Processes of breaking of large-amplitude unsteady lee waves leading to turbulence



[1] The transition to turbulence after excitation of large-amplitude (~200 m) unsteady lee waves in Amchitka Pass, Alaska, is investigated using a nonhydrostatic vertically two-dimensional model with realistic topography. The model resolves motions two orders smaller than a large-amplitude unsteady lee wave, which is excited in the lee of the ridge, and shows that transition processes near the ridge top and downstream of the first trough of the unsteady lee wave are different. Near the ridge top, three stages of transition are identified. In the first stage, convection begins on the upstream sides (forward wave breaking) and downstream sides (backward wave breaking) of the crests of the unsteady lee wave. In the next stage, Kelvin-Helmholtz (KH) waves develop in regions of enhanced shear between statically unstable regions and downslope flow on the bottom. In the last stage, Tollmien-Schlichting (TS) waves develop on the bottom, under the KH waves, and form vortices, which finally break down. To the best of the authors’ knowledge, this is the first paper to report on the occurrence of backward wave breaking and the possibility of TS wave excitation in the ocean. Downstream of the first trough of the unsteady lee wave, flow is separated from the bottom by an adverse pressure gradient attributed to the unsteady lee wave. The separated flow forms vortices, which are shed quasi-periodically. Diapycnal mixing is enhanced by the development of KH and TS waves and flow separation, as well as by convection due to overturning isopycnals induced by the unsteady lee wave.