Abstract

This paper seeks to exploit a recent data set obtained off the coast of New Jersey, USA in the summer of 1996. The two major objectives are to gain insight into mechanisms controlling the vertical structure, particularly the temperature and density, and into the subtidal frequency dynamics. The setting was strongly affected by three conditions: stratification was high, the bottom slope was comparable to the isopycnal slope, and buoyancy forcing from an upshelf freshwater source was, on average, of the same strength as wind forcing. Bottom mixed layer (bml) and surface mixed layer (sml) thicknesses increased offshore, opposite the sense predicted by standard formulations for stratified conditions. Maps of sml and bml thickness were coherent with maps of bml density, not with maps of vertical stratification. It appears that horizontal buoyancy flux from the Hudson coastal current sustains the stratified interior even very near shore. Well resolved vertical profiles at a mooring in 19 m water depth showed that above 1 m height off the bottom the Richardson number was very large, rendering vertical stress and eddy coefficients very small. In contrast at a deeper site offshore beyond the reach of freshwater intrusion a thick bml was present and vertical stresses and eddy coefficients were at expected levels. Moored instruments there revealed temporal variations in bml thickness which correlated with the strength and direction of the interior alongshelf current, but with the opposite sense found on the California shelf: thicker during upwelling and thinner during downwelling. The difference is well explained by the action of interior density changes at the bml edge produced by displacement of interior isopycnals. This action is significant when the ratio of isopycnal slope to bottom slope is of order one, as here. The subtidal dynamics featured high levels of bottom friction, despite the thin bml thicknesses nearshore. The large values of drag coefficient found are consistent with the large effective roughness heights induced by the orbital wave motion in this shallow water regime on an open coast. Bottom stress was comparable to wind stress both in mean values and in variations. No bottom stress reduction was evident, despite the sloping bottom and stratified interior. Volume transport in the bml rarely balanced the surface Ekman transport based on the alongshelf wind and rarely was consistent with bottom Ekman transport based on bottom stress. This implies that the flow field was seldom in a state of alongshelf invariance ("two-dimensional flow"). Despite the very shallow water depths, thermal wind balance was satisfied for the vertical shear of the alongshelf current.

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