Diapycnal Diffusivity Inferred from Scalar Microstructure Measurements near the New England Shelf/Slope Front

Abstract

Conductivity microstructure was used to estimate the diapycnal thermal eddy diffusivity KT near the New England shelf/slope front in early August 1997. Two datasets were collected with a towed vehicle. One involved several horizontal tows in and above a warm, salty layer near the seafloor, and the other was from a tow-yo transect that sampled most of the water column. In the bottom layer, KT derived from microstructure is a factor of about 5 smaller than estimates derived from tracer dispersion at the same density level, and the diffusivity decreases sharply as the buoyancy frequency N increases: KT ? N?3.1. With several assumptions, this behavior is consistent with laboratory results for shear-driven entrainment across a density interface. The bottom layer cools as it moves up the shelf mainly due to diapycnal mixing, and a simplified temperature budget of the layer yields a diffusivity of 3 ? 10?6 m2 s?1, which is between the values derived from microstructure and tracer dispersion. In the tow-yo transect, KT and the thermal variance dissipation rate ?T were high in a frontal region where intrusions were observed at several depths. Averaged over the entire transect, however, KT was slightly lower in water favorable for diffusive layering than it was in either water favorable for salt fingers or diffusively stable water. The eddy diffusivity estimated throughout the water column behaved as KT ? N?1.3±0.8, decreasing less sharply for increasing stratification than near the bottom.