John Marshall

Cecil and Ida Green Professor of Oceanography, MIT

Dynamics of isolated convective regions in the ocean

Dynamics of isolated convective regions in the ocean.

(Visbeck, M and Marshall, J and Jones, H), JOURNAL OF PHYSICAL OCEANOGRAPHY, vol. 26, no. 9, pp. pages, 1996.

Abstract

An initially resting ocean of stratification N is considered, subject to buoyancy loss at its surface of magnitude B-0 over a circular region of radius r, at a latitude where the Coriolis parameter is f. Initially the buoyancy loss gives rise to upright convection as an ensemble of plumes penetrates the stratified ocean creating a vertically mixed layer. However, as deepening proceeds, horizontal density gradients at the edge of the forcing region support a geostrophic rim current, which develops growing meanders through baroclinic instability. Eventually finite-amplitude baroclinic eddies sweep stratified water into the convective region at the surface and transport convected water outwar and away below, setting up a steady state in which lateral buoyancy flux offsets buoyancy loss at the surface. In this final state quasi-horizontal baroclinic eddy transfer dominates upright “plume” convection. By using “parcel theory” to consider the energy transformations taking place, it is shown that the depth, h(final), at which deepening by convective plumes is arrested by lateral buoyancy flux due to baroclinic eddies, and the time t(final) it takes to reach this depth, is given by h(final) = gamma (B(0)r*(1/3)/N, t(final) = beta (r*2)/B-0)*(1/3), both independent of rotation. Here gamma and beta are dimensionless constants that depend on the efficiency of baroclinic eddy transfer. A number of laboratory and numerical experiments are then inspected and carried out to seek confirmation of these parameter dependencies and obtain quantitative estimates of the constants. It is found that gamma = 3.9 +/- 0.9 and beta = 12 +/- 3. Finally, the implications of our study to the understanding of integral properties of deep and intermediate convection in the ocean are discussed.

doi = 10.1175/1520-0485(1996)0262.0.CO;2