The general circulation and open-ocean deep convection in the Labrador Sea: A study using subsurface floats.

摘要

A study of the seasonal evolution of the general circulation and water mass formation in the Labrador Sea is presented. To examine the localized process of open-ocean deep convection and the context in which this process occurs, a new data set consisting of hydrographic measurements and horizontal and vertical velocity measurements from more than 200 neutrally-buoyant, subsurface P-ALACE and SOLO floats is analyzed.; The occurrence and extent of Labrador Sea deep convection in winters 1996–97 and 1997–98 is described. Deep convection, identified by deep mixed layers and high vertical velocity variance, was concentrated in the western basin, although deep mixed layers were also observed in the northern basin and southwest of Cape Farewell. Deep convection occurred as late as April, after surface forcing had ceased. Floats were subject to a sampling bias toward horizontally convergent regions that precludes estimation of mean vertical velocity in the convection region. In a one-dimensional heat balance model, the winter surface heat flux cannot be balanced by plume-scale motions alone, but may include a contribution from vertical motion at longer time scales.; The mean circulation at 700 m depth in the Labrador and Irminger Seas is described from objectively-analyzed float drift velocity data. Unanticipated recirculations of the mid-depth cyclonic boundary currents occur in both basins, comprising an anticyclonic flow in the Labrador Sea. Floats were rapidly transported to the Irminger Sea in this anticyclonic flow, and also eastward into the subpolar gyre. Float trajectories did not clearly depict the deep western boundary current. Rather, flow along the boundary near Flemish Cap is dominated by eddies that transport water offshore.; Features of the seasonal-mean hydrography and general circulation of the Labrador Sea are described. A sudden surface freshening measured by floats in late winter is attributed to float drift toward freshwater sources on the continental shelves and in the northern basin. Calculation of the 1997 seasonal-mean heat budget indicates a balance of terms in three of four seasons. The Labrador Sea accounts for 22% of the net heat transport into the North Atlantic Ocean, and is therefore a significant component of the meridional overturning circulation.