Mixing of chlorophyll from the Middle Atlantic Bight cold pool into the Gulf Stream at Cape Hatteras in July 1993

In July 1993 we collected hydrographic data and information on chlorophyll distribution on the continental shelf north of Cape Hatteras and across the shelf break at Cape Hatteras. The data show that a warm, transparent mixed layer lies over much colder, euphotic, chlorophyll-rich bottom water on the shelf. This layer has temperature and salinity properties characteristic of the Middle Atlantic Eight (MAB) cold pool, a distinctive mass of cold bottom water formed when cold water from the Gulf of Maine and Scotian Shelf is isolated from surface water by vernal warming and seasonal stratification [Houghton et nl., 1982]. The constant density of this chlorophyll-rich water (sigma(theta) = 25.0-25.6) combined with a strong chlorophyll gradient along the 25 sigma(theta) isopycnal at the shelf break indicates that chlorophyll advected off the shelf at Cape Hatteras in July 1993. TS diagrams further indicate that cold pool water, and the chlorophyll it contained, mixed into upper levels of the Gulf Stream. Thus the MAB may contribute to the nutrient budget of Atlantic surface waters through a long loop of circulation that transports deep water from the Labrador Sea to Cape Hatteras.