The Mean and Seasonal Circulation off Southwest Nova Scotia

Abstract

Long-term measurements off southwest Nova Scotia reveal the following features of the mean circulation: (a) a westward longshore coastal current (4–10 cm s−1, (b) an anticyclonic gyre around Brown Bank (5–15 cm s−1, and (c) an upwelling circulation off Cape Sable (1–2 cm s−1, at bottom). The gyre circulation appears permanent but the coastal current and upwelling exhibit annual variations of the same order as the means. There is a distinct annual signal in the longshore transport at Cape Sable (maximum westward in winter), whereas the mean transport (0.14 × 106 m4 s−1) is consistent with both geostrophic estimates and budget requirements in the Gulf of Marine. Strong seasonal cycles are also found in the salinity and density fields at Cape Sable which appear to be controlled both by buoyancy input from the coastal current and local mixing effects. A linear diagnostic model indicates that the primary dynamical balance for the circulation is between a longshore pressure gradient and longshore mean density and stratification gradients which have summer maxima. Lesser contributions arise from longshore wind, offshore density gradient and centrifugal upwelling. Tidal rectification, deduced from coherent modulations of the semidiurnal tidal streams and low-frequency currents, supports the Browns Bank gyre circulation and drives both westward and offshore components of the near-bottom flow off Cape Sable. Thus the “centrifugal upwelling” hypothesis fails and the main driving force for Cape Sable upwelling appears to be the longshore density variations maintained by tidal mixing.