Acceleration, creation, and depletion of wind-driven, baroclinic Rossby waves over an ocean ridge

The influences of topography on the propagation. spatial patterns, and ampl itude variations of long baroclinic Rossby waves are investigated with a wi nd-forced. two-layer model above a midocean ridge. With steep topography th e evolution equation for the baroclinic mode is shown to differ from that f or a flat bottom in several ways: 1)The phase speed is systematically faste r by the factor H/H-2, where H is the total ocean depth and H-2 is the lowe r layer thickness, though the propagation remains westward and nearly nondi spersive; 2) an effectively dissipative transfer to the barotropic mode occ urs 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 lon gitudinally varying topography, which can be a large factor, but the amplif ication is only by the modest factor H/H-2 for a latitudinally varying topo graphy. Effects 2 and 3 are the result of energy exchanges to and from the barotropic mode, respectively. Effect 3 causes freely propagating, baroclin ic Rossby waves to be generated west of the ridge. These effects collective ly cause distortions of the baroclinic wave pattern as it traverses the rid ge. These effects account qualitatively for several features seen in altime tric measurements in the vicinity of major topographic features: an increas e in variance of baroclinic signals on the west side, an enhanced phase spe ed overall (compared to hat-bottom waves), and an abrupt change In the phas e speed at midocean ridges.