Current structure and mixing in the shelf/slope water front south of New England

Horizontal and vertical currents in the front along the edge of the continental shelf south of New England during the late spring were measured by tracking special neutrally buoyant floats which tagged different water masses. The mean current (10–20 cm s−1) was westward along the shelf and was confined completely to the overlying shelf water with a geostrophic transport of about 0.7 sverdrups between the 80- and 300-m isobaths. No mean cross-shelf or vertical currents could be reliably measured. Higher-frequency currents were detected and are described. Numerous conductivity-temperature-depth lowerings were made over the drifting floats to examine the interleaving process and possible mixing between shelf water and slope water. The layers are thinned by vertical shear, and they gain and lose both heat and salt on a time scale of 1–3 days. This plus the rich microstructure suggests the existence of vertical turbulent exchange between layers having a turbulent diffusivity of the order of 5 cm2 s. Estimates of net annual heat and salt input to the shelf are made, and they appear to be significant in determining the heat and salt budget of the shelf water mass.