Advection of North Atlantic Deep Water from the Labrador Sea to the southern hemisphere

Abstract

Recently formed Labrador Seawater (LSW) and overflow water from Denmark Strait (DSOW) are main components of the Atlantic Meridional Overturning Circulation. Both exhibit a distinct chlorofluorocarbon (CFC) maximum. Here we use 25 years of CFC observations in the Atlantic to study the main features of the circulation of LSW and DSOW. From the CFC data, the age and fraction of young deep water are inferred. Due to the superior spatial data resolution compared to former attempts, regional differences in the spreading velocity and pathways of young deep water become evident, dependent on the regional circulation. The observed distributions of young LSW and DSOW showed that the DWBC is the fastest pathway to reach the southern hemisphere. The downstream decrease of the fractions of young LSW in the DWBC is slower compared to model studies. From 47 N to 42 N, DWBC transports of young LSW and DSOW decrease by 44% and 49%, respectively. At 26 N, the DWBC transport of young water is still 39% of the LSW formation rate and 44%of the DSOW overflow transport. Interior pathways also exist, especially in the subpolar North Atlantic and in the transition zone between the subpolar and subtropical gyre. Compared to DSOW, the distributions indicate a higher tendency for LSW to follow additional interior pathways. North of 45 N the major part of LSW is younger than 20 years. The general weakening of new LSW formation since the 1990s worked toward a homogenization between the LSW in the western and the eastern subpolar North Atlantic.

Dynamics of North Atlantic western boundary currents

“…We find that the dLSW layer salinity is very similar from the Labrador Sea to the Flemish Cap, but is increased at the Tail of the Grand Banks, and increased further at We also show that the salinity minimum arrives at Line W before arriving at Bermuda. As Line W and Bermuda are both about 4500km downstream from the Labrador Sea, this indicates that dLSW properties travel faster along the boundary than in the interior pathway to Bermuda, which is also consistent with other studies (Bower et al, 2009;Rhein et al, 2015;Smith et al, 2016).…”

“…Recently, Rhein et al (2015) compiled 25 years of CFC data to make maps of age and fraction of young deep water. Their results imply a CFC transit time of 11 years for LSW to Line W along the boundary, which is, again, longer than our estimate.…”

“…Consequently, deep water can be tracked throughout the world ocean using its PV signature, its characteristic temperature and salinity properties and anthropogenic tracer concentrations, such as Chloroflourocarbons (CFCs), which are imprinted on it by the atmosphere at its source. Many studies have taken advantage of these unique tracer properties to track the equatorward spreading of North Atlantic deep water along the western boundary of the North Atlantic (Lynn and Reid, 1968;Talley and McCartney, 1982;Pickart et al, 1989;McCartney, 1992;Doney and Jenkins, 1994;Molinari et al, 1998;Smethie, 1993;Smethie et al, 2000;Smethie and Fine, 2001;Stramma et al, 2004;LeBel et al, 2008;Kieke et al, 2009;Rhein et al, 2002Rhein et al, , 2015Van Sebille et al, 2011). CFC studies of the DWBC, such as Pickart et al (1989) found along-boundary tracer spreading rates that were several times slower than mean speeds in the DWBC ( and a mean deep water transport of 25 Sv, with a range of comparable size (Toole et al, 2011).…”

North Atlantic Deep Water formation inhibits high Arctic contamination by continental perfluorooctane sulfonate discharges

“…Temporal changes are expected to be more variable for complex food webs with mixed prey items and biota that forage from deeper seawater such as North Atlantic pilot whales (Globicephala melas) that mainly consume squid between 400 and 700 m depth [Hoydal and Lastein, 1993;Li et al, 2015]. Rahmstorf et al, 2015;Rhein et al, 2015;Yashayaev, 2007]. Rahmstorf et al, 2015;Rhein et al, 2015;Yashayaev, 2007].…”

Relative timing of precipitation and ocean circulation changes in the western equatorial Atlantic over the last 45 ky

“…Oceanographic studies have shown that the southward transport of northern-sourced waters in the equatorial Atlantic mainly 20 takes place between ~1300 and 4000 m depth in a ~ 100 km wide Deep Western Boundary Current (DWBC) (Lux et al, 2001;Rhein et al, 2015). Using hydrographic, geochemical, and direct velocity measurements acquired in 1993 to inverse an ocean circulation model, (Lux et al, 2001) estimated that the volumetric flow of upper North Atlantic Deep Water (NADW) occupying water depths between ~1300 and 2300 m at 4.5° S within the DWBC is 11.2 Sv (1 Sv = 10 6 m 3 s -1 ).…”