Transport Variability of the Irminger Sea Deep Western Boundary Current From a Mooring Array

Abstract

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.

Mean Conditions and Seasonality of the West Greenland Boundary Current System near Cape Farewell

“…the OSNAP East line northeast of Cape Farewell using the first two years of data(Hopkins et al 2019). Using only the first two years of WG data, to be consistent with theHopkins et al (2019) study, the boundary current overflow water transport is 8.8 6 2.8 Sv at OSNAP WG.…”

Moored Observations of the Iceland‐Scotland Overflow Plume Along the Eastern Flank of the Reykjanes Ridge

“…The observations reported here are from the first 4 years (2014)(2015)(2016)(2017)(2018) of the OSNAP Iceland Basin array, and further updates to these results will be forthcoming as OSNAP is extended; it is currently planned to continue until at least 2024. Besides the observations of the ISOW plume described here, the basin-wide OSNAP array provides continuous measurements across the other major deep boundary current systems of the subpolar gyre-including the DWBC off east Greenland (Hopkins et al, 2019), west Greenland (Pacini et al, 2020), and in the Labrador Sea (Zantopp et al, 2017)-so that an integrated, long-term analysis of the transports of the deep limb of the AMOC around the entire subpolar North Atlantic can be achieved. The subpolar North Atlantic is currently undergoing remarkable changes, having recently experienced the largest freshening event in the upper eastern subpolar basin observed in the past 120 years (Holliday et al, 2020).…”

Cyclonic eddies in the West Greenland Boundary Current System

“…Additionally, the detided, nongridded data from the OSNAP EG moorings are used to compare cyclone statistics between the east and west sides of Cape Farewell. Details on the processing of these data, as well as the mean conditions and seasonality at the array site, can be found in Le Bras et al (2018) and Hopkins et al (2019). The detiding has been performed using the same harmonic tidal routine as the OSNAP WG data.…”

“…The deep part of the boundary current system advects roughly equal amounts of Northeast Atlantic Deep Water (NEADW) and Denmark Strait Overflow Water (DSOW), both of which are important components of the AMOC (Dickson and Brown 1994). NEADW represents overflow waters emanating from the eastern part of the Greenland-Scotland Ridge (Lee and Ellett 1965), and DSOW represents overflow waters emanating from the Denmark Strait (Dickson and Brown 1994;Tanhua et al 2005;Hopkins et al 2019). DSOW is denser, colder, and fresher than NEADW, and neither water mass exhibits seasonality in properties or The Labrador Sea has long been identified as a region with high eddy kinetic energy (e.g., Gascard and Clarke 1983;Lilly et al 1999Lilly et al , 2003Eden and Böning 2002;Prater 2002;Chanut et al 2008).…”