Convective Modifications of a Geostrophic Eddy Field

Sonya Legg Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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James C. McWilliams Institute for Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California

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Abstract

Ocean convection often occurs in regions of mesoscale eddy activity, where convective mixing and geostrophic eddy dynamics interact. The authors examine the interactions between a group of geostrophic eddies and convective mixing induced by surface buoyancy loss through a series of numerical simulations using a nonhydrostatic Boussinesq model. The eddies are initially baroclinic, with a surface-intensified density anomaly and sheared flow, but they are stable to baroclinic instability because of their small size. In the absence of buoyancy loss, eddy mergers occur much as in previous studies of geostrophic turbulence. With the addition of surface buoyancy loss, the surface stratification is eroded by small-scale convection. The convective mixing is highly heterogeneous, being deeper in regions of weaker initial stratification and shallower in more strongly stratified regions. The deformation radius is reduced in mixing regions and the weakly stratified eddies become baroclinically unstable. The barotropic component of kinetic energy increases as convection proceeds, largely due to the conversion of the available potential energy of the eddies in the baroclinic instability process. The convective forcing therefore provides a means of increasing the barotropic component of the eddy kinetic energy, by enabling the baroclinic instability. The fluid is efficiently homogenized by the energetic eddy field, leading to a few isolated eddies separated by a well-mixed fluid. These simulations provide a possible explanation for energetic eddy fields observed during convective periods in the Labrador Sea.

Corresponding author address: Sonya Legg, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.

Email: slegg@whoi.edu

Abstract

Ocean convection often occurs in regions of mesoscale eddy activity, where convective mixing and geostrophic eddy dynamics interact. The authors examine the interactions between a group of geostrophic eddies and convective mixing induced by surface buoyancy loss through a series of numerical simulations using a nonhydrostatic Boussinesq model. The eddies are initially baroclinic, with a surface-intensified density anomaly and sheared flow, but they are stable to baroclinic instability because of their small size. In the absence of buoyancy loss, eddy mergers occur much as in previous studies of geostrophic turbulence. With the addition of surface buoyancy loss, the surface stratification is eroded by small-scale convection. The convective mixing is highly heterogeneous, being deeper in regions of weaker initial stratification and shallower in more strongly stratified regions. The deformation radius is reduced in mixing regions and the weakly stratified eddies become baroclinically unstable. The barotropic component of kinetic energy increases as convection proceeds, largely due to the conversion of the available potential energy of the eddies in the baroclinic instability process. The convective forcing therefore provides a means of increasing the barotropic component of the eddy kinetic energy, by enabling the baroclinic instability. The fluid is efficiently homogenized by the energetic eddy field, leading to a few isolated eddies separated by a well-mixed fluid. These simulations provide a possible explanation for energetic eddy fields observed during convective periods in the Labrador Sea.

Corresponding author address: Sonya Legg, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.

Email: slegg@whoi.edu

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