Circulation of slopewater

Abstract

The recently conducted Mid-Atlantic Slope and Rise (MASAR) experiment yielded much new information on the structure and behavior of slopewater, the water mass occupying the upper “Slope Sea”, a narrow band of ocean between the Gulf Stream and the continental shelf from Cape Hatteras to the Grand Banks. The results of this experiment, combined with earlier evidence, have been used to construct a new empirical schema of slopewater circulation. Key features are: (1) inflow of Coastal Labrador Sea Water (CLSW) across the Grand Banks at the rate of 4 × 10 6 m 3 s −1, and isopycnal advection from the Gulf Stream thermocline at the rate of 6 × 10 6 m 3 s −1, the total draining eastward; (2) a closed cyclonic gyre in the western Slope Sea, transporting approximately 3 × 10 6 m 3 s −1 along the New Jersey coast southward; and (3) seasonal formation of a pycnostad by convective overturn and its flushing in approximately 6 months. Some of the CLSW inflow retroflects and turns eastward following entry into the Slope Sea, but a significant fraction flows westward along the coast and rounds the western gyre before draining eastward. The circulation just described reaches to an approximate depth of 500 m. Deeper layers move through the Slope Sea southwestward. The layers of the Gulf Stream thermocline in contact with slopewater along isopycnals encompass the Antarctic Intermediate Water (AAIW) core, as well as the nutrient maximum and oxygen minimum layers. Shoreward advection of these layers and seasonal overturn to 200 m establish conditions favoring productivity in the upper slopewater, as well as on the shelf.