we investigate the pathways and properties of the plume. Four plume pathways for export of freshwater from the western tropical North Atlantic are identified. These consist of direct and indirect pathways to the northwest, and eastward pathways toward the subtropical gyre and toward Africa in the North Equatorial Counter Current. Because of the seasonality and cooccurrence of these pathways, plume characteristics are highly variable. Two pathways export water to the Caribbean, however the time scales associated with those direct and indirect pathways (3 versus 61 months) differ, leading to different salinity characteristics of the plume water. Models results show that the Amazon river and tropical precipitation have similar magnitude impact on the observed seasonal cycle of freshwater within the western tropical Atlantic and at the 8 N, 38 W PIRATA mooring. Freshwater associated with the Amazon also influences surface salinity in winter as far as 20W in the model. The mean plume salinity minimum leads maximum discharge, highlighting the importance of currents and advection rather than discharge in maintaining plume properties. Plume pathways are tied to the underlying current structure, with the North Equatorial Counter Current jet preventing direct freshwater transport into the southern hemisphere. The plume influences underlying currents as well, generating vertical current shear that leads to enhanced eddy stirring and mixing in the model simulations.

Wave directions and mean currents observed for two 1 month long periods in 7 and 2 m water depths along 11 km of the southern shoreline of Martha's Vineyard, MA, have strong tidal modulations. Wave directions are modulated by as much as 70° over a tidal cycle. The magnitude of the tidal modulations in the wavefield decreases alongshore to the west, consistent with the observed decrease in tidal currents from 2.1 to 0.2 m/s along the shoreline. A numerical model (SWAN and Deflt3D‐FLOW) simulating waves and currents reproduces the observations accurately. Model simulations with and without wave‐current interaction and tidal depth changes demonstrate that the observed tidal modulations of the wavefield primarily are caused by wave‐current interaction and not by tidal changes to water depths over the nearby complex shoals.

Field-tested numerical model simulations are used to estimate the effects of an inlet, ebb shoal, wave height, wave direction, and shoreline geometry on the variability of bathymetric change on a curved coast with a migrating inlet and strong nearshore currents. The model uses bathymetry shoreline depends more strongly on its radius of curvature (a proxy for the separation of tidal flows from the coast) than on the presence of the inlet, the ebb shoal, or wave height and direction. As the radius of curvature decreases (as the corner sharpens), tidal asymmetry of nearshore currents is enhanced, leading to more sediment transport near the shoreline over several tidal cycles. The results suggest that feedbacks between shoreline geometry and innershelf flows can be important to coastal erosion and accretion in the vicinity of an inlet.3

The dispersal of sand from submerged mounds in the nearshore is driven by the interplay of processes such as converging and recirculating flows, changing roughness, bed slope effects and wave focusing/refraction. This morphological diffusivity is key to understanding sand bars in shallow seas, tidal inlets, estuaries, and the nearshore response to human interventions such as nourishments and dredging. Most of the work on the evolution of submerged mounds has been based on fluvial studies, focusing on flow without waves. In these cases, circular mounds tend to deform to crescentic (barchan) shapes. In contrast, observations of sandbars and berms in the nearshore subjected to waves show much more complex translation and deformation behavior. This contribution introduces the laboratory MOrphological Diffusivity Experiment (MODEX) aimed at examining morphological diffusivity under different forcing conditions. The experiment particularly addresses the linkages between small scale (local) effects (e.g. bed slope, bedforms) on the adjustment of sandy mounds.

No abstract