Export of Labrador Sea Water from the subpolar North Atlantic: A Lagrangian perspective

Results from 59 acoustically tracked RAFOS floats obtained during 2003–2008 are analyzed here to study the spreading pathways of Labrador Sea Water (LSW) from the subpolar to subtropical North Atlantic. An earlier paper based on a subset of these float data presented evidence of a potentially important interior pathway for the southward spreading of LSW. Here those results are reinforced with the full data set and new information on LSW pathways around the Grand Banks is presented. About 70% of the RAFOS floats were expelled into the ocean interior east of the Grand Banks and meandered slowly eastward toward the mid-Atlantic ridge, a pathway observed previously with profiling floats. Less than 10% was advected continuously around the Grand Banks by the Deep Western Boundary Current (DWBC). A larger fraction (∼17%) drifted into the subtropical interior from the Tail of the Grand Banks, suggesting that this pathway is at least if not more important than the DWBC pathway for the export of LSW to the subtropics. RAFOS floats released closest to the continental slope at 700 m depth were more likely to rapidly reach subtropical latitudes, mainly because they drifted through Flemish Pass, a 1100 m deep channel between the Grand Banks and the Flemish Cap, which protects fluid parcels from being swept off the continental slope by meanders of the North Atlantic Current. A statistical comparison of the RAFOS trajectories with more than 5000 simulated floats obtained from the Family of Linked Atlantic Models Experiment (FLAME) high-resolution ocean general circulation model reveals a similar pattern of LSW spreading. The RAFOS and simulated floats are also used to show that contrary to a previous modeling study, the isobaric RAFOS floats do not underestimate the amount of LSW that continuously follows the DWBC around the Grand Banks compared to isopycnal floats. These results have implications for the connectivity of the deep limb of the Atlantic Meridional Overturning Circulation between the subpolar and the subtropical North Atlantic.