Observations of a Gulf Stream frontal eddy on the Georgia continental shelf, April 1977

Abstract

Satellite, hydrographic, and data from moored current meters are used to show the effect of Gulf Stream frontal disturbances on low-frequency current and temperature variability, water exchange, and nutrient flux in the outer region of the Georgia shelf. Perturbations of the Gulf Stream cyclonic front are commonly observed as folded wave patterns in routine satellite-derived analyses of the western boundary of the Gulf Stream between Cape Hatteras and Miami. The disturbances consist of southward-flowing warm filaments or streamers of near-surface Gulf Stream water, 15 to 20 m deep, which can extend 35 to 40 km over the outer shelf around a cold upwelled core. Downstream dimensions of the filaments reach 100 to 200 km in the region from Jupiter, Florida, to Charleston, South Carolina, 10 to 50 km south of Jupiter, and 200 to 300 km between Charleston and Cape Hatteras. The features are defined as cyclonic, cold-core frontal eddies due to their flow and water mass properties. They appear to form from amplified waves in the Gulf Stream cyclonic front on an annual average of one every two weeks but with considerable monthly variability. They can persist up to three weeks and travel to the north with the same phase speed as the waves, approx. 40 cm s −1. The cyclonic circulation in frontal eddies provides a means for rapid shelf-Gulf Stream water exchange. The eddies appear to control the residence time of the outer shelf waters, defined as the mean separation time between eddy events, or approx. two weeks. Upwelling in the cold core extended into the euphotic zone (45 m) and shoreward (35 to 40 km) beneath the southward-flowing warm filament in a bottom intrusion layer 20 m thick. The annual nitrogen input to the shelf waters by this process is estimated as 55,000 tons each year, about twice all other estimated nitrogen sources combined; it can support an annual carbon production by phytoplankton of 32 to 64 g C m −2y −1 with no nitrogen recycling.