Turbulent convection into a linearly stratified fluid: the generation of ‘subsurface anticyclones’

Abstract

Penetrative turbulent convection from a localized circular top source into a rotating, linearly stratified ambient fluid of strength N has been investigated in a laboratory tank. Initially, the induced three-dimensional convective flow penetrated rapidly into the stratified water column until it reached an equilibrium depth at which the convective flow began to propagate radially outward. At this stage, the usual cyclonic vortices were generated around the convection source at the edge of the radially propagating flow. Soon after, a thin ‘subsurface anticyclone’ was formed at the level of equilibrium depth beneath the convection source. Later, this anticyclone dominated the central part of the convective regime and did not allow new cyclones to be injected into the system. After reaching its maximum mean diameter Da/R ≈10(R0;R)2/3 and swirl velocity va ≈(B0R)1/3, an anticyclone became unstable and split into two new vortices that left the area beneath the source, allowing a new anticyclone to form at its original place (here, R0,R =(B0/f3R2) 1/2 is the Rossby number based on R the radius of the source, B0 is the surface negative buoyancy flux, and f is the Coriolis parameter). These observations provide crucial evidence that many of the ‘subsurface anticyclonic’ vortices detected in the stratified pycnocline of the central Arctic Ocean are indeed generated as a result of convective processes occurring in this region.