Lagrangian Observations of the Deep Western Boundary Current in the North Atlantic Ocean.

In this study, the authors analyze the trajectories of 18 RAFOS floats, launched in the deep western boundary current (DWBC) between the Grand Banks and Cape Hatteras to investigate the kinematics and dynamics in the region where the DWBC crosses under the Gulf Stream, near 36°N (the “crossover region”). Floats deployed in the chlorofluorocarbon (CFC) maximum associated with upper Labrador Sea Water (depth ∼800 m) illustrate the entrainment process of this water mass into the Gulf Stream. The behavior of the floats (and fluid parcels) in the crossover region is strongly dependent on the meandering of the Gulf Stream. When the stream is close to its mean position, fluid parcels entrained from the upper DWBC travel along the northern edge of the stream. When a meander trough is present downstream of the entrainment location, DWBC fluid parcels cross the Gulf Stream and sometimes are expelled on the south side. This represents a previously unrecognized mechanism for transporting upper Labrador Sea Water properties across the Gulf Stream. Floats deployed in the DWBC near the deep CFC maximum that identifies overflow water from the Nordic seas (depth ∼3000 m) show a bifurcation in fluid parcel trajectories in the crossover region: fluid parcels that intersect the stream farther west tend to cross more directly and smoothly under the stream, while fluid parcels that hit the stream farther east exhibit more eddy motion and are more likely to be diverted into the interior along the Gulf Stream path. The deep float observations also reveal directly that the deep DWBC crosses under the Gulf Stream while conserving potential vorticity by sliding down the continental slope, as first conceptualized in a steady, two-layer model of the crossover. While potential vorticity is conserved along the deep float tracks on the short timescales associated with crossing under the Gulf Stream (up to a month), potential vorticity decreases over the longer timescales required for fluid parcels to transit the entire crossover region (several months to a year), consistent with what would be expected from eddy mixing.