The nonlinear response of an equatorial ocean to oscillatory forcing
Abstract:The effect of upper ocean variability on deep equatorial flow is simulated by prescribing a forcing wave field near the eastern boundary in a one and a half layer shallow water model. Equatorially trapped long Rossby waves are generated by the forcing, have approximately linear dynamics, and propagate westward. Near the western boundary the dynamics are nonlinear, and there is a large mean flow. With forcing symmetric about the equator, the nonlinear response has two distinct phases. When the interior flow in the vicinity of the equator is westward near the western boundary, a poleward flowing western boundary current forms. This flow separates from the boundary several deformation radii away from the equator. When the interior flow is eastward, a recirculation gyre sets up. This gyre has dynamics similar to the mid-latitude recirculation of the Gulf Stream. The zonal scale of the gyre depends not only on the amplitude of the interior wave field, but also on the period of oscillation and the magnitude of the viscosity. The meridional structure and amplitude of the zonal flow can be understood using a model of a constant potential vorticity zonally elongated gyre.
The net Lagrangian circulation resulting from the combination of the interior wave field and the nonlinear flow near the western boundary is found by tracking floats in the model. In the interior, fluid parcels move westward along the equator in the Stokes drift of the Rossby waves. The potential vorticity of fluid parcels is altered near the western boundary so that the floats are returned to the interior poleward of the equator. Significant mixing of fluid parcels occurs near the western boundary.
Document Type: Research Article
Publication date: 1993-08-01
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