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

During late spring 1997, a Gulf Stream warm-core ring (WCR) strongly influe nced water mass and chlorophyll distributions alone, 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 wa ter encircled nearly the entire WCR, and horizontal convergence toward the central southern flank was evident. Scotian Shelf water (SSW) extended acro ss Northeast Channel, onto Georges Bank near the northeast peak, and along the southern flank between the 60-m isobath and the shelfbreak front. SSW e xtended furthest southwest along the southern flank shelfbreak. Satellite i magery from the ocean color and temperature sensor (OCTS) showed that by la te May, in addition to the high chlorophyll concentrations within the entra ined shelf water encircling the WCR, pigment rich bands of chlorophyll deve loped along the 60- and 100-m isobaths. Where this biological enhancement d eveloped along > 100 km of the shelfbreak (100-m isobath), shelf waters ext ended furthest seaward due to entrainment by the WCR. The enhanced shelfbre ak chlorophyll was sampled in situ alone two transects separated by approxi mate to 40 km along shelf Along both transects, the enhanced chlorophyll co incided with divergent cross-shelf flow, maximum along-shelf flow and minim um surface temperature. Coincidence of the highest surface chlorophyll and lowest surface temperature with divergent cross-shelf flow is consistent wi th upwelling. Coincidence with along-shelf jets and their associated vertic al shear is consistent with turbulent vertical mixing. The importance of tu rbulent vertical mixing was supported. Maximum velocity at the shelfbreak c oincided with the subsurface temperature minimum of SSW near 30-m depth. Es timated gradient Richardson numbers (Ri) above the SSW were below critical ( < 0.25) within the shelfbreak chlorophyll maximum along both transects, a nd surface chlorophyll was significantly, inversely correlated with Ri alon g the southern flank. WCR entrainment of shelf water continued into June as the WCR propagated southwestward. On June 11, high chlorophyll concentrati ons along the shelfbreak north of the WCR were observed by the OCTS. Satell ite sea-surface temperature showed that in addition to WCR entrainment, bre aking waves (<lambda> approximate to 30 km) of the shelf-slope front coinci ded directly with the locally enhanced chlorophyll. The waves propagated we st at approximate to 12 km day(-1). Vertical mixing due to these breaking f rontal waves may have contributed to local nutrient enrichment of near-surf ace waters at the shelfbreak. (C) 2000 Published by Elsevier Science Ltd.