Ridge waves

Abstract

Second-class wave propagation along mid-ocean ridges is investigated in an effort to explain subinertial peaks found in the velocity spectra over the Juan de Fuca Ridge (JdFR, 4 days) and the Iceland-Faeroe Ridge (IFR, 1.8 days). Topographic cross sections of the ridges are fit by a double-exponential depth profile and the linearized shallow water equations are solved with the simplified topography. In the northern hemisphere the western ridge flank supports an infinite set of modes for a topographically trapped northward propagating wave and the eastern flank supports southward propagating modes. The eigenfunctions are calculated and dispersion curves are examined for a variety of ridge profiles. Increasing the slope of a ridge flank increases the frequencies of the modes it supports. In addition, the waves travelling along the flanks 'feel' the topography of the opposite side so t hat increasing the width or steepness of the eastern slope decreases the frequencies of the modes supported by the western side (and vice versa). The dispersion characteristics of the trapped nondivergent oscillations allow a zero group velocity (ZGV) so that energy may accumulate along the ridge as long as the ridge does not approach the isolated shelf profile. Including divergence lowers the frequencies of the longest waves so that a ZGV may be found for all ridge profiles. The nature of the effects of stratification, represented by a two-layer model, are explored by a perturbation procedure for weak stratification. The 0(1) barotropic basic state is accompanied by an 0(E2) baroclinic perturbation. The frequencies of the barotropic modes are increased and the velocities are bottom-trapped. For reasonable values of stratification, however, this effect is small. Plugging the JdFR topography into the models produces an approximate 4-day ZGV wave with wavelengths between 1500 and 4500 km. The IFR oscillation, however, appears to be better modelled by a topographic-Rossby mode model. (Miller et al., 1996) The ridge wave models discussed here also predict the observed anticyclonic velocity ellipses over the ridge and horizontal decay away from the ridge crest.