THE DRIFT OF MIDOCEAN JETS

The migration of nonlinear frontal jets is examined using an inviscid ''reduced gravity'' model. Two cases are considered in detail. The fir st involves the drift of deep jets situated above a sloping bottom, an d the second addresses the zonal beta-induced migration of meridional jets in the upper ocean. Both kinds of jets are shallower on their lef t-hand side looking downstream (in the Northern Hemisphere). For the f irst case, exact nonlinear analytical solutions are derived, and for t he second, two different methods are used to calculate the approximate migration speed. It is found that deep oceanic jets migrate along iso baths (with the shallow ocean on their right-hand side) at a speed of g' S/f0 (where g' is the reduced gravity, S the slope of the bottom, a nd f0 the Coriolis parameter). This speed is universal in the sense th at all jets migrate at the same rate regardless of their details. By c ontrast, upper-ocean meridional jets on a beta plane drift westward at a speed that depends on their structure. Specifically, it is shown th at this drift is the average of the two long planetary wave speeds on either side of the front: namely, C = -beta(R(d+)2 + R(d-)2)/2, where R(d+)(R(d-)) is the deformation radius based on the undisturbed depth east (west) of the jet; for frontal jets the above formula gives half the long Rossby wave speed. Both kinds of drift occur even if the jets in question are slanted; that is, it is not necessary that the deep j ets be directly oriented uphill (or downhill) or that the upper-ocean jets be oriented in the north-south direction. For the drifts to exist , it is sufficient that the deep jets have an uphill (or downhill) com ponent and that the beta-plane jets have a north-south component. Poss ible application of this theory to the jet observed during the Local D ynamic Experiment, which has been observed to drift westward, is discu ssed.