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

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.