A Lagrangian View of the Estuarine Neap/Spring Transition.

A recent series of dye studies conducted in the Hudson River estuary demonstrated a marked change in the estuarine advective/diffusive transport process over the neap/spring cycle. These studies consisted of four injections of fluorescein dye into the bottom mixed layer of the Hudson that were tracked for up to 4 days after injection. During neap tide dye remained in a coherent patch in the bottom mixed layer and moved rapidly though salinity space as it advanced 20-30 km upriver over four days toward the landward limit of the salt field. Dye freshened most rapidly during the flood at a rate of 0.66 psu/hour?a rate that was 3 times that observed during the ebb. Reduced mixing during neap tide ebb enabled the water column to become strongly stratified by tidal straining. Increased stratification at the end of ebb facilitates the freshening on flood because it provides appreciably fresher water aloft for the bottom boundary layer to entrain it to on flood. However, the observed growth of the bottom mixed layer during the flood is less than the an estimate of entrainment made with salinity and suggests that fluid in the bottom mixed layer is actually divergent during the flood. During spring tide a radically different trajectory of the dye was observed, and appears to be related to a reduction in the flood/ebb asymmetry in vertical mixing. During spring tides the freshening of the dye on flood was only 50 percent greater than that on ebb. Subsequently, mixing on ebb competed with tidal straining and reduced vertical stratification at the end of ebb. Reduced stratification at the end of ebb allowed the bottom boundary layer on the subsequent flood to penetrate the entire water column. This suggests that the key to the neap/spring transition is associated with enhanced mixing on ebb. Furthermore, the increased mixing on spring tide rapidly mixes the dye vertically where it is readily exported seaward as tidal current amplitude decreases and the estuary restratifies when the lower layer is renewed with more saline bottom water.