Frontal Instabilities of Baroclinic Oceanic Currents with Applications to the Gulf Stream

Abstract

The frontal instability of major baroclinic ocean currents such as the Gulf Stream is numerically studied here, using a three-dimensional, primitive-equation model. The model current is driven by a buoyant discharge near the surface from the light water side of the front. Subsequent geostrophic adjustment produces a baroclinic analysis in which a prescribed mean current is subject to eddy dissipation with no provision for its maintenance. At low latitudes small-amplitude unstable waves are generated. The eddy fluxes are shown to be largely upgradient and responsible for the maintenance of the front. At higher latitudes the model produces anticyclonic barotropic instability. The triggering mechanism for instability is related to the outward surge of the front during the initial stage of geostrophic adjustment, which in turn is related to the inertial oscillations of surface isopycnals. The outward surge is surface-trapped. To trigger instability, the surge must be shallow and intense in lower latitudes, and becomes deeper and weaker in higher latitudes. The Richardson number decreases below 0.25 shortly before and after onset of instability, and increases again after unstable waves have fully developed. The instability is initially of grid scale, and subsequently evolves into larger scales through nonlinear cascading processes, rendering themselves to baroclinic instability.