Mean circulation and high-frequency flow amplification in the Sable Gully

Abstract

The Sable Gully, a broad, shelf break submarine canyon approximately 40km east of Sable Island on the eastern Scotian Shelf, separates Banquereau and Sable Island Banks. Unique among canyons on the eastern Canadian continental shelf because of its depth, steep slopes and extension far onto the shelf, its ecological significance and increasing human pressures led to its designation in 2004 under Canada's Oceans Act as the first Marine Protected Area (MPA) in the Atlantic Region. To improve the state of knowledge of the Gully MPA, a multi-disciplinary field program was carried out in 2006–07; the physical oceanographic component consisted of the deployment (April 2006) and recovery (August 2007) of four current meter moorings and CTD surveys. Analysis of this 16-month mooring deployment demonstrates that the mean circulation above the canyon rim (~200m) is characterized by a southwestward flow that appears unaffected by the canyon topography. There is also some indication of the existence of an eddy at rim depth. Below 500m, the circulation is dominated by an upcanyon flow (of order 0.02ms−1) at the mooring array (halfway between the canyon head and mouth). The mean, 200m-bottom transport towards the head of the Gully was estimated as 35,500m3s−1, implying an upwelling velocity of 1.7×10−4ms−1 (14md−1) over the area. Results also show bottom-intensified tidal flows and non-linear constituents due to the interaction of K1, O1, M2 and S2 components along the thalweg of the canyon; the strong overtides and compound tides observed in the Gully make it unique among canyons. Further analyses provide evidence of enhanced mixing in the Gully (Kv~180×10−4m2s−1), which is approximately 20 times that observed on the adjoining Scotian Shelf. Total variance of the currents in the Gully is about 2.5 times greater than that observed on the nearby continental slope with an equivalent water depth.