Tracing the thermohaline circulation in the western North Atlantic using chlorofluorocarbons

Abstract

Measurement of chlorofluorocarbons (CFCs), F-11 and F-12, were obtained from vertical sections extending across the Deep Western Boundary Current (DWBC) along the western margin of the North Atlantic Ocean between 32°–45°N in 1983 and 1986. These sections revealed an upper and a deep CFC maximun with highest concentrations along the continental slope. The upper maximum had a potential temperature ranging from 4.2°–4.6°C, a core density of σ 1.5=34.56, and occurred at a depth of about 800m adjacent to the continental slope. Water with classical Labrador Sea Water θ/S characteristics occurs just above the CFC minimum between the upper and lower maxima and indicates that a significant volume of classical Labrador Sea Water has not entered the region south of the Grand Banks for the last 3 decades. The deep CFC maximum is from dense water overflowing the Greenland-Iceland-Scotland ridge. It occurred at about 3500m depth and had a potential temperature range of 2.02°–2.18°C and a core density of σ 4=45.87. A source function of F-11 and F-12 for the deep maximum was estimated using temperature, salinity and tracer data from previous studies. The core of the DWBC defined by the deep CFC maximum was estimated to consist of roughly 2/3 Gibbs Fracture Water and 1/3 Denmark Strait Overflow Water, however 80% or more of the F-11 and F-12 signal was derived from Denmark Strait Overflow Water. Ages for the upper and lower CFC maxima in the DWBC were estimated from the CFC data. These ages ranged from 11–12 years at the northernmost section to 18 years at the southernmost section for both features. This corresponds to apparent mean current speeds of about 0.8cm s −1 for the upper maximum and 1.4cm s −1 for the deep maximum. An age of 8–9 years with a corresponding apparent mean current speed of 2.2–2.5cm s −1 was also estimated for the deep CFC maximum from the observed increase in the F-11 concentration at 41°N between 1983 and 1986. The apparent mean current speeds are significantly less than current speeds measured with long-term current meter moorings. Two reasons for this are (1) mixing between the DWBC core and adjacent water causes ages estimated from CFC data to be overestimated and hence the associated current speed to be underestimated (PICKART, HOGG and SMETHIE, 1989) and (2) water propagating along the western boundary spends some time in recirculation gyres. The northern recirculation gyre south of the eastern North American continental margin is clearly apparent in the lateral distributions of F-11 and F-12. The apparent mean current speed in the northern recirculation gyre was estimated to be 1.3cm s −1 from the CFC data. Seaward of the DWBC north of the New England Seamount chain, high CFC/low silica parcels of water were observed interspersed with low CFC/high silica parcels of water along the bottom. The high CFC/low silica water is overflow water with a slightly greater component of Denmark Strait Overflow Water than contained in the DWBC deep CFC maximum. The low CFC/high silica water is of southern origin and has a component of Antarctic Bottom Water.