Baroclinic Rossby Waves in a Square Basin

Abstract

The properties of forced baroclinic, quasigeostrophic Rossby waves in an ocean basin are discussed, with emphasis on the apparent phase speed. The response to a traveling wave wind stress curl consists of three parts, two of which propagate westward and one that tracks the wind. The apparent phase speed in the basin interior depends on the relative sizes of the amplitudes of these three terms, which vary with forcing frequency and scale and with the size of the deformation radius. With low-frequency, large-scale forcing, one westward wave dominates and its phase speed is the long baroclinic Rossby wave speed. With smaller forcing scales, the component that tracks the wind is comparably strong, and the superposition with the dominant westward wave can produce augmented apparent phase speeds. At frequencies larger than the maximum free wave frequency (proportional to the deformation radius), the westward waves are trapped in deformation-scale boundary layers, yielding a weak, directly forced interior. The choice of boundary conditions is found to affect strongly the response. In contrast to wind forcing, forcing by an imposed boundary oscillation yields propagation at the long-wave speed over a wide range of forcing frequencies.