Three-dimensional hydrographic and velocity structure and isopycnal float trajectories in the vicinity of fronts on George Bank

J. Barth, D. Hebert and D. Ullman

We conducted high-resolution hydrographic, bio-optical and velocity surveys of the Northeast Peak Front in March-April and of the Tidal Mixing Front (TMF) and Shelfbreak Front (SBF) on the southern flank of Georges Bank in June, 1999. The thermohaline and bio-optical fields were mapped using the towed undulating vehicle SeaSoar equipped with a CTD, a fluorometer and a transmissometer; velocity was measured with a shipboard ADCP. Sampling strategies included repeated 3D mesoscale surveys (40 by 40 km) and a set of 10 by 20 km butterfly patterns during which a subsurface, isopycnal float was acoustically tracked from the ship.

During the spring cruise (March-April), the water column was well-mixed over the Bank and the deeper, offbank stratified fluid moved with the tide on and off the Bank. Cold, fresh Scotian shelf water intruded into the region from the northeast, reaching the 100-m isobath at one time. Bio-optical property distributions were highly coupled to the thermohaline fields with an indication of increased penetration to depth of the shallow high-chlorophyll water near the edge of the Bank.

During June, there was considerable mesoscale variability between the TMF and SBF including an intrusion of warm, saline slope water onto the Bank up to the 65-m isobath. Early in the cruise, water over the Bank shallower than 55-60 m was well mixed while two weeks later considerable temperature stratification was present in the upper water column throughout the measurement region. High fluorescence was found throughout the water column (sometimes excluding a 5-10 m surface layer) inshore of the TMF while offshore, high values were only found at 20-50 m associated with the spatially variable pycnocline. The foot of the SBF closely followed the 95-100 m isobaths. A subsurface isopycnal float released near the foot of the front moved 15-km seaward as it rose from 80 to 40 m depth along the sloping frontal isopycnals over a 2-day deployment. The float's average horizontal velocity was 0.09 m/s, while a drifter drogued at 15 m released at the same location moved westward essentially alongfront at 0.18 m/s.