ZONAL JETS OVER TOPOGRAPHY ON A BETA-PLANE, WITH APPLICATIONS TO THE KUROSHIO EXTENSION OVER THE SHATSKY RISE

The response of an initially zonal barotropic or baroclinic jet to an isolated seamount or a meridional ridge is investigated here using a three-dimensional primitive equation model with a free surface. An eastward jet with an axial speed U and a half-width L(y) over a beta plane is supercritical if the Froude/Rossby number (U/betaL(y)2) is greater than about 0.25, and subcritical otherwise. A westward jet with a negative U and hence a negative U/betaL(y)2 is always supercritical. Following this definition proposed by Armi, an obstacle renders barotropic zonal jets subcritical or less supercritical especially to the west of the obstacle. For an initially supercritical eastward jet, a significant obstacle projects its influence upstream through a migrating ''hydraulic jump,'' broadening and weakening the incoming jet. For subcritical and westward jets, the westward influence is achieved by the propagation of long Rossby waves. In the barotropic regime, the westward influence increases with \U/betaL(y)2\ and the height and width of the topography, and travels faster for westward jets. In the baroclinic regime, an eastward upper ocean jet is unlikely to be rendered subcritical and subject to upstream influences by a topography remotely below. However, the aforementioned barotropic dynamics is carried over to the baroclinic regime if the baroclinic jet does not vanish at great depths. A weak current at great depths, whether eastward or westward, can strengthen the barotropic response that leads to extensive jet bifurcation propagating upstream, with a minor branch to the north and a major branch to the south. Some meridional variations of topography would enhance the bifurcation. It is speculated that the 1979-80 anomalous bifurcation of the Kuroshio Extension far west of the Shatsky Rise may be a manifestation of barotropic upstream influences exerted by the rise.