The Mid-Atlantic Bight (MAB) continental shelf and Georges Bank, off the northeast U.S., are among the most productive of marine ecosystems. Within the 8 years of satellite ocean color imagery from the Coastal Zone Color Scanner (CZCS), there is clear evidence of annual enhancement of chlorophyll at the shelfbreak of this highly productive region. From the CZCS archive (1979-1986) and in situ observations, I defined fundamental attributes of this ecological feature. It develops during the spring transition from well-mixed to stratified conditions. As defined by ocean color, its temporal window is between mid-April and the end of June, and expression is intermittent intraannually. Enhanced shelfbreak chlorophyll averages more than twice that of adjacent shelf and slope waters. It is typically tens of km cross-shelf, and its alongshelf scale ranges between ca. 100 and 1100 km. With only one exception, all occurrences coincided with seaward entrainment of shelf water, most frequently by Gulf Stream warm-core rings (WCRs).

Enhanced shelfbreak chlorophyll during the spring transition consistently coincided with the shelfbreak front, the dynamic boundary between shelf and slope waters. Because enhanced chlorophyll develops when near-surface waters are normally nutrient-depleted, local nutrient supply via frontal processes is implied. Through combination of in situ observations from three research programs with satellite ocean color observations from the Ocean Color and Temperature Sensor (OCTS), I examined frontal processes associated with shelfbreak chlorophyll enhancement during the spring transition of 1997.

Two separate occurrences of shelfbreak chlorophyll enhancement developed during May of 1997. The first occurrence developed along more than 200 km of the MAB shelfbreak in association with meanders of the shelfbreak front. The meanders entrained shelf water seaward. Observations at two meanders captured 20-25 m upwelling of shelf water along isopycnals of the shelfbreak front. The deep shelf water is relatively nutrient-rich during late spring, and its upwelling defined the basis for enhanced shelfbreak chlorophyll. The second occurrence developed along more than 100 km of the southern flank of Georges Bank in association with seaward entrainment of shelf water by a WCR. Satellite observations were consistent with a role for upwelling along isopycnals of the shelfbreak front. In situ observations showed a significant
inverse correlation between gradient Richardson numbers (Ri) and chlorophyll over the southern flank of the bank. The enhanced chlorophyll was in a region of low (some sub-critical) Ri, supporting the role of nutrient enrichment by turbulent
vertical mixing.

Annual enhancement of chlorophyll along the shelfbreak of the MAB and Georges Bank is an important aspect of regional ecology. It is based in the influence of frontal structure on circulation at the shelfbreak. Understanding the processes driving this annual biological enhancement requires the integration of satellite and in situ observation.