Mid-depth recirculation observed in the interior Labrador and Irminger seas by direct velocity measurements

The Labrador Sea is one of the sites where convection exports surface water to the deep ocean in winter as part of the thermohaline circulation. Labra dor Sea water is characteristically cold and fresh, and it can be traced at intermediate depths (500-2,000 m) across the North Atlantic Ocean, to the south and to the east of the Labrador Sea(1-3). Widespread observations of the ocean currents that lead to this distribution of Labrador Sea water hav e, however, been difficult and therefore scarce. We have used more than 200 subsurface floats to measure directly basin-wide horizontal velocities at various depths in the Labrador and Irminger seas. We observe unanticipated recirculations of the mid-depth (similar to 700 m) cyclonic boundary curren ts in both basins, leading to an anticyclonic flow in the interior of the L abrador basin. About 40% of the floats from the region of deep convection l eft the basin within one year and were rapidly transported in the anticyclo nic flow to the Irminger basin, and also eastwards into the subpolar gyre. Surprisingly, the float tracks did not clearly depict the deep western boun dary current, which is the expected main pathway of Labrador Sea water in t he thermohaline circulation. Rather, the flow along the boundary near Flemi sh Cap is dominated by eddies that transport water offshore. Our detailed o bservations of the velocity structure with a high data coverage suggest tha t we may have to revise our picture of the formation and spreading of Labra dor Sea water, and future studies with similar instrumentation will allow n ew insights on the intermediate depth ocean circulation.