The Drift of Midocean Jets

Abstract

The migration of nonlinear frontal jets is examined using an inviscid ?reduced gravity? model. Two cases are considered in detail. The first involves the drift of deep jets situated above a sloping bottom, and the second addresses the zonal ?-induced migration of meridional jets in the upper ocean. Both kinds of jets are shallower on their left-hand side looking downstream (in the Northern Hemisphere). For the first case, exact nonlinear analytical solutions are derived, and for the second, two different methods are used to calculate the approximate migration speed. It is found that deep oceanic jets migrate along isobaths (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, and f0 the Coriolis parameter). This speed is universal in the sense that all jets migrate at the same rate regardless of their details. By contrast, upper-ocean meridional jets on a ? plane drift westward at a speed that depends on their structure. Specifically, it is shown that this drift is the average of the two long planetary wave speeds on either side of the front: namely, C = ??(R2d+ + R2d?)/2, where Rd+(Rd?) 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 jets 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) component and that the ?-plane jets have a north?south component. Possible application of this theory to the jet observed during the Local Dynamic Experiment, which has been observed to drift westward, is discussed.