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

Part II: The Gulf Stream–Deep Western Boundary Current Crossover

Amy S. Bower
and
Heather D. Hunt

Abstract

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.

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Amy S. Bower
and
Heather D. Hunt

Abstract

Twenty-six RAFOS floats were deployed in the deep western boundary current (DWBC) of the North Atlantic Ocean between the Grand Banks and Cape Hatteras in 1994–95 and tracked acoustically for up to two years. Half of the floats were launched in the upper chlorofluorocarbon (CFC) maximum associated with upper Labrador Sea Water (∼800 m), and the other half near the deep CFC maximum that identifies the overflow water from the Nordic seas (∼3000 m). The float observations reveal the large-scale pathways of these recently ventilated water masses in the subtropics. The shallow float tracks show directly that upper Labrador Sea Water is diverted away from the western boundary and into the interior at the location where the DWBC encounters the Gulf Stream near 36°N (the “crossover region”), consistent with previous hydrographic studies. East of the crossover region, only one upper Labrador Sea Water float out of seven (∼15%) “permanently” crossed to the south side of the stream in two years, caught in a cold core ring formation event. The other shallow floats recirculated north of the Gulf Stream, apparently confined by the mean potential vorticity gradient aligned with the stream. The deep floats closely followed the topography to the crossover region, then revealed a bifurcation in fluid parcel pathways. One branch continues equatorward along the western boundary, and the other turns first eastward along the Gulf Stream path, then southward. The deep float pathways, including the bifurcation in the crossover region, can be explained in terms of the deep potential vorticity distribution. Comparison of the float results with results from recent modeling studies suggests that the deep flow is strongly influenced by both the depth of the main pycnocline and bottom depth. The effective spreading rates of upper Labrador Sea Water and overflow water estimated directly from the float data, southward at 0.6 ± 0.2 cm s−1 and 1.4 ± 0.4 cm s−1, respectively, agree well with tracer-derived spreading rates. Mean velocities in the DWBC, equatorward at 2–4 cm s−1 (upper Labrador Sea Water) and 4–5 cm s−1 (overflow water), are consistent with other in situ measurements. One deep float drifted almost 4000 km along the western boundary in two years, revealing a “fast track” for the spreading of overflow water in the DWBC. These observations emphasize the importance of the crossover region in the spreading and mixing of recently ventilated water masses, addressed in Part II of this study.

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