The importance of lateral variability on exchange across the inner shelf south of M artha's V ineyard, MA

This study examines the spatial variability of transport within the inner‐shelf south of Martha's Vineyard Massachusetts, its time and space dependence, and its importance to the total volume exchanged between the nearshore and the coastal ocean. The exchange of water across the inner shelf is often considered to be driven primarily by wind forcing, yet the effects of small‐scale O(1–10 km) variability on the total exchange have not been well quantified. Using a combination of high‐resolution HF radar‐based surface currents and a dense array of moorings to document the lateral variability of across‐shelf exchange, the cumulative wind‐driven across‐shelf transport over the summer stratified period was less than the volume of the inner‐shelf onshore of the 25 m isobath. Along‐shelf variations in the wind‐driven exchange were as large as the spatial mean of the wind‐driven exchange. A spatially varying time‐averaged circulation caused by tidal rectification resulted in across‐shelf exchange larger in magnitude than, and independent of, the integrated wind‐forced exchange. Coherent submesoscale eddies also occurred frequently within the domain due to flow‐topography effects onshore and horizontal density gradients offshore, generally with lifespans shorter than 10 h, diameters smaller than 6 km, and vertical depths shallower than 10 m. The across‐shelf volume transport due to eddies, estimated by seeding particles within the surface current fields, was more than half the wind‐driven depth‐dependent exchange. Thus, accounting for the potential coherent along‐shelf variability present over the inner shelf can significantly increase estimates of the across‐shelf transfer of water masses and particles.