Topographic Upwelling off Southwest Nova Scotia

Abstract

The waters off Cape Sable in southwestern Nova Scotia have anomalously low temperatures and high nutrients during summer. Using a three-dimensional tidal model as well as field observations obtained between 1978 and 1985, a new mechanism is proposed to explain the origin of these anomalous properties. The numerical model predicts several areas of strong tidally induced residual upwelling and downwelling. On the submarine ridge off Cape Sable, upwelling occurs over the eastern flank and downwelling over the western flank. This upward vertical transport is very effective in supplying cold, saline, and nutrient-rich water from deep to shallow layers. The predicted upwelling and downwelling are induced by a tidal rectification process resulting from tidal currents flowing over complex bottom topography. Hence, the process is named ?topographic upwelling and downwelling.? The upwelling (downwelling) is generated by the residual currents flowing from deep to shallow (shallow to deep) waters. This process is three-dimensional so that the water parcels advected onshore from deep to shallow zones in the upwelling regime may remain in the lower layer and be carried away from the upwelling region by a longshore coastal current. Thus, the upward transfer of cold and saline water in the three-dimensional topographic upwelling may not be as efficient as the classical upwelling associated with two-dimensional (on-off shore) circulation. However, efficient vertical transfer can be accomplished in conjunction with the topographic upwelling if the cross-isobath transport associated with the upwelling carries deep water to a shallow region where there is strong vertical mixing. This combination of topographic upwelling and strong tidal mixing provides the mechanism for producing the observed cold water anomaly off Cape Sable. Topographic upwelling occurs on the eastern side of the cape, and strong tidal mixing produces a well-mixed area off the cape. These processes are verified by hydrographic, current meter, and Lagrangian drift data collected in the surrounding area. The mechanism is expected to be important in other coastal regions where strong tidal currents and large variations of bottom topography are found.