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TURBULENCE PATCHES AND FINITE-AMPLITUDE INTERNAL WAVES IN STRATIFIED FLUID WITH STABLE SHEAR

Hieu T. Pham
Department of Mechanical & Aerospace Engineering Scripps Institution of Oceanography University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093

Sutanu Sarkar
Mechanical and Aerospace Engineering, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093

Kraig B. Winters
Department of Mechanical & Aerospace Engineering Scripps Institution of Oceanography University of California, San Diego 9500 Gilman Dr., La Jolla, CA 92092

Résumé

Direct Numerical Simulation is performed to investigate the evolution of internal waves and turbulence inside a stratified medium with stable background shear. The upper-ocean flow consists of a shear layer associated with a sub-surface jet located below a free-slip surface. Inside the shear layer, a well-mixed layer is located on top of a linearly stratified layer in which the gradient Richardson number is larger than 0.25. Holmboe instability is observed to grow at the base of the mixed layer, eject thin wisps of heavy fluid into the mixed layer and subsequently generate turbulence. Holmboe instability also excites internal waves into the linearly stratified region. The waves propagate downward in the stratified sheared medium and are reflected upward when the back- ground velocity is too large to support propagating internal waves according to linear wave theory. Intermittent bursts of turbulence with the dissipation rate of at least two orders of magnitude larger than that of the waves are observed in the linearly stratified region. Horseshoe vortices originated from the mixed layer penetrating downward are found to cause the bursts.