Introduction

Understanding the processes responsible for the position and structure of the Shelf-Slope Water front, a continuous feature along the shelfbreak of the northeast continental margin from Nova Scotia to Cape Hatteras, has been a focus of much research. Typical structure in the late winter of the front separating cold, fresh Shelf Water from denser warm, salty Slope Water is shown in Fig. 1. Numerical model calculations by Gawarkiewicz and Chapman (1992) and Chapman and Lentz (1994) highlight the dynamical role of the bottom Ekman layer (BBL) by which the offshore flow is arrested at a convergent zone generated by the cross-shelf buoyancy flux and then separates from the bottom to shoal along the frontal boundary.

This convergent flow has not been detected. In fact all mean Eulerian cross-shelf velocities measured by current meters moored in the BBL on the outer shelf and upper slope are offshore ranging 0.01-0.04 m/s (Beardsley et al., 1985; Aikman et al., 1988; Butman, 1988; Butman et al., 1988; Houghton et al., 1994). A continuous offshore flow across the shelfbreak without any convergence is difficult to reconcile with the presence of a persistently narrow front. The fact that the foot of the front often undergoes cross-shelf excursions greater than 20 km (Houghton et al., 1994) makes observation of a small scale convergent flow at the frontal boundary by an array of current meters neither feasible nor cost effective. For these reasons a new technique, a dye tracer, was proposed as a means of observing the convergence and perhaps even the detachment of the BBL flow at the frontal boundary. The results of a pilot cruise which not only tested this technique but also produced some striking observations are presented here.

Lamont-Doherty Earth Observatory of Columbia University