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
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.
D. Jarvis Belinne
Lamont-Doherty Earth Observatory of Columbia University