Influence of a Gulf Stream warm-core ring on water mass and chlorophyll distributions along the southern flank of Georges Bank

Abstract

During late spring 1997, a Gulf Stream warm-core ring (WCR) strongly influenced water mass and chlorophyll distributions along the southern flank of Georges Bank. Entrainment of Georges Bank shelf water by the WCR, centered at the central southern flank, persisted through May. By late May, shelf water encircled nearly the entire WCR, and horizontal convergence toward the central southern flank was evident. Scotian Shelf water (SSW) extended across Northeast Channel, onto Georges Bank near the northeast peak, and along the southern flank between the 60-m isobath and the shelfbreak front. SSW extended furthest southwest along the southern flank shelfbreak. Satellite imagery from the ocean color and temperature sensor (OCTS) showed that by late May, in addition to the high chlorophyll concentrations within the entrained shelf water encircling the WCR, pigment rich bands of chlorophyll developed along the 60- and 100-m isobaths. Where this biological enhancement developed along >100 km of the shelfbreak (100-m isobath), shelf waters extended furthest seaward due to entrainment by the WCR. The enhanced shelfbreak chlorophyll was sampled in situ along two transects separated by ≈40 km along shelf. Along both transects, the enhanced chlorophyll coincided with divergent cross-shelf flow, maximum along-shelf flow and minimum surface temperature. Coincidence of the highest surface chlorophyll and lowest surface temperature with divergent cross-shelf flow is consistent with upwelling. Coincidence with along-shelf jets and their associated vertical shear is consistent with turbulent vertical mixing. The importance of turbulent vertical mixing was supported. Maximum velocity at the shelfbreak coincided with the subsurface temperature minimum of SSW near 30-m depth. Estimated gradient Richardson numbers (Ri) above the SSW were below critical (<0.25) within the shelfbreak chlorophyll maximum along both transects, and surface chlorophyll was significantly, inversely correlated with Ri along the southern flank. WCR entrainment of shelf water continued into June as the WCR propagated southwestward. On June 11, high chlorophyll concentrations along the shelfbreak north of the WCR were observed by the OCTS. Satellite sea-surface temperature showed that in addition to WCR entrainment, breaking waves ( λ≈30 km) of the shelf-slope front coincided directly with the locally enhanced chlorophyll. The waves propagated west at ≈12 km day −1 . Vertical mixing due to these breaking frontal waves may have contributed to local nutrient enrichment of near-surface waters at the shelfbreak.