Shallow Convection and Buoyancy Equilibration in an Idealized Coastal Polynya*

Abstract

The recent theoretical approach of Visbeck, Marshall, and Jones is used to examine shallow convection and offshore transport of dense water from an idealized coastal polynya. A constant negative buoyancy flux is applied in a half-elliptical region adjacent to a coastal boundary, surrounded by a forcing decay region with uniform width W over which the imposed buoyancy flux decreases smoothly to zero. Initially, the density beneath the forcing increases linearly with time. A baroclinically unstable front forms at the edge of the forcing region. The width of the front is imposed by the width of the forcing decay region, provided this distance is larger than the baroclinic Rossby radius. Baroclinic eddies, whose velocities are inversely proportional to W, develop along the front and exchange dense water from the forcing region with ambient water, eventually reaching an equilibrium in which the lateral buoyancy flux by eddies balances the prescribed surface buoyancy flux. The time to reach equilibrium t_e and the equilibrium density anomaly ρ_e are given by