Current Shear–Inertial Wave Interaction in the Sargasso Sea

Abstract

Acoustic Doppler profiler measurements of inertial waves embedded within the high shell region of a cold core ring in the Sargasso Sea are described. By repeatedly traversing the same point from different directions, a sampling pattern resembling an asterisk was obtained. The data reveal the presence of two different wave signals which are advected through the test region: a strong, monochromatic, downgoing wave, and a less well defined ensemble of mostly downgoing waves. Calculations of the phase of the vertical shear and coherence in the vertical and horizontal planes establishes the horizontal and vertical wavenumbers. These are 33 m and 11.8 km, respectively, and the wave propagates in a nearly cross-stream direction. The weaker ensemble of waves advected through the test region later in the experiment has similar dominant scales: ?30 m in the vertical and a horizontal wavelength in the range 11.6?30.0 km. For all of these waves, the ratio of vertical to horizontal wavelength is small and the intrinsic frequency is 1.09f, so that the waves are of near-inertial frequency. By examining individual terms in the near-inertial ray equations, it is seen that the horizontal crosscurrent variations cause the horizontal wavenumber to be rotated so as to propagate perpendicular to the current. Consideration of the full ray equations for all internal wave frequencies in a barotropic current shows a filtering effect that also preserves waves propagating across the current. Both of these results are consistent with the observation that the waves are propagating nearly cross stream.