Search

The Deep Western Boundary Current in the subpolar North Atlantic is the lower limb of the Atlantic Meridional Overturning Circulation and a key component of the global climate system. Here, a mooring array deployed at 60°N in the Irminger Sea, between 2014 and 2016, provides the longest continuous record of total Deep Western Boundary Current volume transport at this latitude. The 1.8‐year averaged transport of water denser than σθ = 27.8 kg/m3 was −10.8 ± 4.9 Sv (mean ± 1 std; 1 Sv = 106 m3/s). Of this total, we find −4.1 ± 1.4 Sv within the densest layer (σθ > 27.88 kg/m3) that originated from the Denmark Strait Overflow. The lighter North East Atlantic Deep Water layer (σθ = 27.8–27.88 kg/m3) carries −6.5 ± 7.7 Sv. The variability in transport ranges between 2 and 65 days. There is a distinct shift from high to low frequency with distance from the East Greenland slope. High‐frequency fluctuations (2–8 days) close to the continental slope are likely associated with topographic Rossby waves and/or cyclonic eddies. Here, perturbations in layer thickness make a significant (20–60%) contribution to transport variability. In deeper water, toward the center of the Irminger Basin, transport variance at 55 days dominates. Our results suggest that there has been a 1.8 Sv increase in total transport since 2005–2006, but this difference can be accounted for by a range of methodological and data limitation biases. Plain Language Summary: A network of currents in the Atlantic Ocean plays an important role in the global climate system, redistributing heat, salt, nutrients, and carbon around the globe. It is made up of a northward flow of warm, salty water in the upper layers of the Atlantic, and a deep, southward flow of colder, denser water. Dense water is formed at high latitudes when surface waters release heat to the atmosphere and sink toward the seafloor. This forms the Deep Western Boundary Current that moves southward to the east of Greenland. Knowing how much water is carried and whether the current is stable is vital to our understanding of global climate. Using measurements of temperature, salinity, and current speed from instruments deployed between 2014 and 2016, we find that the Deep Western Boundary Current transports on average 10.8 × 106 m3 of water a second. This transport varies in time. It increases and decreases from day to day and month to month. Eddies, waves, and other ocean currents all contribute to these fluctuations. Our results suggest that transport has increased since 2005–2006, but it is possible to account for this change by considering the limitations of previous data sets and the methods used to calculate the transport. Key Points: Volume transport of the Deep Western Boundary Current in the Irminger Sea between 2014 and 2016 was ‐10.8 +/‐ 4.9 SvTransport variability shifts from high (2‐8 days) to low (55 days) frequencies with distance down slopeNo significant long‐term trend in transport was identified [ABSTRACT FROM AUTHOR]

Abstract: Repeat glider sections obtained during 2014–2016, as part of the Overturning in the Subpolar North Atlantic Program, are used to quantify the circulation and transport of North Atlantic Current (NAC) branches over the Rockall Plateau. Using 16 glider sections collected along 58°N and between 21°W and 15°W, absolute geostrophic velocities are calculated, and subsequently the horizontal and vertical structure of the transport are characterized. The annual mean northward transport (± standard deviation) is 5.1 ± 3.2 Sv over the Rockall Plateau. During summer (May to October), the mean northward transport is stronger and reaches 6.7 ± 2.6 Sv. This accounts for 43% of the total NAC transport of upper‐ocean waters (σO<27.55 kg/m3) estimated by Sarafanov et al. (2012, https://doi.org/10.1029/2011JC007572) along 59.5°N, between the Reykjanes Ridge and Scotland. Two quasi‐permanent northward flowing branches of the NAC are identified: (i) the Hatton Bank Jet (6.3 ±  2.1 Sv) over the eastern flank of the Iceland Basin (20.5°W to 18.5°W) and (ii) the Rockall Bank Jet (1.5 ±  0.7 Sv) over the eastern flank of the Hatton‐Rockall Basin (16°W to 15°W). Transport associated with the Rockall Bank Jet is mostly depth independent during summer, while 30% of the Hatton Bank Jet transport is due to vertical geostrophic shear. Uncertainties are estimated for each individual glider section using a Monte Carlo approach, and mean uncertainties of the absolute transport are less than 0.5 Sv. Although comparisons with altimetry‐based estimates indicate similar large‐scale circulation patterns, altimetry data do not resolve small mesoscale current bands in the Hatton‐Rockall Basin which are strongly needed for the right transport estimates. [ABSTRACT FROM AUTHOR]