Physical Processes at the Shelf Edge in the Northwest Atlantic

Abstract

Abrupt depth changes at the edges of continental shelves leading to important physical processes in the ocean are described with reviews on the progress in understanding three such processes: (1) interaction of offshore currents and eddies with the continental margin; (2) wind-driven upwelling at the shelf break; (3) generation of internal tides and nonlinear waves by the M2 surface tide. Two possible modes of interaction between offshore currents and coastal waters are contrasted. The first is characterized by eddy exchange created by a current guided along the shelf edge (as on the Labrador and Newfoundland shelves or in the United States South Atlantic Bight) while the second involves remote forcing by low-frequency topographic Rossby waves radiated from offshore meanders and rings to the shelf edge. In addition, wind-driven upwelling from depths of 400 m or more results from moderate (10-15 m S-1) but persistent alongshore wind. Complex bathymetry of the shelf may cause anomalously high currents in a fully three-dimensional circulation. Large-amplitude internal waves driven by the M2 surface tide are a ubiquitous feature of the shelf edge circulation. The intense vertical mixing associated with these waves promotes high levels of biological productivity at the shelf edge by continuously supplying nutrients to the surface layers.