Acceleration, Creation, and Depletion of Wind-Driven, Baroclinic Rossby Waves over an Ocean Ridge

Abstract

The influences of topography on the propagation, spatial patterns, and amplitude variations of long baroclinic Rossby waves are investigated with a wind-forced, two-layer model above a midocean ridge. With steep topography the evolution equation for the baroclinic mode is shown to differ from that for a flat bottom in several ways: 1) The phase speed is systematically faster by the factor H/H2, where H is the total ocean depth and H2 is the lower layer thickness, though the propagation remains westward and nearly nondispersive; 2) an effectively dissipative transfer to the barotropic mode occurs whenever the baroclinic mode is locally parallel to f/H contours, where f is the Coriolis frequency; and 3) the wind-forced response is amplified in proportion to the topographic steepness, (f/H)(dH/dx)/(df/dy), for a longitudinally varying topography, which can be a large factor, but the amplification is only by the modest factor H/H2 for a latitudinally varying topography. Effects 2 and 3 are the result of energy exchanges to and from the barotropic mode, respectively. Effect 3 causes freely propagating, baroclinic Rossby waves to be generated west of the ridge. These effects collectively cause distortions of the baroclinic wave pattern as it traverses the ridge. These effects account qualitatively for several features seen in altimetric measurements in the vicinity of major topographic features: an increase in variance of baroclinic signals on the west side, an enhanced phase speed overall (compared to flat-bottom waves), and an abrupt change in the phase speed at midocean ridges.