EASTWARD JETS OVER DIVERGING ISOBATHS, WITH APPLICATIONS TO THE GULF-STREAM PAST THE GRAND-BANKS

A three-dimensional primitive-equation model is employed to investigat e how an initially eastward jet responds to isobaths that diverge east ward. This problem is investigated using first a barotropic model and then a baroclinic model. In the barotropic regime, the diverging isoba ths force an initially supercritical jet to widen and become subcritic al after a hydraulic jump. Increasing eastward advection of vorticity moves the diverging point eastward and narrows the jet locally but wid ens the jet farther downstream. The incoming jet axis is fixed at a pr eferred latitude due to the steep meridional bottom slope upstream. In the baroclinic regime, the eastward divergence of the jet becomes muc h more pronounced. This strong divergence leads to the bifurcation of the jet downstream. The northern branch flows northeastward and the ma in branch continues eastward. The former is a western boundary current along the northeastward continental slope. The bifurcation of barocli nic jets is caused by hydraulic jumps reinforced by instabilities of t he baroclinic current system. Among possible applications, the model i s particularly relevant to the bifurcation of the Gulf Stream to the s outh of the Grand Banks, where the essentially zonal isobaths begin to bifurcate eastward.