ライブラリ登録: Guest
TSFP DL Home アーカイブ 執行委員会
TSFP DIGITAL LIBRARY ONLINE


ISSN オンライン: 2642-0554

ROLE OF SHEAR INSTABILITIES IN THE UPPER EQUATORIAL UNDERCURRENT

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

要約

A high-resolution LES model with dynamic procedure is used to investigate the role of shear instabilities on near-N isopycnal oscillations and deep-cycle turbulence, the two phenomena consistently observed in the upper Equatorial Undercurrent (EUC). The model simulated at oceanographic scale has a velocity profile, which consists of a westward surface current and an eastward undercurrent, and a density profile having a surface mixed layer on top of a pycnocline, similar to the observed profiles. Results from the model indicate that a Holmboe shear instability that develops at the base of the surface mixed layer can lead to isopycnal oscillations and enhanced dissipation rate at depth in the EUC where the gradient Richardson number is greater than 0.25. The process generating turbulence consists of the primary Holmboe shear instability, secondary Kelvin-Helmholtz (KH)-like shear instability and downward vortex penetration to depths in the EUC. As the Holmboe instability grows to finite amplitude, the shear in the region immediately below the surface mixed layer is enhanced and induces secondary KH shear instability. Vortices associated with the secondary shear instability penetrate into deeper region of the EUC, are stretched the local shear and broken down into turbulence. The vortex penetrations break the oscillations down into turbulence. The dissipation rate at depth recorded during the downward penetration is larger than that seen in the surface layer. The results from the model suggest that the near-N oscillations are remnants of the Holmboe shear instability and the deep-cycle turbulence is caused by secondary shear instabilities and the downward vortex penetration.