Historical and recent evidence of Scotian Shelf Water on southern Georges Bank

Historical data from 1912–1987 indicate that low salinity (less than 32.0 psu) near-surface water occurs occasionally on southern Georges Bank during May, while none occurs during April. Composite monthly plots of historical near-surface salinity and estimated advection rates show that the southwestern Scotian Shelf is the immediate upstream source of this low salinity water. Optimally interpolated satellite-derived SST and hydrographic data reveal that very cold (less than 2.0°C), low salinity (less than 32.0 psu) Scotian Shelf Water (SSW), initially located south of the 200-m isobath off southern Georges Bank in early March 1992, moved north onto southern Georges Bank during April 1992. SSW was not as extensive during the same period in 1993, as evidence by significantly higher temperatures and salinities. These differences show large interannual variability in the transport and/or properties of SSW flowing onto Georges Bank. Lower (higher) salinities measured during spring 1992 (1993) on southern Georges Bank are consistent with higher (lower) St Lawrence River discharge noted during spring 1991 (1992) and the ∼ nine month lag between annual discharge maxima from rivers located upstream and minimum salinity at Cape Sable in February. However, comparisons between historical occurrences of low salinity (less than 32.0 psu) SSW on southern Georges Bank noted for May 1966, 1971, and 1978, and cumulative St Lawrence River discharge from the spring prior to each occurrence, show no relationship. This suggests that the occurrence of low salinity water on southern Georges Bank is not directly related to variations in upstream river discharge. gh an idealized model of wind forcing on a bank-trapped density front shows that near-surface flow is related to superposition of an Ekman layer and the along-bank gyral circulation, evidence from late winter-spring 1992 and 1993 on Georges Bank shows that except for on-bank movement of the shelf-slope front during late spring 1992, simple Ekman theory does not explain on-bank movement of the SSW plume during 1992 nor the immobile nature of the shelf-slope front during late winter 1992 and late winter-spring 1993. Surface heat flux and satellite-derived SST from Georges Bank and the Scotian Shelf together with a slab mixed-layer model allows estimation of the advective heat flux and transport of SSW flowing onto Georges Bank from the Scotian Shelf during late winter-spring 1992. Mean advective heat flux onto Georges Bank during March–May 1992 is −14.5 mW cm−2 with an estimated error of ∼3.5 mW cm−2, demonstrating that advection of cold SSW onto Georges Bank is required. Mean velocity and transport of SSW flowing onto Georges Bank during March–May 1992 are 13.4 cm s−1 and 0.21 Sv, assuming a SSW depth (width) of 40 m (40 km), demonstrating that flow of SSW onto Georges Bank was robust during the period. At this time, the dynamics that control the flow of SSW across Northeast Channel are unknown.