Diapycnal diffusivity inferred from scalar microstructure measurements near the New England shelf/slope front

Conductivity microstructure was used to estimate the diapycnal thermal eddy diffusivity K-T near the New England shelf/slope front in early August 199 7. Two datasets were collected with a towed vehicle. One involved several h orizontal 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, K-T derived from microstructure is a factor of about 5 sm aller than estimates derived from tracer dispersion at the same density lev el, and the diffusivity decreases sharply as the buoyancy frequency N incre ases: K-T proportional to N--3,N-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 x 10(-6) m(2) s(-1), which is between the values derived from microstructure and tracer dispersion. In the tow-yo transect, K-T and the thermal variance dissipation rate chi(1) were high in a frontal region where intrusions were observed at several depths. Averaged over the entire transect, hoe ever, K-T was slightly lower in water favorable for d iffusive layering than it was in either water favorable for salt fingers or diffusively stable water. The eddy diffusivity estimated throughout the wa ter column behaved as K-T proportional to N-13+/-0.8, decreasing less sharp ly for increasing stratification than near the bottom.