Data assimilative hindcast on the Southern Flank of Georges Bank during May 1999: frontal circulation and implications

The circulation of the tidal front along the Southern Flank of the Georges Bank region during May 1999 is examined for the purpose of understanding the transport of larval fish for periods of days to a week. Assimilation of shipboard ADCP data from several Georges Bank cruises into 3-D models is used to produce the hindcast circulation. Adjustments to boundary nodal elevations are estimated to minimize the misfit between model and observations both in frequency and time domains as described in Lynch and Hannah (J. Atmos. Oceanic Technol 18 (2001) 962) and Lynch and Naimie (Cont. Shelf Res. 22 (2002) 2191). An intercomparison of different methods of applying the adjustments is completed using drifter records and dye patch trajectories to provide a measure of skill. The computed flow fields show a skill of 2.4kmday-1 when compared to (unassimilated) drifter trajectories. The tracking of a dye-patch recovers an observed near-bottom cross-bank flow component that is not present without assimilation. Using these flow fields we then investigate the importance of the front as a retention mechanism using passive particle simulations. Wind, heat flux, and tides control the circulation across and along the tidal mixing front. Three 3-day time periods, before, during and after a wind event during May 1999 were studied. Model simulations suggest a highly variable cell-like circulation in the frontal region (onshore near the bottom, upwelling shoalward of the front, offshore at mid-depth and downwelling seaward of the front) that controls the exchange and retention of particles. During the periods when the cell-like circulation is active the highest accumulation of particles occurs in the areas surrounding the front.