Shear instability and gravity wave saturation in an asymmetrically stratified jet

Motivated by the mean current and stratification structure associated with the equatorial undercurrent (EUC), we examine the stability and wave propagation characteristics of a highly idealized model flow: the asymmetrically stratified jet. This is a parallel shear flow in which the depth-varying current has the sech(2) form of a Bickley jet. The stratification has a step function structure: the buoyancy frequency takes uniform values above and below the center of the jet, with the larger value occurring below. The spectrum contains three classes of unstable normal modes. Two are extensions of the sinuous and varicose modes of the unstratified Bickley jet; the third has not been described previously. The asymmetric stratification structure allows instabilities to radiate gravity wave energy from the upper flank of the jet to the lower flank, where it encounters a critical layer. From here, wave energy may be reflected, absorbed or transmitted. Absorption results in wave saturation and momentum transfer to the mean flow, in close analogy with the breaking of orographic gravity waves in the middle atmosphere. Transmission beyond the lower flank may partly account for wave signals observed in the deep equatorial oceans. All of these processes exert zonal forces on the jet that alter its speed and shape. The wave structures and associated fluxes developed by the idealized model are compared with observations of the EUC. (C) 2002 Elsevier Science B.V. All rights reserved.