Jack Barth, Dave Hebert, Dave Ullman and Andy Dale

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 (Figure 1). 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 (Mar-Apr), the water column was well-mixed over the Bank and the deeper, off-bank 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 (Figure 2). See the report by Dale and Barth for more details of the vertical structure of the water column over a tidal cycle and for a brief discussion of the influence of Scotian Shelf crossovers.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 the June cruise we conducted surveys across the TMF in the vicinity of the Schlitz/NMFS moored array and also studied the SBF farther offshore (Figure 1). 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 (Figure 3). 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 along-front at 0.18 m/s.

We continue to analyze our SeaSoar and ADCP data, paying particular attention to ordering the sections with respect to the tidal cycle and attempting to align multiple sections using bathymetric and/or frontal features as reference points. To create sections of a qualitative indicator of vertical mixing, we compute a quasi-Cox number from conductivity sampled at 24 Hz.The space-time distribution of this parameter relative to the fronts over the tidal cycle is being investigated in order to identify the important locations and times of small scale mixing.The relationship of the quasi-Cox number with estimates of Richardson number and with hydrographic structures is also being pursued.