Summer surface circulation on the Newfoundland shelf and grand banks: The roles of local density gradients and remote forcing

Abstract

Summer surface circulation on the Newfoundland Shelf and Grand Banks is diagnosed from observed density profiles using the method proposed recently by Sheng and Thompson (unpublished manuscript). To assess the accuracy of the predictions they are compared against all available near-surface current measurements for the region. In accord with earlier studies, local density gradients alone cannot account for the summer surface circulation in this region. The surface currents diagnosed from the density field are much weaker than the observations, particularly near the shelf break. To explain the discrepancy we use a simple inverse method to infer the optimal inflow boundary conditions for a barotropic model from the differences between the observed and diagnosed currents. The surface circulation is then modelled as the sum of the diagnosed baroclinic component and the remotely-forced barotropic component. Overall the predicted circulation pattern agrees reasonably well with the current observations and maps of the mean surface circulation based on a variety of data sources. The predicted circulation pattern has identifiable inshore and offshore branches of the Labrador Current that split and merge as they encounter the main banks and saddles of the region. The mean square of the observed currents (Jobs) is reduced by 52% on removal of the combined effect of local density gradients and remote forcing. Removing the baroclinic component alone reduces Jobs by 40%. Our conclusion is that both remote-forcing and local density gradients make significant contributions to the circulation in this region. For example the strong shelf-break flow on the northeast Newfoundland Shelf appears to be due mainly to remote-forcing while the circulation over Flemish Cap, and the narrow coastal jet off southeast Newfoundland, are due mainly to local density gradients. © 1996 Taylor & Francis Group, LLC.