Structure and dynamics of the midshelf front in the New York Bight

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The structure and variability of the wintertime midshelf front in the New York Bight is examined using moored observations of currents and hydrography during 2007. The front is located near the 50 m isobath, inshore of the shelf break front and offshore of estuarine outflow plume fronts. It spans the water column and is the boundary between cooler, fresher, and less dense inner shelf water and warmer, saltier, and denser outer shelf water. The mean hydrographic front slopes upward offshore, and there is an associated equatorward along-shelf 5-10 cm/s surface-intensified current jet. The mean across-shelf circulation is offshore near the surface and onshore at depth. The across-shelf velocity is convergent and strengthens across-shelf property gradients with a time scale for gradient doubling of approximately 8 days. Tidal analysis of currents and temperatures suggests that tidal shear dispersion is not an important mechanism for midshelf front formation. The frontal structure and location are roughly consistent with the theory of bottom boundary layer advected fronts, suggesting that multiple estuarine outflows are collectively responsible. Pulses of strong offshore wind cause the breakdown of the across-shelf thermal wind balance, which has been commonly assumed to hold in shelf environments. Observed shear significantly exceeds thermal wind shear at middepths, where observation of low-gradient Richardson number indicates the importance of vertical mixing. The response of currents to wind fluctuations is asymmetric, whereby the combination of upwelling favorable and offshore winds or downwelling favorable and onshore winds produces a stronger response than winds from the remaining quadrants. Copyright 2012 by the American Geophysical Union.

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Journal of Geophysical Research: Oceans