations provide a possible explanation for energetic eddy fields observed du ring convective periods in the Labrador Sea.