Description: Long‐term observations from a 17 year long mooring array at the exit of the Labrador Sea at 53°N are compared to the output of a high‐resolution model (VIKING20). Both are analyzed to define robust integral properties on basin and regional scale, which can be determined and evaluated equally well. While both, the observations and the model, show a narrow DWBC cyclonically engulfing the Labrador Sea, the model's boundary current system is more barotropic than in the observations and spectral analysis indicates stronger monthly to interannual transport variability. Compared to the model, the observations show a stronger density gradient, hence a stronger baroclinicity, from center to boundary. Despite this, the observed temporal evolution of the temperature in the central Labrador Sea is reproduced. The model results yield a mean export of North Atlantic Deep Water (NADW) (33.0 +/‐ 5.7 Sv), which is comparable to the observed transport (31.2 +/‐ 5.5 Sv) at 53°N. The results also include a comparable spatial pattern and March mixed layer depth in the central Labrador Sea (maximum depth ∼ 2000 m). During periods containing enhanced deep convection (1990's) our analyses show increased correlation between LSW and LNADW model transport at 53°N. Our results indicate that the transport variability in LSW and LNADW at 53°N is a result of a complex modulation of wind stress and buoyancy forcing on regional and basin wide scale.

Description: SPNA transport 2020 The scientific program of the MARIA S. MERIAN MSM94 expedition was dedicated to studies on the intensity of water mass transformation and the transport of water masses in the boundary current systems off Labrador and at the southwestern tip of Greenland. During the expedition we redeployed 11 moorings and recovered 1 bottom lander. Measurements of the vertical structure of temperature, salinity, density, oxygen, optical properties and the flow along selected sections have been surveyed during the MSM94 expedition. Close to the surface, permanent registrations are carried out with the thermosalinograph (temperature, salinity) and meteorological data are continuously collected. Flow measurements up to 1000m depth are performed with the ships installed ADCPs. Argo floats (IFREMER, BSH) and surface drifter (NOC, UK) were also deployed. The expedition is a contribution to international projects (OSNAP, Blue Action, EuroSea). The expedition was conducted during the COVID-19 pandemic and modifications to the science program had to be applied (e.g. Start/end-port Emden, Germany, reduced science crew and respective cut’s in data acquisition).

Description: The Labrador Sea in the North Atlantic Ocean is one of the few regions globally where oxygen from the atmosphere can reach the deep ocean directly. This is the result of wintertime deep convection, which homogenizes the water column to a depth of up to 2000 m and brings deep water undersaturated in oxygen into contact with the atmosphere. In this study, we analyze how the intense oxygen uptake during Labrador Sea Water (LSW) formation affects the properties of the outflowing deep western boundary current, which ultimately feeds the upper part of the North Atlantic Deep Water layer in much of the Atlantic Ocean. Seasonal cycles of oxygen concentration, temperature, and salinity from a 2-year time series collected by sensors moored at 600 m nominal depth in the outflowing boundary current at 53∘ N show a cooling, freshening, and increase in oxygen content of the water flowing out of the basin between March and August. Analysis of Argo float data suggests that this is preceded by an increased input of LSW into the boundary current about 1 month earlier. This input is the result of newly ventilated LSW entering from the interior, as well as LSW formed directly within the boundary current. Together, these results imply that the southward export of newly formed LSW primarily occurs in the months following the onset of deep convection, from March to August, and that this direct LSW export route controls the seasonal oxygen increase in the outflow at 600 m depth. During the rest of the year, properties of the boundary current measured at 53∘ N resemble those of Irminger Water, which enters the basin with the boundary current from the Irminger Sea. The input of newly ventilated LSW increases the oxygen concentration from 298 µmol L−1 in January to a maximum of 306 µmol L−1 in April. As a result of this LSW input, an estimated (1.60 ± 0.42) × 1012 mol yr−1 of oxygen are added to the outflowing boundary current, mostly during spring and summer, equivalent to 50 % of the wintertime uptake from the atmosphere in the interior of the basin. The export of oxygen from the subpolar gyre associated with this direct southward pathway of LSW is estimated to supply 42 %–71 % of the oxygen consumed annually in the upper North Atlantic Deep Water layer in the Atlantic Ocean between the Equator and 50∘ N. Our results show that the formation of LSW is important for replenishing oxygen to the deep oceans, meaning that possible changes in its formation rate and ventilation due to climate change could have wide-reaching impacts on marine life.

Description: The North Atlantic Deep Water (NADW) is a crucial component of the Atlantic meridional overturning circulation and is therefore an important factor of the climate system. In order to estimate the mean relative contributions, sources, and pathways of the NADW at the southern exit of the Labrador Sea, a Lagrangian particle experiment is performed. The particles were seeded according to the strength of the velocity field along the 53∘ N section and traced 40 years backward in time in the three-dimensional velocity and hydrography field. The resulting transport pathways, their sources and corresponding transit timescales were inferred. Our experiment shows that, of the 30.1 Sv of NADW passing 53∘ N on average, the majority of this water is associated with a diapycnal mass flux without contact to the atmosphere, accounting for 14.3 Sv (48 %), where 6.2 Sv originate from the Labrador Sea, compared to 4.7 Sv from the Irminger Sea. The second-largest contribution originates from the mixed layer with 7.2 Sv (24 %), where the Labrador Sea contribution (5.9 Sv) dominates over the Irminger Sea contribution (1.0 Sv). Another 5.7 Sv (19 %) of NADW crosses the Greenland–Scotland Ridge within the NADW density class, where about two-thirds pass the Denmark Strait, while one-third crosses the Iceland–Scotland Ridge. The NADW exported at 53∘ N is hence dominated by entrainment through the diapycnal mass flux and mixed-layer origin in the Labrador Sea.

Description: A hierarchy of global 1/4° (ORCA025) and Atlantic Ocean 1/20° nested (VIKING20X) ocean/sea-ice models is described. It is shown that the eddy-rich configurations performed in hindcasts of the past 50–60 years under CORE and JRA55-do atmospheric forcings realistically simulate the large-scale horizontal circulation, the distribution of the mesoscale, overflow and convective processes, and the representation of regional current systems in the North and South Atlantic. The representation, and in particular the long-term temporal evolution, of the Atlantic Meridional Overturning Circulation (AMOC) strongly depends on numerical choices for the application of freshwater fluxes. The interannual variability of the AMOC instead is highly correlated among the model experiments and also with observations, including the 2010 minimum observed by RAPID at 26.5° N pointing at a dominant role of the forcing. Regional observations in western boundary current systems at 53° N, 26.5° N and 11° S are explored in respect to their ability to represent the AMOC and to monitor the temporal evolution of the AMOC. Apart from the basin-scale measurements at 26.5° N, it is shown that in particular the outflow of North Atlantic Deepwater at 53° N is a good indicator of the subpolar AMOC trend during the recent decades, if the latter is provided in density coordinates. The good reproduction of observed AMOC and WBC trends in the most reasonable simulations indicate that the eddy-rich VIKING20X is capable in representing realistic forcing-related and ocean-intrinsic trends.

Description: Observations of the eastern subpolar North Atlantic in the 2010s show exceptional freshening and cooling of the upper ocean, peaking in 2016 with the lowest salinities recorded for 120 years. Published theories for the mechanisms driving the freshening include: reduced transport of saltier, warmer surface waters northwards from the subtropics associated with reduced meridional overturning; shifts in the pathways of fresher, cooler surface water from the Labrador Sea driven by changing patterns of wind stress; and the eastward expansion of the subpolar gyre. Using output from a high-resolution hindcast model simulation, we propose that the primary cause of the exceptional freshening and cooling is reduced surface heat loss in the Labrador Sea. Tracking virtual fluid particles in the model backwards from the eastern subpolar North Atlantic between 1990 and 2020 shows the major cause of the freshening and cooling to be an increased outflow of relatively fresh and cold surface waters from the Labrador Sea; with a minor contribution from reduced transport of warmer, saltier surface water northward from the subtropics. The cooling, but not the freshening, produced by these changing proportions of waters of subpolar and subtropical origin is mitigated by reduced along-track heat loss to the atmosphere in the North Atlantic Current. We analyse modelled boundary exchanges and water mass transformation in the Labrador Sea to show that since 2000, while inflows of lighter surface waters remain steady, the increasing output of these waters is due to reduced surface heat loss in the Labrador Sea beginning in the early 2000s. Tracking particles further upstream reveals that the primary source of the increased volume of lighter water transported out of the Labrador Sea is increased recirculation of water, and therefore longer residence times, in the upper 500–1000 m of the subpolar gyre.

Description: Because new observations have revealed that the Labrador Sea is not the primary source for waters in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) during the Overturning in the Subpolar North Atlantic Programme (OSNAP) period, it seems timely to re-examine the traditional interpretation of pathways and property variability for the AMOC lower limb from the subpolar gyre to 26.5 degrees N. In order to better understand these connections, Lagrangian experiments were conducted within an eddy-rich ocean model to track upper North Atlantic Deep Water (uNADW), defined by density, between the OSNAP line and 26.5 degrees N as well as within the Labrador Sea. The experiments reveal that 77% of uNADW at 26.5 degrees N is directly advected from the OSNAP West section along the boundary current and interior pathways west of the Mid-Atlantic Ridge. More precisely, the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea, while particles connecting OSNAP East to 26.5 degrees N are exclusively advected from the Iceland Basin and Rockall Trough along the eastern flank of the Mid-Atlantic Ridge. Although the pathways between OSNAP West and 26.5 degrees N are only associated with a net formation of 1.1 Sv into the uNADW layer, they show large density changes within the layer. Similarly, as the particles transit through the Labrador Sea, they undergo substantial freshening and cooling that contributes to further densification within the uNADW layer. Key Points: - The large majority of upper North Atlantic Deep Water (uNADW) sourced from the Irminger Sea transits through the Labrador Sea before reaching 26.5°N - Interior pathways along the eastern flank of the Mid-Atlantic Ridge connect the Iceland Basin and Rockall Trough to 26.5°N - Though uNADW is mainly sourced in the eastern subpolar gyre, its transit in the Labrador Sea is associated with further property changes