Internal waves and mixing along the New England shelf-slope front

Abstract

A 48-day early spring data set from the outer continental shelf south of New England is used to describe the internal wave climate of the shelf-slope front. Current data from three near-bottom and one mid-depth instruments in water depths between 80 and 120 m indicate that internal waves provided a significant portion of the near-bottom current variance in the frontal zone. The data show that the internal wave energy was enhanced in the frontal zone relative to the shelf. At these depths, at least during the observational program, surface waves did not generally cause noticeable near-bottom water motions. Spectra describe an internal wave field of low vertical mode propagating perpendicular to the isobaths onto the shelf. This suggests significant refraction of the waves as they propagated onshore from the isotropic internal wave field of the continental slope. There appeared to be very little along-shelf variation in internal wave energy over distances on the order of 100 km. Onshore, however, there was a factor of two decrease in internal wave energy density over a distance of about 15 km. Assuming frictional dissipation within a bottom boundary layer, this decrease in energy translates into a friction velocity of about 2 cm s −1. Thus, internal wave dissipation could result in bottom stress levels comparable to values deduced for shallower waters under the influence of storm-driven surface waves. This dissipation rate is shown to produce a significant bottom mixed layer which can, under some circumstances, result in sizeable cross-frontal exchange of shelf and slope waters.