Subtropical to subpolar pathways in the North Atlantic: Deductions from Lagrangian trajectories

Surface waters of the Gulf Stream and North Atlantic Current are thought to replenish deep water formation sites at high latitudes with warm and salty subtropical waters, thereby closing the large-scale meridional overturning circulation. In recent studies of this subtropical to subpolar throughput, far fewer surface drifters were transported from the subtropical to subpolar gyre than expected on the basis of Eulerian estimates of intergyre exchange. Here, in an attempt to reconcile these Lagrangian observations with the Eulerian-based expectation of significant surface throughput, synthetic drifters launched within an eddy-resolving ocean general circulation model are used to examine Lagrangian pathways from the subtropical to subpolar gyre. The results of this study indicate that drifters in the observational record are limited in their ability to measure intergyre exchange and its temporal variability by their short lifetimes, their variable launch locations, and, importantly, their inability to follow the 3-D flow field. Synthetic floats launched within the ocean model and advected with the 3-D flow field indicate that subtropical to subpolar exchange in the North Atlantic is primarily located subsurface; while less than 5% of floats launched at 15 m reach the eastern subpolar gyre within 4 years, close to 30% of drifters launched at 700 m do so. The transport of subsurface waters to high latitudes is shown to occur primarily along density surfaces as they shoal northward toward subpolar latitudes.