Equatorward Currents in Temperatures 1.8 - 6.0°C on the Continental Slope in the Mid-Atlantic Bight

Abstract

A large historical set of near-bottom current meter measurements along the continental slope and rise off the northeastern US coast between Cape Hatteras and Georges Banks is used to determine the mean Deep Western Boundary Current (DWBC) velocity and temperature structure and to estimate transports of several water types. While essentially all the mean flow is equatorward in the depth range 800 to 4000 m, there are three distinctly higher speed bands imbedded within a surrounding mean flow of lower speed. The banding can best be categorized according to the mean potential temperature, θ, as follows: Near bottom on the continental rise in 3400–4500 m, there is a thin relatively swift (6 cm s −1 ) cold (θ ≤ 2.0°C) band of flow. From θ = 2.1 to 2.7°C the flow is weak (3 cm s −1 ). Above this, near 2500 m depth and θ = 2.9 to 3.0°C is another band of swifter flow (7 cm s −1 ). Shallower and warmer yet, from θ = 3.1 to 3.9°C, the mean equatorward flow is again weak (3 cm s−1 ). The warmest (θ = 4.0 to 4.3°C) and shallowest (1100 to 800 m) waters included in this study are in another relatively swift band of equatorward flow (6 to 10 cm s −1 ). There are three striking points to note about this structure, that differ from traditional concepts regarding the equatorward flow of DWBC waters. Firstly, the “deep” western boundary current extends up to as shallow as 800 m in the Mid-Atlantic Bight. Secondly, water of “traditional” Labrador Sea Water (LSW) characteristics (θ = 3.3 to 3.9°C) is relatively absent (ie., has weak flow) here, but is flanked above and below by swifter bands of flow. Other water-mass tracer properties are quoted from the literature to support this banded nature of the flow, the absence of LSW, and predominance of older North Atlantic Deep Water (NADW) in its place. Thirdly, the observed speeds would imply a remarkably short transit time from a northern source (4000 km) of about 2 to 4 years. Thus the evidence suggests that some components of the DWBC, a t least within the Mid-Atlantic Bight, arrive there by some longer indirect route rather than a continuous ribbon hugging the western boundary. Approximate transports are presented for the various temperature bands and water types.