Dynamic Excitation of Internal Gravity Waves in the Equatorial Oceans

Abstract

It is proposed that shear instability of the upper flank of the equatorial undercurrent may generate, under a broad range of conditions, downward propagating internal gravity waves (IGW) of large amplitude. The generation mechanism is shown to require only that the background stratification is weak where the shear is large (i.e., in the mixing region) and that the stratification is sufficiently large in the far field (i.e., near the thermocline). In a series of studies, the generation of IGW from unstable shear flows is examined. Linear theory is used to predict under what circumstances the generation of IGW may be large, and fully nonlinear simulations restricted to two dimensions are employed to provide estimates of the degree of vertical mixing and of the vertical transport of horizontal momentum by IGW. In particular, the simulations demonstrate that, when large amplitude IGW are generated by shear instability, the mean flow itself is significantly decelerated in the mixing region. The momentum flux associated with the radiating IGW is large, and it is proposed that these may act in part as a momentum source to the deep equatorial countercurrents.