Diapycnal flow through a tidal front: a dye tracer study on Georges Bank

A fluorescent dye tracer, fluorescein, injected into the bottom mixed layer (BML) at the off-bank edge of the tidal front on Georges Bank in late May and early June 1999 has provided the first quantitative measurement of an on-bank diapycnal Lagrangian flow through the front. From the warming of the dye patch, 1.9×10−6 and 7.6×10−6 °C/s on the south flank and northeast peak, respectively, as it passed through the frontal temperature gradient, we infer a mean on-bank flow of 1.6 cm/s on the south flank and 3.1 cm/s on the northeast peak. The heat flux required for this warming is predominantly due to vertical mixing within the tidal front. From the dispersion of the dye patch, we estimate a vertical diffusivity of 1.6 and 3.1×10−3 m2/s and a cross-front diffusivity of 18 and 30 m2/s on the south flank and northeast peak, respectively. The evolution of a second dye injection on the south flank was distinctly different from the first because of the proximity of the foot of shelfbreak front now displaced onto the bank by a transient meander to within 6 km of the tidal front. Although the dispersion of this patch was the same as for the first injection, its cross-bank displacement was much less reflecting the divergence of the cross-bank flow in this region.