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Equatorial Wave-Mean Flow Interaction: The Long Rossby Waves

Jeffrey A. ProehlAdvanced Study Program, National center for Atmospheric Research, Boulder, Colorado

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Abstract

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 changing of wave scales expected from slowly varying wave theory, the effect of strongly sheared mean flows is to cause strong scattering of wave energy. This scattering can cause large structural and dispersional changes to the wave solutions. As a result, for realistic mean flows, wave energy input near the equator 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 subundercurrent equatorial focusing of wave energy input at higher latitudes.

Results for westward flow show that 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 equatorial confined critical layer does not preclude the radiation of wave activity into the deep ocean as it does for the Kelvin waves. The wave induced mean accelerations for both the resting ocean and for an eastward undercurrent in this model are dominated by Fictional dissipation of wave energy and are relatively weak. The induced accelerations for westward flows, where the intrinsic phase speed (cU) ?? 0, show that large divergences of the wave action 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.

Abstract

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 changing of wave scales expected from slowly varying wave theory, the effect of strongly sheared mean flows is to cause strong scattering of wave energy. This scattering can cause large structural and dispersional changes to the wave solutions. As a result, for realistic mean flows, wave energy input near the equator 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 subundercurrent equatorial focusing of wave energy input at higher latitudes.

Results for westward flow show that 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 equatorial confined critical layer does not preclude the radiation of wave activity into the deep ocean as it does for the Kelvin waves. The wave induced mean accelerations for both the resting ocean and for an eastward undercurrent in this model are dominated by Fictional dissipation of wave energy and are relatively weak. The induced accelerations for westward flows, where the intrinsic phase speed (cU) ?? 0, show that large divergences of the wave action 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.

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