Spring-summer frontogenesis at the mouth of Block Island Sound: 2. Combining acoustic Doppler current profiler records with a general circulation model to investigate the impact of subtidal forcing

The use of a one-layer inverse model to improve the fidelity of a three-dimensional primitive equation model is explored in the region just south of Block Island Sound in the Middle Atlantic Bight. The inverse model, based on the theory of Bennett and McIntosh, produces a solution to the linear, shallow-water equations that minimizes model-data misfit and the resulting open boundary velocity variance. The Massachusetts Institute of Technology general circulation model is configured regionally and includes realistic tidal forcing, surface warming, and wind stress, as well as climatological buoyancy gradients between the estuary and the shelf. A 15 day record from two acoustic Doppler current profiler moorings, deployed in spring 2000 south of the entrance of Block Island Sound, allows assessment of the forward model and provides data to improve subtidal boundary conditions. Model-data misfit decreases notably for both the depth-averaged and vertically sheared flow, indicating that improvements to the depth-averaged circulation positively impact the three-dimensional circulation in this shallow shelf environment. The role of wind stress in modulating the position of the buoyant plume that characterizes the regional hydrography is also examined. A distinctly three-dimensional circulation is observed, with a growing offshore plume at the mouth of the sound during northward stress followed by rapid alongshelf advection during periods of southwestward or weak winds. Although, on the timescales considered here, wind stress forcing dominates the buoyancy-driven motion, it is overwhelmed by the tidally driven rectified flow just south of Montauk Point, which provides the inshore boundary of the front even in downwelling favorable conditions.