One of the overall goals of the NW Atlantic/Georges Bank GLOBEC Study is "to determine the processes that control the Georges Bank circulation and transport of biological, chemical, and geological materials in a strongly tidal- and buoyancy-driven system and to determine the physical and biological processes that control the population dynamics of the target species For Phase III of this program, the primary focus of research will be on cross-frontal exchange processes. For this project, the investigators will determine the three-dimensional circulation associated with the Northeast Peak Front (in late March/early April), the Shelf/Slope Front, and the Tidal Mixing Front on the southern flank of Georges Bank (in mid-June). The dynamics of these different fronts over a fortnightly period will be studied. The effect of the variability in the tidal currents (both at the semi-diurnal and fortnightly time-scales) on the cross-frontal exchange processes will be examined. An attempt will be made to determine the dynamics of the cross-frontal exchange and the rate of cross-frontal exchange for each front. To address these goals, high-resolution SeaSoar surveys will be made around an isopycnal Lagrangian float placed initially in the center of the front. The SeaSoar will have dual temperature and conductivity sensors on the CTD, a WET LABS ac-9 optical package (absorption and attenuation data collected at 9 frequencies), a fluorometer and MicroSoar. MicroSoar has fast temperature and conductivity sensors allowing estimates of the temperature variance dissipation rate (i.e., temperature microstructure) to be made. A shipboard Acoustic Doppler Current Profiler (ADCP) will provide horizontal velocity data. The trajectory of the isopycnal float will provide the cross-frontal exchange pathway for one location for each deployment. Analysis of the CTD and ADCP data will be used to determine the dynamics of this exchange pathway. The salinity, temperature and ac-9 data will be used to characterize the water masses present. Combinations of the nine absorption and attenuation channels can be used as surrogate "tracers" to track cross-frontal exchange pathways. Repeated surveys may allow the exchange rates to be determined. The microstructure data will provide information if turbulent mixing processes are important and, if so, where they occur. The results of this work will provide insight on how water (and other properties of the water) are exchanged across the different types of fronts found on Georges Bank. With the measurements of biological processes and distributions at the same location made by other GLOBEC PIs, the effect of physical forcing on these biological processes can be addressed.
