A coastal current in winter: Autonomous underwater vehicle observations of the coastal current east of Cape Cod

Abstract

Evolution of the coastal current structure on the shallow continental shelf east of Cape Cod was studied using autonomous underwater vehicle (AUV) surveys and moored observations during the winters of 2005 and 2006. A coastally bounded plume of relatively fresh water, characteristic of a coastal current, persisted throughout both winters despite strong mixing. Nondimensional parameter analysis classified the plume as a bottom‐trapped gravity current over a moderately steep slope, placing it in the context of other buoyant coastal currents. The range of water properties within the coastal current, its spatial extent and temporal variability were characterized on the basis of the data from repeat hydrographic sections. Along‐shore freshwater transport was dominated by highly variable barotropic flow driven by local wind and basin‐wide pressure gradients. It eventually contributed substantially to the average southward along‐shore freshwater transport, estimated at 1.1 ± 0.3 × 103 m3 s−1 in February and 1.8 ± 0.4 × 103 m3 s−1 in the first half of March 2006. The contribution of baroclinic buoyancy‐driven freshwater transport was typically an order of magnitude lower during both winters. Despite the relative weakness of the baroclinic freshwater transport, the coastal current potentially had a major impact on water mass modification during the winter. Continual presence of the low‐salinity plume prevented the formation of cold dense water near the coast and its export offshore. The coastal current effectively isolated the inner‐shelf zone, reducing its potential role in ventilation of the intermediate layers of the Wilkinson Basin of the Gulf of Maine.