Circulation and Deep-Water Export at the Western Exit of the Subpolar North Atlantic

Abstract

The current system east of the Grand Banks was intensely observed by World Ocean Circulation Experiment (WOCE) array ACM-6 during 1993-95 with eight moorings, reaching about 500 km out from the shelf edge and covering the water column from about 400-m depth to the bottom. More recently, a reduced array by the Institut für Meerskunde (IfM) at Kiel, Germany, of four moorings was deployed during 1999-2001, focusing on the deep-water flow near the western continental slope. Both sets of moored time series, each about 22 months long, are combined here for a mean current boundary section, and both arrays are analyzed for the variability of currents and transports. A mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper-layer extrapolation and isopycnal subdivision of the mean transports into deep-water classes. The offshore part of the combined section is dominated by the deep-reaching North Atlantic Current (NAC) with currents still at 10 cm s Ϫ1 near the bottom and a total northward transport of about 140 Sv (Sv ϵ 10 6 m 3 s Ϫ1 ), with the details depending on the method of surface extrapolation used. The mean flow along the western boundary was southward with the section-mean North Atlantic Deep Water outflow determined to be 12 Sv below the ϭ 27.74 kg m Ϫ3 isopycnal. However, east of the deep western boundary current (DWBC), the deep NAC carries a transport of 51 Sv northward below ϭ 27.74 kg m Ϫ3 , resulting in a large net northward flow in the western part of the basin. From watermass signatures it is concluded that the deep NAC is not a direct recirculation of DWBC water masses. Transport time series for the DWBC variability are derived for both arrays. The variance is concentrated in the period range from ϳ2 weeks to 2 months, but there are also variations at interannual and longer periods, with much of the DWBC variability being related to fluctuations and meandering of the NAC. A significant annual cycle is not recognizable in the combined current and transport time series of both arrays. The moored array results are compared with other evidence on deep outflow and recirculation, including recent models of different types and complexity.

Interdecadal variability of the meridional overturning circulation as an ocean internal mode

“…7): one along the deep western boundary from east of Newfoundland and the other entering the eastern North Atlantic basin and traveling southward from both sides of the Mid-Atlantic Ridge, which is more clear in the integrated mass transport by the LSW (not shown). These two simulated pathways are consistent with observational data (e. g., Schott et al 2004). However, it may also be a feature of coarse-resolution models.…”

Mechanisms for decadal scale variability in a simulated Atlantic meridional overturning circulation

“…Dickson and Brown (1994) estimated that the dense overflows double in volume transports due to entrainment, which is somewhat more efficient than found in this model. Observational estimates of EODW (8.9 Sv; Schott et al 2004) are similar to the modelled values.…”

Variability in the North Atlantic Deep Western Boundary Current : upstream causes and downstream effects as observed at Line W

“…Over the years, observations at the exit of the subpolar gyre as well as along the continental slope in the western North Atlantic consistently showed the existence of a deep boundary current carrying dense poorly stratified waters southward (eg. Swallow and Worthington (1961), Schott et al (2004), Pickart and Smethie (1998), Joyce et al (2005), Toole et al (2010)), reinforcing the existing view of a continuous DWBC that connects the northern North Atlantic and the subtropics. But more recently, Fischer and Schott (2002) float ob-servations, and later work by Bower et al (2009), both with float trajectories and numerical simulations, suggested that a larger fraction of the LSW that reaches the subtropics does so via an interior pathway.…”