Coastal recipes: Internal waves, turbulence and mixing on the New England continental shelf.

摘要

Turbulent dissipation and associated diapycnal diffusivity during the stratified season on a continental shelf are shown to vary by over four orders of magnitude and depend sensitively on the changing dynamics of forcing mechanisms; at different times and depths turbulence is caused by surface wind-stress, bottom friction, direct instability of energy-containing internal waves, and wave-wave interaction between small- and large-scale internal waves. Turbulence, in turn, shapes the evolving spring stratification and is a strong drain of energy from the internal-wave field, especially for low-mode waves that have sub-critical gradient Richardson numbers. This thesis focuses on observations and analysis of internal waves and turbulence taken on the New England continental shelf during late summer, 1996 and late spring, 1997 as part of the ONR-sponsored Coastal Mixing and Optics experiment.; In late summer, 1996, baroclinic energy and shear were primarily associated with low-mode near-inertial and semidiurnal internal waves and, at times, high-frequency solibores. The average turbulent dissipation rate and diapycnal diffusivity were 5–50 × 10−9 W kg−1 and 0.05–0.2 × 10−4 m2 s−1. Half the thermocline dissipation was due to six solibores that cumulatively lasted less than a day, but contained 100-fold elevated turbulence. Non-solibore, mid-column dissipation was strongly correlated with shear from low-frequency internal waves; this dissipation was not well parameterized by Gregg-Henyey scaling. A new parameterization is developed, based on the original analytical model of Henyey et. al. '86, modified to account for the non-steady-state nature of coastal internal waves.; In late spring, 1997, we observed the return of seasonal stratification, and the rise of low-mode, near-inertial internal waves forced by passing storms. There was a local energy balance between wind input, bottom-drag, and local turbulent dissipation. The largest turbulent dissipation away from surface and bottom boundary layers was coincident with shear from mode-one, near-inertial waves generated by a strong storm; 4-m Richardson numbers were well below 0.25 and diapycnal diffusivity exceeded 10−3 m 2s−1. After stratification strengthened, the Richardson number from low-mode waves became stable, and turbulent dissipation was consistent with the new turbulence parameterization.