Conclusions

The results of this pilot cruise demonstrated the utility of a dye tracer to investigate mixing and circulation at the foot of the shelfbreak front in the Middle Atlantic Bight. It resolved water displacements of 3-4 km relative to the frontal boundary even when during the course of the observation the front was displaced 12-15 km onshore. It has revealed a heretofore undetected onshore flow responsible for convergence within the frontal boundary. The reason that this onshore flow was not detected by moored current meters is now clear. In the Chapman and Lentz (1994) model the onshore flow is weak and confined to the foot of the front while on both sides of the front the BBL flow is offshore. Since the front undergoes cross-shelf excursions that exceeds its width any moored current meter will predominantly sample the offshore flow regime.

Details of the flow field suggested in Fig. 4 requires further confirmation. A subsequent cruise is scheduled involving more careful and controlled dye injection into the BBL on both sides of the convergence zone to refine the flow patterns inferred from this pilot cruise.

These observations and their apparent confirmation of model predictions by Chapman and Lentz (1994) indicate the importance of the BBL to both frontal dynamics and to shelf-slope exchange processes. It is in the BBL at the foot of the front where large T-S gradients imposed by the frontal boundary and turbulent energy derived from bottom boundary friction combine to induce strong diapycnal mixing.

Lamont-Doherty Earth Observatory of Columbia University