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Cross-Frontal Larvae Transport

The water exchanges across the tidal mixing front on the southern flank of Georges Bank (GB) were examined using a 2-D primitive equation model. The numerical domain features a cross-frontal transect corresponding to a June 1999 hydrographic/ADCP study as part of the US GLOBEC Program. The model was initialized with temperature and salinity fields measured at the first hydrographic section on the 11th of June and ran prognostically with tidal forcing, real-time measured winds, and surface heat flux. The model results show that the wind fluctuation plus tidal mixing play an essential role in the transport of water from the stratified region to the mixed region on GB in early summer when stratification just developed and a thin thermocline formed at about 10 m below the surface. This process happened following two scenarios. At first, the wind-induced onbank Ekman transport caused an onbank migration of the upper part of the tidal mixing front and particles in the surface mixed layer. Then, when the wind relaxed or changed its direction, the water in the onbank extended part of the front was mixed quickly by the tidal motion. As a result, particles, which were advected along the front, stayed in the previously-mixed region of the bank. Such an onbank transport of particles tends to enhance as the surface heat flux is added. Since the wind-induced cross-frontal, onbank transport of water occur episodically associated with fluctuating wind from atmospheric frontal passages and also since this is much more significant than the tide-induced cross-frontal flux of water near the bottom, this wind-induced process can be important in the cross-frontal, onbank flux of the copepods and other zooplankton species observed on the southern flank of GB in early summer.

Animation of particle trajectories for the cases with (1) only tidal forcing and (2) tidal forcing plus a constant wind stress averaged over the cruise period.

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Animation of particle trajectories for the case with tidal forcing plus real-time wind stress measured over the cruise period.

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Animation of particle trajectories for the case with tidal forcing, real-time wind stress and estimated heat flux.

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