Mixing on the Late-Summer New England Shelf—Solibores, Shear, and Stratification

Abstract

Observations are presented of microstructure and velocity measurements made on the outer New England shelf in the late summer of 1996 as part of the Coastal Mixing and Optics Experiment. The depth- and time-averaged turbulent dissipation rate was 5?50 (? 10?9 W kg?1). The associated average diapycnal diffusivity in stratified water was 5?20 (? 10?6 m2 s?1), comparable to observed open-ocean thermocline values and too low to explain the strong variability observed in local water properties. Dissipation rates and diffusivity were both highly episodic. Turbulent boundary layers grew down from the surface and up from the bottom. The dissipation rate within the bottom boundary layer had an average of 1.2 ? 10?7 W kg?1 and varied in magnitude with the strength of near-bottom flow from the barotropic tide, an along-shelf flow, and low-frequency internal waves. The average dissipation rate in the peak thermocline was 5 ? 10?8 W kg?1; one-half of the thermocline dissipation was due to the strong shear and strain within six solibores that cumulatively lasted less than a day but contained 100-fold elevated dissipation and diffusivity. Nonsolibore, midcolumn dissipation was strongly correlated with shear from low-frequency internal waves. Dissipation was not well parameterized by Gregg?Henyey-type scaling. An alternate scaling, modified to account for observed coastal internal wave properties, was in good agreement with measured dissipation rates. At the end of the observational period Hurricane Edouard passed by, producing strong dissipation rates (4 ? 10?6 W kg?1) and consequent mixing during and for several days following the peak winds.