The importance of lateral variability on exchange across the inner shelf.

This work describes the characteristics, occurrences and drivers of spatially- variable circulation over the inner part of the continental shelf having scales of 100s of meters to 10s of kilometers in order to understand the relative importance of lateral mechanisms of exchange and stirring on the total cross-shelf transport. While evidence of additional transport due to major capes or headlands, bathymetric irregularities, or non-uniform wind fields has been found in most coastal areas, the relative impact of these sources of variability on exchange and dispersion across the shelf have not been quantified. Using observations of high-resolution HF radar surface currents and dense observations of subsurface velocity and hydrography collected in the coastal ocean south of Martha's Vineyard, MA, relative importance of spatial variability in stratification, wind, and bathymetry on exchange are quantified. In the study area, coherent vortices or eddies, squirts or jets, density intrusions, and a spatially variable mean circulation due to tidal rectification all contribute to the total exchange across the shelf. Eddies alone lead to exchange equivalent to 1/3 of the wind-driven depth-dependent exchange, but in the opposite direction. Using Lagrangian trajectory analyses in addition to the Eulerian-based transport studies, this effort examines the implications of variations in the dominant type of exchange on coastal systems.