Mean circulation and variability on the eastern Canadian continental shelf

Abstract

The circulation and variability on eastern Canadian continental shelves is reviewed with emphasis on horizontal and vertical structure in the water column. Mean and seasonal currents, sheared by stratification and friction, generally follow bathymetry and are driven primarily by surface wind stress and buoyancy fluxes. In certain near-resonant embayments, nonlinear interactions of the tide also make significant contributions to the mean and low-frequency currents. Direct forcing of low-frequency shelf circulation by offshore currents and pressure fields is of lesser importance according to recent observations and models. At higher frequencies, the wintertime response to atmospheric forcing occurs in three important frequency bands: surface wave (periods of 2–20 s), inertial (17–22 h) and synoptic or subtidal (2–10 days). Most of the subtidal energy in the subsurface pressure (SSP) field is attributable to direct local forcing by alongshore wind and to remote forcing by coastal-trapped waves (CTW). The subtidal current variability is less coherent with these sources primarily because of small scale circulations created by vortex stretching, topographic steering and scattering by the rugged shelf bathymetry. Bottom friction and scattering control the ratio of locally and remotely forced subtidal energy as a function of position on the shelf. Intermittent inertial currents in stratified shelf waters, often associated with mesoscale atmospheric forcing, exhibit a 180° phase change across the pycnocline and are inhibited near the coast. Surface waves generated by storm winds are strongly dissipated in shallow water, where they enhance bottom stresses that: (1) balance the surface wind stress; and (2) may contribute to the damping of inertial waves. Specific examples of these phenomena are drawn from observations on the Scotian Shelf and in the Gulf of Maine.