Equatorial wave-mean flow interaction: the long Rossby waves
Proehl, Jeffrey A., 1958-
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The interaction of long equatorial Rossby waves with mean zonal currents in the ocean is investigated in a continuously stratified finite difference numerical model. The model allows for realistic specification of the mean state including both vertical and meridional shear of the mean flow.In addition to stretching of wave scales expected from slowly varying wave theory, one effect of strongly sheared mean flows is to cause meridional scattering of wave energy. This scattering, which cannot be treated by slowly varying theory, causes wave energy input at the surface in an equatorially confined structure to be sent poleward. This scattering of wave activity, depicted by reflection of the Eliassen-Palm flux off the mean shear, causes structural changes to the no-flow wave solutions. As a result, for realistic mean flows, wave energy appears at higher latitudes and a shadow zone occurs below the Equatorial Undercurrent in the near equatorial zone. The lateral shear of the Undercurrent also causes equatorward focusing of wave energy input at higher latitudes.The case of westward flow shows that the changes in resonant phase speeds and wave structures can be very large. Due to the scattering nature of rapidly varying flow, the presence of an equatorially confined critical layer does not preclude the radiation of wave activity into the deep ocean as it does for the Kelvin waves (McPhaden et al., 1986). However, wave activity which reaches the deep ocean does so at higher latitudes.The wave induced mean accelerations for both the resting ocean and for an eastward Undercurrent in this model are dominated by frictional dissipation of wave energy and, therefore, are relatively weak. The induced accelerations for westward flows, where (c-U) $\to$ O, show behavior similar to Kelvin waves in eastward flow in that large divergences of the EP flux exist leading to large momentum transfers from wave to mean states. The primary difference from the Kelvin waves is that the wave-induced residual circulation is important locally and leads to significant accelerations. These accelerations are primarily due to Coriolis torque with a lesser contribution from the residual advection of mean momentum.
- Oceanography