Woods Hole Oceanographic InstitutionAnthony Kirincich

 Anthony Kirincich and Glen Gawarkiewicz, Along-shelf circulation dynamics along the Outer Cape Coastal Current: using Reynolds Stress Observations to infer the role of the Along-shelf Pressure Gradient., J. Geophysical Res., (submitted)The role of along-shelf pressure gradients in the circulation along the Outer Cape Coastal Current was investigated during the spring transitional period using observations of temperature hydrography and acoustic Doppler current profiler (ADCP)-based velocities made in 2010 and 2011.  Deployed across the inner shelf offshore of Cape Cod, Massachusetts, the observations captured the coastal circulation at a critical linkage between the Gulf of Maine and the Mid-Atlantic Bight.  In situ observations of turbulent momentum flux, or Reynolds stresses, were estimated from the ADCP-based velocity observations using recently developed methods.  The observed Reynolds stresses, consistent with simple models of circulation during across-shelf wind forcing, required the existence of an along-shelf pressure gradient that scales with the wind forcing during times of along-shelf wind forcing.  Using an inversion of the momentum balance to infer the barotropic pressure gradient, a mean, or background along-shelf pressure gradient of d$\eta$/dy $\sim$ 2$\times$10$^{-7}$ was estimated for the Spring of 2011, while a gradient of  1$\times$10$^{-7}$ for the Fall of 2010.  Both are similar to gradients inferred for the Mid-Atlantic Bight.  Additionally, the inferred pressure gradient contained a sizable component not driven by the along-shelf winds. This portion often opposed the regional pressure gradient, suggesting the main along-shelf current exiting the Gulf of Maine is forced offshore during spring.  This was confirmed by an analysis of historical drifter observations.  A prediction of the along-shelf velocity, based on forcing from the local winds, geostrophy, and the along-shelf pressure gradient was similar to the observed along-shelf velocity.FILE