On-bank flow in the bottom mixed layer across the tidal front on Georges Bank: observations from a dye tracer release

R. W. Houghton

Preliminary results from three fluorescent dye injections into the bottom mixed layer at the tidally mixed front during the 1999 Phase III field program will be presented. There were two injections on the south flank May 22-31 and a final one at the northeast peak June 2-5. Several of these injections were coincident with pycnocline injections conducted by Jim Ledwell.

The first injection was 5 m above the bottom in 6.8C water on the seaward side of the front. Over 4 days the dye tagged water warmed roughly linearly to 7.4C implying a cross-frontal Lagrangian velocity of 1.6 cm/s. The mean along-bank velocity was 4.1 cm/s

A second dye injection was in 7.0C water in virtually the same location with respect to the tidal front but the subsequent spreading was strikingly different. Temperature change of the dye tagged water over 4 days was negligible, <<0.1C, implying no cross-frontal flow. The hydrographic difference was the proximity of the foot of the shelfbreak front now only 5 km off bank of the dye patch, in contrast to > 20 km during injection #1. This suggests a divergence or null in the cross-bank flow near the midpoint between the tidal and shelfbreak fronts.

The third injection was in 6.85C water seaward of the tidal front on the northeast peak at 66.9W. The dye tagged water warmed rapidly and linearly to 8.5C in just over 2 days implying a cross-frontal velocity of 3.7 cm/s. There is an indication that once dye tagged water passed entirely through the front and mixed throughout the water column over the bank it began to flow off-bank in the pycnocline.

From the increase in the cross-bank width of the dye patch we estimate a Fickian lateral diffusivity of 50-75 m²/s on the south flank and 75-100 m²/s at the northeast peak.

The dye injection experiment has provided unambiguous evidence of an on-bank flow through the tidal front in the bottom mixed layer of Georges Bank.