On the formation of surface to bottom fronts over steep topography

Abstract

Imposed horizontal density differences in a laterally unbounded rotating fluid across a bottom of variable depth lead to the formation of surface to bottom fronts with enhanced asymmetry. In a two‐layer fluid, the down‐slope spillage of heavy fluid leads to greater lower layer penetration than the extent of upper layer advance in the opposite direction. These finite amplitude movements generate barotropic flows that have the same direction as the velocity in the layer that thins as a result of the movement. Lower‐layer movements up the depth gradient enhance the generation of lower‐layer flow due to vortex foreshortening and favor a barotropic flow in the same direction. In contrast, lower‐layer movements down the depth gradient enhance the upper‐layer flow for the same reason and favor a barotropic flow in the direction of that flow. Extended down‐slope movement of heavy fluids also leads to the formation of isolated heavy fluid lens over the bottom, within which the flow is reminiscent of observed bottom currents such as that associated with the cold, dense Norwegian Sea water south of the Denmark Strait (Smith, 1976). In addition to bodily displacing the front in the Ekman sense, uniform surface stress directed with deep (shallow) fluid to the right flattens (sharpens) the front. The displacement of the front as a whole also gives rise to additional vortex stretching that leads to further deviation from symmetry in the cross‐front distribution of the barotropic transport.