Continuous Flow of Upper Labrador Sea Water around Cape Hatteras

Magdalena Andres,Mike Muglia,John Bane,Frank Bahr

doi:10.1038/s41598-018-22758-z

Abstract

Six velocity sections straddling Cape Hatteras show a deep counterflow rounding the Cape wedged beneath the poleward flowing Gulf Stream and the continental slope. This counterflow is likely the upper part of the equatorward-flowing Deep Western Boundary Current (DWBC). Hydrographic data suggest that the equatorward flow sampled by the shipboard 38 kHz ADCP comprises the Upper Labrador Sea Water (ULSW) layer and top of the Classical Labrador Sea Water (CLSW) layer. Continuous DWBC flow around the Cape implied by the closely-spaced velocity sections here is also corroborated by the trajectory of an Argo float. These findings contrast with previous studies based on floats and tracers in which the lightest DWBC constituents did not follow the boundary to cross under the Gulf Stream at Cape Hatteras but were diverted into the interior as the DWBC encountered the Gulf Stream in the crossover region. Additionally, our six quasi-synoptic velocity sections confirm that the Gulf Stream intensified markedly at that time as it approached the separation point and flowed into deeper waters. Downstream increases were observed not only in the poleward transport across the sections but also in the current’s maximum speed.

Highlights

The “attached” Gulf Stream—the western boundary current of the wind-driven North Atlantic subtropical gyre— follows the shelf break of the southeastern United States to Cape Hatteras, where it separates abruptly from the continental margin and proceeds to the northeast (Fig. 1)
It is generally difficult to separate transports into the throughput and local recirculations associated with mesoscale eddies that impinge on the offshore edge of the Gulf Stream
Shipboard observations from AR-15 and the argo.whoi. edu (Argo) float deployed during that cruise, suggest that flow in the Deep Western Boundary Current (DWBC)’s Upper Labrador Sea Water (ULSW) and Classical Labrador Sea Water (CLSW) layers is continuous along the boundary around the Cape, at least under some circumstances

Summary

Introduction

The “attached” Gulf Stream—the western boundary current of the wind-driven North Atlantic subtropical gyre— follows the shelf break of the southeastern United States to Cape Hatteras, where it separates abruptly from the continental margin and proceeds to the northeast (Fig. 1). Both interior and DWBC pathways are thought to play a role in the export of AMOC cold limb waters from the subpolar to the subtropical gyre on its equatorward route[6]. Previous studies have shown that the Gulf Stream’s detachment latitude may be influenced by the strength of the DWBC here[13] Those DWBC waters that pass under the separating Gulf Stream in the broad “crossover region” around Cape Hatteras are thought to move to deeper isobaths, conserving their potential vorticity as they continue equatorward along the boundary[14]. Tracer studies suggest that there must be a DWBC “fast track” that allows some young ULSW (recently ventilated waters with high chlorofluorocarbon concentrations) to flow along the boundary and reach 26°N with little dilution by older interior waters[7]

Results

Discussion

Conclusion