Lagrangian observations of the deep western boundary current in the North Atlantic Ocean. part II: The Gulf Stream-deep western boundary current crossover

In this study, the authors analyze the trajectories of 18 RAFOS floats, lau nched in the deep western boundary current (DWBC) between the Grand Banks a nd Cape Hatteras to investigate the kinematics and dynamics in the region w here the DWBC crosses under the Gulf Stream, near 36 degrees N (the "crosso ver region"). Floats deployed in the chlorofluorocarbon (CFC) maximum assoc iated with upper Labrador Sea Water (depth similar to 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 depen dent 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 o f the entrainment location, DWBC fluid parcels cross the Gulf Stream and so metimes are expelled on the south side. This represents a previously unreco gnized mechanism for transporting upper Labrador Sea Water properties acros s the Gulf Stream. Floats deployed in the DWBC near the deep CFC maximum th at identifies overflow water from the Nordic seas (depth similar to 3000 m) show a bifurcation in fluid parcel trajectories in the crossover region: f luid parcels that intersect the stream farther west tend to cross more dire ctly 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 i nto the interior along the Gulf Stream path. The deep float observations al so reveal directly that the deep DWBC crosses under the Gulf Stream while c onserving potential vorticity by sliding down the continental slope, as fir st conceptualized in a steady, two-layer model of the crossover. While pote ntial vorticity is conserved along the deep float tracks on the short times cales associated with crossing under the Gulf Stream (up to a month), poten tial vorticity decreases over the longer timescales required for fluid parc els to transit the entire crossover region (several months to a year), cons istent with what would be expected from eddy mixing.