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Observing System Experiments (OSEs) are carried out over a one-year period to quantify the impact of Argo observations on the Mercator-Ocean 1=4° global ocean analysis and forecasting system. The reference simulation assimilates sea surface temperature (SST), SSALTO/DUACS altimeter data and Argo and other in situ observations from the Coriolis data center. Two other simulations are carried out where all Argo and half of Argo data sets are withheld. Assimilating Argo observations has a significant impact on analyzed and forecast temperature and salinity fields at different depths. Without Argo data assimilation, large errors occur in analyzed fields as estimated from the differences when compared with in situ observations. For example, in the 0-300m layer RMS differences between analyzed fields and observations reach 0.25 psu and 1.25 °C in the western boundary currents and 0.1 psu and 0.75 °C in the open ocean. The impact of the Argo data in reducing observation-model forecast error is also significant from the surface down to a depth of 2000 m. Differences between independent observations and forecast fields are thus reduced by 20% in the upper layers and by up to 40% at a depth of 2000m when Argo data are assimilated. At depth, the most impacted regions in the global ocean are the Mediterranean outflow and the Labrador Sea. A significant degradation can be observed when only half of the data are assimilated. All Argo observations thus matter, even with a 1=4° model resolution. The main conclusion is that the performance of global data assimilation systems is heavily dependent on the availability of Argo data. [ABSTRACT FROM AUTHOR]

Reconstructing the opening of the Labrador Sea and Baffin Bay between Greenland and North America remains controversial. Recent seismic data suggest that magnetic lineations along the margins of the Labrador Sea, originally interpreted as seafloor spreading anomalies, may lie within the crust of the continent-ocean transition. These data also suggest a more seaward extent of continental crust within the Greenland margin near the Davis Strait than assumed in previous full-fit reconstructions. Our study focuses on reconstructing the full-fit configuration of Greenland and North America using an approach that considers continental deformation in a quantitative manner. We use gravity inversion to map crustal thickness across the conjugate margins, and assimilate observations from available seismic profiles and potential field data to constrain the likely extent of different crustal types. We derive end-member continental margin restorations following alternative interpretations of published seismic profiles. The boundaries between continental and oceanic crust (COB) are restored to their prestretching locations along small circle motion paths across the region of Cretaceous extension. Restored COBs are fitted quantitatively to compute alternative total-fit reconstructions. A preferred full-fit model is chosen based on the strongest compatibility with geological and geophysical data. Our preferred model suggests that (i) the COB lies oceanward of magnetic lineations interpreted as magnetic anomaly 31 (70 Ma) in the Labrador Sea, (ii) all previously identified magnetic lineations landward of anomaly 27 reflect intrusions into continental crust, and (iii) the Ungava fault zone in Davis Strait acted as a leaky transform fault during rifting. This robust plate reconstruction reduces gaps and overlaps in the Davis Strait and suggests that there is no need for alternative models proposed for reconstructions of this area including additional plate boundaries in North America or Greenland. Our favored model implies that break up and formation of continent-ocean transition (COT) first started in the southern Labrador Sea and Davis Strait around 88Ma and then propagated north and southwards up to onset of real seafloor spreading at 63Ma in the Labrador Sea. In the Baffin Bay, continental stretching lasted longer and actual break up and seafloor spreading started around 61Ma (Chron 26). [ABSTRACT FROM AUTHOR]

We have investigated dominant mechanisms of the Atlantic Meridional Overturning Circulation (AMOC) variability at 26.5 N (without the Ekman component) on monthly timescales using 1 and 1/4 NEMO model data. All data were detrended and the seasonal cycle removed. The spatial lead-lag correlations of different hydrodynamic fields with the AMOC time series were calculated. The analysis shows that the AMOC depends on the strength of wind over the North Atlantic on different time scales. At ~1 yr the January-June difference of mean sea level pressure between high and mid-latitudes in the North Atlantic defines (according to different model runs) 35-50% of the annual AMOC variability. At interannual time scales ~4 yr after strong (weak) winds over the North Atlantic the AMOC transport becomes higher (lower) by means of an increase (a decrease) in deep water formation in the North Atlantic subpolar gyre. The analysis of the 1/4 NEMO model shows that about 30% of the AMOC variability is due to density changes in the top 1000m in the Labrador and Irminger seas occurring about 4 yr early. [ABSTRACT FROM AUTHOR]

The article presents titles and abstracts of various research articles, related to geography. "A Canoe Journey in Southeastern Labrador," by Henry G. Bryant is about Labrador, which with its large unexplored areas and comparative accessibility, presents an attractive field for the pioneer explorer. The Atlantic seaboard is desolate owing to climatic conditions produced by the Labrador current. In addition to that, the article presents information on various other abstracts related to geography.

Boundary Exchange (BE - exchange of elements between continental margins and the open ocean) has recently been emphasized as a key process in the oceanic cycle of neodymium (Nd). We here use a regional eddy-permitting resolution Ocean General Circulation Model (1/4°) of the North Atlantic basin to simulate the distribution of the Nd isotopic composition, considering BE as the only source. Results show good agreement with the data, confirming previous results obtained using the same parameterization of the source in a coarse resolution global model (Arsouze et al., 2007), and therefore the major control played by the BE processes in the Nd cycle on the regional scale. We quantified the exchange rate of the BE, and found that the time needed for the continental margins to significantly imprint the chemical composition of the surrounding seawater (further referred as characteristic exchange time) is of the order of 0.2 years. However, the timescale of the BE may be subject to large variations as a very short exchange time (a few days) is needed to reproduce the highly negative values of surface waters in the Labrador Sea, whereas a longer one (up to 0.5 years) is required to simulate the radiogenic influence of basaltic margins and distinguish the negative isotopic signatures of North Atlantic Deep Water from the more radiogenic southern origin water masses. This likely represents geographical variations in erosion fluxes and the subsequent particle load onto the continental marings. These exchange times are significantly lower than the previous evaluations using a low resolution model (6 months to 10 years), but however in agreement with the available seawater Nd isotope data, highlighting the importance of the model dynamics in simulating the BE process. [ABSTRACT FROM AUTHOR]

This paper investigates the link between the Labrador Sea Water (LSW) formation rate and the strength of the Atlantic Meridional Overturning Circulation within the Labrador Sea. LSW is formed in the Labrador Sea through deep wintertime ocean convection and is then carried out of the basin by, among other currents, the Deep Western Boundary Current (DWBC) that flows southward along the east coast of Canada. We used an eddy‐permitting simulation (at 1/12°, horizontal resolution) with an Arctic and Northern Hemisphere Atlantic configuration of the Nucleus for European Modeling of the Ocean (NEMO) model that covers the period from 2002 to 2017. In this study, the formation rate of LSW is estimated using an instantaneous kinematic subduction approach by analyzing the vertical transport of a water mass through the base of the mixed layer. We computed the local Meridional Overturning Circulation (MOC) index and the transport of LSW within the DWBC at the Atlantic Repeat hydrographic section 7 West (AR7W). Results show that an increase in the formation rate of LSW entails an increase in the LSW transport in the DWBC within 1 year. This is followed by an enhancement of the overturning in the Labrador Sea. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) is of paramount importance for the climate. The AMOC contributes to the redistribution of heat, salt, and anthropogenic carbon in the world's oceans. The upper limb of the AMOC transports warm, surface water northward. The lower limb of the AMOC is associated with dense, cold, and deep water formation and is carried by a vigorous Deep Western Boundary Current (DWBC). The Labrador Sea Water (LSW) is one of the water masses formed by deep convection in the subpolar gyre and then carried out of the basin by, among other currents, the DWBC that flows southward along the east coast of Canada. In this paper, we use a simulation to investigate how the change in the formation rate of the LSW could impact the strength of the local AMOC in the Labrador Sea. We found that an increase in the formation rate of LSW entails an increase in the LSW transport in the DWBC within 1 year. This is followed by an enhancement of the overturning in the Labrador Sea. Key Points: Strong convection events in the Labrador Sea entail a high formation rate of Labrador Sea WaterPart of the Labrador Sea Water is exported out of its formation region through the Deep Western Boundary Current within 1 yearStrong, deep convection events are related to a higher local MOC index in the Labrador Sea along the AR7W section [ABSTRACT FROM AUTHOR]

Abstract: The paper considers steric level fluctuations in the northern Atlantic Ocean. We use a method that combines AVISO altimetry and GRACE gravity measurements to evaluate steric level fluctuations and obtain estimates of steric variations in the Labrador and Irminger seas for 2003-2015. The range of steric fluctuations in the Labrador Sea is from -11 to 10 cm, and in the Irminger Sea is from -11 to 12 cm. We estimate trends of steric fluctuations, which indicate a significant increase in the steric component of variability in the level of the North Atlantic. We propose a method for determining regions of deep convection in the Labrador and Irminger seas based on the minimum values of the steric level anomalies (the seasonal component has been excluded) from combined satellite measurement data. Possible spots of deep convection are determined in the Labrador and Irminger seas and shown on steric level fluctuation maps for different years. We demonstrate that deep convection was not manifested in the Labrador Sea in 2006, and there was a general weakening in deep convection processes in the North Atlantic after 2008. [ABSTRACT FROM AUTHOR]

This paper summarizes the rationale behind developing and implementing an Autonomous Observing System for the Labrador Sea and details a recent successful glider deployment in the Labrador Sea in 2016 as part of the VITALS (Ventilation, Interactions and Transports Across the Labrador Sea) project. The glider carried a novel scientific sensor and operated using various sampling modes novel to the glider community. Some of the glider data and mission segments are shown as well as a discussion on how this effort fits in with the larger quest to implement the Canadian component of a global ocean observing system. [ABSTRACT FROM AUTHOR]

In this paper we introduce a parameter for the retrieval of the thickness of undeformed first-year sea ice that is specifically adapted to compact polarimetric (CP) synthetic aperture radar (SAR) images. The parameter is denoted as the "CP ratio". In model simulations we investigated the sensitivity of the CP ratio to the dielectric constant, ice thickness, ice surface roughness, and radar incidence angle. From the results of the simulations we deduced optimal sea ice conditions and radar incidence angles for the ice thickness retrieval. C-band SAR data acquired over the Labrador Sea in circular transmit and linear receive (CTLR) mode were generated from RADARSAT-2 quad-polarization images. In comparison with results from helicopter-borne measurements, we tested different empirical equations for the retrieval of ice thickness. An exponential fit between the CP ratio and ice thickness provides the most reliable results. Based on a validation using other compact polarimetric SAR images from the same region, we found a root mean square (rms) error of 8 cm and a maximum correlation coefficient of 0.94 for the retrieval procedure when applying it to level ice between 0.1 and 0.8m thick. [ABSTRACT FROM AUTHOR]

The Canadian Arctic Archipelago (CAA) is a complex area formed by narrow straits and islands in the Arctic. It is an important pathway for freshwater and sea-ice transport from the Arctic Ocean to the Labrador Sea and ultimately to the Atlantic Ocean. The narrow straits are often crudely represented in coupled sea-ice-ocean models, leading to a misrepresentation of transports through these straits. Unstructured meshes are an alternative in modelling this complex region, since they are able to capture the complex geometry of the CAA. This provides higher resolution in the flow field and allows for more accurate transports (but not necessarily better modelling). In this paper, a finite element sea-ice model of the Arctic region is described and used to estimate the sea-ice fluxes through the CAA. The model is a dynamic-thermodynamic sea-ice model with elastic-viscous-plastic rheology and is coupled to a slab ocean, where the temperature and salinity are restored to climatology, with no velocities and surface elevation. The model is spun-up from 1973 to 1978 with NCEP/NARR reanalysis data. From 1979 to 2007, the model is forced by NCEP/DoE reanalysis data. The large scale sea-ice characteristics show good agreement with observations. The total sea-ice area agrees very well with observations and shows a sensitivity to the Arctic oscillation (AO). For 1998-2002, we find estimates for the sea-ice volume and area fluxes through Admunsen Gulf, McClure Strait and the Queen Elizabeth Islands that compare well with observation and are slightly better than estimates from other models. For Nares Strait, we find that the fluxes are much lower than observed, due to the missing effect of topographic steering on the atmospheric forcing fields. The 1979-2007 fluxes show large seasonal and interannual variability driven primarily by variability in the ice velocity field and a sensitivity to the AO and other large-scale atmospheric variability, which suggests that accurate atmospheric forcing might be crucial to modelling the CAA. [ABSTRACT FROM AUTHOR]

This paper extends A. Bracco and J. Pedlosky’s investigation of the eddy-formation mechanism in the eastern Labrador Sea by including a more realistic depiction of the boundary current. The quasigeostrophic model consists of a meridional, coastally trapped current with three vertical layers. The current configuration and topographic domain are chosen to match, as closely as possible, the observations of the boundary current and the varying topographic slope along the West Greenland coast. The role played by the bottom-intensified component of the boundary current on the formation of the Labrador Sea Irminger Rings is explored. Consistent with the earlier study, a short, localized bottom-trapped wave is responsible for most of the perturbation energy growth. However, for the instability to occur in the three-layer model, the deepest component of the boundary current must be sufficiently strong, highlighting the importance of the near-bottom flow. The model is able to reproduce important features of the observed vortices in the eastern Labrador Sea, including the polarity, radius, rate of formation, and vertical structure. At the time of formation, the eddies have a surface signature as well as a strong circulation at depth, possibly allowing for the transport of both surface and near-bottom water from the boundary current into the interior basin. This work also supports the idea that changes in the current structure could be responsible for the observed interannual variability in the number of Irminger Rings formed. [ABSTRACT FROM AUTHOR]

In this paper, it is shown that coherent large-scale low-frequency variabilities in the North Atlantic Ocean—that is, the variations of thermohaline circulation, deep western boundary current, northern recirculation gyre, and Gulf Stream path—are associated with high-latitude oceanic Great Salinity Anomaly events. In particular, a dipolar sea surface temperature anomaly (warming off the U.S. east coast and cooling south of Greenland) can be triggered by the Great Salinity Anomaly events several years in advance, thus providing a degree of long-term predictability to the system. Diagnosed phase relationships among an observed proxy for Great Salinity Anomaly events, the Labrador Sea sea surface temperature anomaly, and the North Atlantic Oscillation are also discussed. [ABSTRACT FROM AUTHOR]

The significant increase in global energy demand has drawn the attention of oil and gas industries to exploration of less-exploited resources. Arctic offshore region is reported to hold a great proportion of un-discovered oil reserves. While this can be a promising opportunity for the industry, more exploration activities will also increase the possibility of oil spill during the entire process including production and transport. A comprehensive risk assessment based on Ecological Risk Assessment (ERA) method is then required during the planning and operation stages of future Arctic oil production facilities. In the exposure analysis stage, ERA needs an evaluation of the oil concentration profile in all media. This paper presents a methodology for predicting the stochastic fate and transport of spilled oil in ice-infested regions. For this purpose, level IV fugacity models are used to estimate the time-variable concentration of oil. A hierarchical Bayesian approach (HBA) is adopted to estimate the probability of time to reach a concentration (TRTC) based on the observations made from a fugacity model. To illustrate the application of the proposed method, a subsea pipeline accident resulting in the release of 100 t of Statfjord oil into the Labrador Sea is considered as the case study. [ABSTRACT FROM AUTHOR]

The response of the Gulf Stream (GS) system to atmospheric forcing is generally linked either to the basin-scale winds on the subtropical gyre or to the buoyancy forcing from the Labrador Sea. This study presents a multiscale synergistic perspective to describe the low-frequency response of the GS system. The authors identify dominant temporal variability in the North Atlantic Oscillation (NAO), in known indices of the GS path, and in the observed GS latitudes along its path derived from sea surface height (SSH) contours over the period 1993-2013. The analysis suggests that the signature of interannual variability changes along the stream's path from 75° to 55°W. From its separation at Cape Hatteras to the west of 65°W, the variability of the GS is mainly in the near-decadal (7-10 years) band, which is missing to the east of 60°W, where a new interannual (4-5 years) band peaks. The latter peak (4-5 years) was missing to the west of 65°W. The region between 65° and 60°W seems to be a transition region. A 2-3-yr secondary peak was pervasive in all time series, including that for the NAO. This multiscale response of the GS system is supported by results from a basin-scale North Atlantic model. The near-decadal response can be attributed to similar forcing periods in the NAO signal; however, the interannual variability of 4-5 years in the eastern segment of the GS path is as yet unexplained. More numerical and observational studies are warranted to understand such causality. [ABSTRACT FROM AUTHOR]

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Abstract: Benthic foraminifera are found in a wide range of environments and may at times be one of few proxies available for the study of palaeoenvironmental conditions. However, the response of benthic foraminifera to changing sea-ice conditions is not well understood. This paper discusses benthic foraminifera as potential sea-ice proxies, with special emphasis on their use in shelf regions of the sub-arctic realm. Four marine sediment records from the Labrador Sea–Baffin Bay region serve as examples; in all four records independent sea-ice proxy will be used for testing the foraminiferal response to changing sea ice conditions. This test suggests that 1) Benthic foraminifera provide information on variations in sea-ice cover, but they are not direct proxies for sea-ice cover and no true sea-ice species has yet been identified. 2) Foraminifera mainly respond to the surplus of food often available at sea-ice edges. 3) Dominance of agglutinated foraminifera may suggest corrosive bottom-water conditions which may at times be linked to seasonal sea-ice cover. [Copyright &y& Elsevier]

Abstract: Results from 59 acoustically tracked RAFOS floats obtained during 2003–2008 are analyzed here to study the spreading pathways of Labrador Sea Water (LSW) from the subpolar to subtropical North Atlantic. An earlier paper based on a subset of these float data presented evidence of a potentially important interior pathway for the southward spreading of LSW. Here those results are reinforced with the full data set and new information on LSW pathways around the Grand Banks is presented. About 70% of the RAFOS floats were expelled into the ocean interior east of the Grand Banks and meandered slowly eastward toward the mid-Atlantic ridge, a pathway observed previously with profiling floats. Less than 10% was advected continuously around the Grand Banks by the Deep Western Boundary Current (DWBC). A larger fraction (∼17%) drifted into the subtropical interior from the Tail of the Grand Banks, suggesting that this pathway is at least if not more important than the DWBC pathway for the export of LSW to the subtropics. RAFOS floats released closest to the continental slope at 700m depth were more likely to rapidly reach subtropical latitudes, mainly because they drifted through Flemish Pass, a 1100m deep channel between the Grand Banks and the Flemish Cap, which protects fluid parcels from being swept off the continental slope by meanders of the North Atlantic Current. A statistical comparison of the RAFOS trajectories with more than 5000 simulated floats obtained from the Family of Linked Atlantic Models Experiment (FLAME) high-resolution ocean general circulation model reveals a similar pattern of LSW spreading. The RAFOS and simulated floats are also used to show that contrary to a previous modeling study, the isobaric RAFOS floats do not underestimate the amount of LSW that continuously follows the DWBC around the Grand Banks compared to isopycnal floats. These results have implications for the connectivity of the deep limb of the Atlantic Meridional Overturning Circulation between the subpolar and the subtropical North Atlantic. [Copyright &y& Elsevier]

Abstract: This paper presents some new concepts in the petroleum systems of the northern Labrador and southern Baffin Island offshore region of eastern Canada. The focus of this work is the region of the Hekja O-71 gas discovery of 1979 by Aquitaine, one of only five wells drilled between 1976 and 1980 within an area covering some 166,000 square kilometers within the Saglek Basin. This study emerged from a broad scale re-examination of the petroleum potential of this area “from the crust up” using regional reflection, refraction and potential field data. An opportunity to use SAR (synthetic aperture radar) data from the RADARSAT-1 Earth observation satellite was taken to incorporate alternative data sources to support this exploratory review and resource assessment. Examination of the final map of interpreted slick-like features on the sea surface, revealed a close correlation of some of these occurrences to several bathymetric features which have underlying seismic signatures similar to previously identified gas hydrate “pipes” or chimney anomalies as seen in data from offshore Nova Scotia and in the Irish Sea. Globally, many active marine hydrocarbon seeps appear on the seafloor as “pockmarks”; in this case no seafloor depressions were seen on conventional multi channel 2-D seismic data associated with active seepage. Instead, very distinct mound-like structures are seen associated with seeps at two locations east of Hudson Strait. The presence of significant cold water coral in close proximity to the active seepage sites may show a link between seafloor petroleum seepage and coral reef development similar to that observed offshore Norway. Close examination of the near surface seismic character led to the interpretation of what was originally seen as seismic noise in vintage data, as likely being fluid escape chimneys or gas pipes, along the eastern edge of the Hekja structural complex, where favourable hydrocarbon trapping conditions are present. No active seeps were seen there; this may be due to other factors, such as the nature of the fluids that are escaping or the possibility of self sealing activity by biological agents such as bacterial matting with associated carbonate hard-ground formation. One seep anomaly was noted relatively close to the major basaltic eruptive complex near the Gjoa G-37 well. This may be due to the presence of volcaniclastic- hosted hydrocarbons similar in style to those recently discovered at the Rosebank field in Paleocene volcanics in the Faroe-Shetland Basin. The occurrence of an active seep in a volcanically-dominated terrain raises the potential for a radical shift in accepted types of petroleum leads and where they may occur. The presence of apparent oil seepage versus the past history of finding only gas-prone accumulations is a strong indicator that a second, oil-prone petroleum system exists in the Saglek Basin. The signs of active petroleum systems as shown by oil seep anomalies with confirming seismic evidence of discrete sea floor structures in close proximity makes a compelling argument for re-examining these areas with an eye towards stratigraphic instead of structural plays. [Copyright &y& Elsevier]

The climate response to idealized changes in the atmospheric CO2 concentration by the new GFDL climate model (CM2) is documented. This new model is very different from earlier GFDL models in its parameterizations of subgrid-scale physical processes, numerical algorithms, and resolution. The model was constructed to be useful for both seasonal-to-interannual predictions and climate change research. Unlike previous versions of the global coupled GFDL climate models, CM2 does not use flux adjustments to maintain a stable control climate. Results from two model versions, Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), are presented. Two atmosphere–mixed layer ocean or slab models, Slab Model versions 2.0 (SM2.0) and 2.1 (SM2.1), are constructed corresponding to CM2.0 and CM2.1. Using the SM2 models to estimate the climate sensitivity, it is found that the equilibrium globally averaged surface air temperature increases 2.9 (SM2.0) and 3.4 K (SM2.1) for a doubling of the atmospheric CO2 concentration. When forced by a 1% per year CO2 increase, the surface air temperature difference around the time of CO2 doubling [transient climate response (TCR)] is about 1.6 K for both coupled model versions (CM2.0 and CM2.1). The simulated warming is near the median of the responses documented for the climate models used in the 2001 Intergovernmental Panel on Climate Change (IPCC) Working Group I Third Assessment Report (TAR). The thermohaline circulation (THC) weakened in response to increasing atmospheric CO2. By the time of CO2 doubling, the weakening in CM2.1 is larger than that found in CM2.0: 7 and 4 Sv (1 Sv ≡ 106 m3 s-1), respectively. However, the THC in the control integration of CM2.1 is stronger than in CM2.0, so that the percentage change in the THC between the two versions is more similar. The average THC change for the models presented in the TAR is about 3 or 4 Sv; however, the range across the model results is very large, varying from a slight increase (+2 Sv) to a large decrease (-10 Sv). [ABSTRACT FROM AUTHOR]

The recent paper in Quaternary International, Rashid et al. (2014) “Signature of the Gold Cove event (10.2 ka) in the Labrador Sea” attributes sediment layers rich in detrital carbonate (DC) in several marine sediment cores in the Labrador Sea to the so-called Gold Cove (GC) glacial advance of the early Holocene. The age estimate for the GC event as well as the correlations between sediment cores in this study are based on radiocarbon dates, mostly from previously published studies. Although this work provides a good overview of an apparently widespread DC layer in the early Holocene, the reported age of 10.2 cal. ka BP for the actual Gold Cove event is too young, due to a mistake made during re-calibration of the original radiocarbon dates used to describe the event. The 14 C dates have been corrected to compensate for the marine reservoir effect two times, leading to an unrealistically large total reservoir age of more than thousand years. By applying such a large marine reservoir offset, the final calibrated age becomes too young. Originally the GC event was described to consist of an advance and retreat phase, in total spanning several centuries. The Gold Cove event should therefore more accurately be reported as an age range, and it occurred most likely ca. 500 years earlier than 10.2 ka BP, the age reported by Rashid et al. (2014). [ABSTRACT FROM AUTHOR]

Pearce (2015) commented about the paper titled “Signature of the Gold Cove event (10.2 ka) in the Labrador Sea” by Rashid et al.(2014) suggesting that due to an oversight error in re-calibration of four 14 C-dates of Kaufman and Williams (1992) and Kaufman et al. (1993), Rashid et al. (2014) misidentified the Gold Cove event. Furthermore, as a result of this error, findings and conclusions in Rashid et al. (2014) are “difficult to justify the correlations of several DC layers in the Labrador Sea”. Here we reply to Pearce (2015) comments. We further elaborate our reasoning behind the identification of 10.2 ka event. [ABSTRACT FROM AUTHOR]

Deep convection and associated deep water formation are key processes for climate variability, since they impact the oceanic uptake of heat and trace gases and alter the structure and strength of the global overturning circulation. For long, deep convection in the subpolar North Atlantic was thought to be confined to the central Labrador Sea in the western subpolar gyre (SPG). However, there is increasing observational evidence that deep convection also has occurred in the eastern SPG south of Cape Farewell and in the Irminger Sea, in particular, in 2015–2018. Here we assess this recent event in the context of the temporal evolution of spatial deep convection patterns in the SPG since the mid‐twentieth century, using realistic eddy‐rich ocean model simulations. These reveal a large interannual variability with changing contributions of the eastern SPG to the total deep convection volume. Notably, in the late 1980s to early 1990s, the period with highest deep convection intensity in the Labrador Sea related to a persistent positive phase of the North Atlantic Oscillation, the relative contribution of the eastern SPG was small. In contrast, in 2015–2018, deep convection occurred with an unprecedented large relative contribution of the eastern SPG. This is partly linked to a smaller north‐westward extent of deep convection in the Labrador Sea compared to previous periods of intensified deep convection, and may be a first fingerprint of freshening trends in the Labrador Sea potentially associated with enhanced Greenland melting and the oceanic advection of the 2012–2016 eastern North Atlantic fresh anomaly. Plain Language Summary: The subpolar North Atlantic is one of the few oceanic regions where deep convection occurs, a process by which surface waters become dense enough to mix downwards. Thereby, heat and carbon dioxide are transported into the deep ocean, helping to slow global warming. For long, deep convection in the subpolar North Atlantic was thought to be confined to the central Labrador Sea. However, recently, deep convection was also observed in the Irminger Sea. In this study, we use ocean model simulations to reconstruct when and where deep convection occurred since the mid‐twentieth century. Our simulations show a large variability in strength and spatial patterns of deep convection, with changing relative importance of the Labrador and Irminger Seas. Most notably, in the mid 2010's, deep convection shifted eastwards, so that the relative importance of the Irminger Sea and Labrador Sea were anomalously large and small, respectively. Freshening trends in the Labrador Sea suggest that this shift partially resulted from enhanced melting of Greenland's glaciers and changes in the North Atlantic circulation. Our results stress the need for monitoring and correctly modeling deep convection variability in the Labrador and Irminger Seas to better understand and predict the looming impact of future Greenland melting. Key Points: Eddy‐rich ocean model simulations (1958–2019) feature large variability of spatial deep convection patterns in subpolar North AtlanticIn 2015–2018, deep convection showed exceptional large and small relative contributions of eastern and western subpolar gyre, respectivelySmall western contribution is potentially associated with enhanced Greenland melting and recent eastern North Atlantic fresh anomaly [ABSTRACT FROM AUTHOR]

We use a 30‐year time series (1986–2016) of dichlorodifluoromethane (CFC‐12) concentrations with a refined transit time distribution (TTD) method, to estimate the temporal variation of anthropogenic carbon (Cant) in the Central Labrador Sea. We determined that the saturation of CFC‐12 and sulfur hexafluroide (SF6) in newly‐formed Labrador Sea Water had departed significantly from 100% and varied systematically with time. Multiple linear regression of the time‐varying saturation, with the tracer's atmospheric growth rate and the wintertime mixed layer depth as independent variables, allowed reconstruction of the saturation history of CFC‐12 and SF6 in wintertime surface waters, which was implemented in the TTD method. Use of the time‐varying saturation for CFC‐12 gave Cant concentrations ∼7 μmol kg−1 larger than estimates obtained assuming a constant saturation of 100%. The resulting Cant column inventories were ∼20% larger and displayed lower interannual variability compared to conventional TTD‐based estimates. The column inventory of Cant increased at an average rate of 1.8 mol m−2 y−1 over the 30‐year period. However, the accumulation rate of Cant was higher than this average in the early 1990s and since 2013, whereas inventories remained almost unchanged between 2003 and 2012. The variation in the Cant accumulation rate is shown to be linked to temporal variability in the relative layer thickness of the annually ventilated Labrador Sea Water and the underlying Deep Intermediate Water. The non‐steady Cant accumulation highlights the importance of sampling frequency, especially in regions of variable deep mixing and high carbon inventories, and potential misinterpretation of Cant dynamics. Plain Language Summary: Since the industrial revolution, humankind has emitted large amounts of carbon dioxide (CO2) to the atmosphere as a result of fossil fuel combustion and cement production. About 40% of this Anthropogenic CO2 (Cant) has been sequestered by the oceans, primarily in polar and subpolar regions. The Labrador Sea has been identified as one of the regions with the highest inventory of Cant. Here, as strong winds blow on the ocean's surface in wintertime, heat is lost to the atmosphere and water density increases. This process, defined as deep water convection, determines the mixing of surface water into the ocean interior and the transport of gases, like Cant, from surface to depth. In this work we estimate the concentrations, column inventories and storage rate of Cant in Labrador Sea between 1986 and 2016 by using gases that mimic Cant behavior. We observed that despite the overall increase in Cant that occurred between 1986 and 2016, the pace at which Cant increased has not been constant over time in the Labrador Sea. In fact, we observed periods with both fast and slow increases of Cant, which were influenced by the persistence of deep or shallow convection, respectively. Key Points: In regions of ventilation, assuming a constant tracers' saturation in the transit time distribution method results in significant negative bias of anthropogenic carbon (Cant) estimatesAnnual estimates of Cant column inventory between 1986 and 2016 reveal a non‐steady accumulation rate of Cant in Labrador SeaThe temporal variability of the Cant storage in the Central Labrador Sea appears to be linked to the strength of convection [ABSTRACT FROM AUTHOR]

The subpolar North Atlantic (SPNA) is a region with prominent decadal variability that has experienced remarkable warming and cooling trends in the last few decades. These observed trends have been preceded by slow-paced increases and decreases in the Labrador Sea density (LSD), which are thought to be a precursor of large-scale ocean circulation changes. This article analyses the interrelationships between the LSD and the wider North Atlantic across an ensemble of coupled climate model simulations. In particular, it analyses the link between subsurface density and the deep boundary density, the Atlantic Meridional Overturning Circulation (AMOC), the subpolar gyre (SPG) circulation, and the upper-ocean temperature in the eastern SPNA. All simulations exhibit considerable multidecadal variability in the LSD and the ocean circulation indices, which are found to be interrelated. LSD is strongly linked to the strength of the subpolar AMOC and gyre circulation, and it is also linked to the subtropical AMOC, although the strength of this relationship is model-dependent and affected by the inclusion of the Ekman component. The connectivity of LSD with the subtropics is found to be sensitive to different model features, including the mean density stratification in the Labrador Sea, the strength and depth of the AMOC, and the depth at which the LSD propagates southward along the western boundary. Several of these quantities can also be computed from observations, and comparison with these observation-based quantities suggests that models representing a weaker link to the subtropical AMOC might be more realistic. [ABSTRACT FROM AUTHOR]

A numerical modeling sensitivity study is carried out within the Labrador Sea by varying the atmospheric conditions. From forcing NEMO simulations with five atmospheric products commonly used in ocean modeling (DFS5.2, ERA-Interim, CGRF, ERA5, and JRA55-do), we calculate the air-sea heat fluxes that occur over the Labrador Sea (2002-2015 annual-average net heat flux: -53.4, -51.0, -46.6, -58.5, and -47.9 W m-2). With differences up to 12 W m-2 in net surface heat flux averaged over a central region of the Labrador Sea, each product supplied different atmospheric conditions. While the salinity-dependent surface buoyancy fluxes were similar across all simulation, differences between each simulation's solar and nonsolar heat flux led to significant changes in the level of stratification (up to 400 J m-3), depth of the mixed layer (up to 300 m), and thickness of Labrador Sea Water (LSW; up to 300 m). Greater buoyancy loss from the Labrador Sea produced LSW with greater density. However, the production rate of LSW was not clearly affected by small changes in the surface buoyancy flux. Plain Language Summary The Labrador Sea, between Canada and Greenland, experiences deep convection, a process where the ocean's surface is cooled to such a point that it becomes denser than the water beneath it, causing the surface layer to mix downwards. The overlying atmospheric conditions, such as wind speed, temperature, and humidity, strongly control this cooling. We carry out five ocean simulations to explore how small changes in atmospheric conditions influence the Labrador Sea. Our simulations show that atmospheric forcing which enhances the cooling of the Labrador Sea causes the deep water formed here to become denser. While a denser water mass is produced, we find that the volume of this water mass is not necessarily enhanced with the additional cooling. [ABSTRACT FROM AUTHOR]

The dense waters formed by wintertime convection in the Labrador Sea play a key role in setting the properties of the deep Atlantic Ocean. To understand how variability in their production might affect the Atlantic Meridional Overturning Circulation (AMOC) variability, it is essential to determine pathways and associated timescales of their export. In this study, we analyze the trajectories of Argo floats and of Lagrangian particles launched at 53°N in the boundary current and traced backward in time in a high-resolution model, to identify and quantify the importance of upstream pathways. We find that 85% of the transport carried by the particles at 53°N originates from Cape Farewell, and it is split between a direct route that follows the boundary current and an indirect route involving boundary-interior exchanges. Although both routes contribute roughly equally to the maximum overturning, the indirect route governs its signal in denser layers. This indirect route has two branches: part of the convected water is exported rapidly on the Labrador side of the basin and part follows a longer route toward Greenland and is then carried with the boundary current. Export timescales of these two branches typically differ by 2.5 years. This study thus shows that boundary-interior exchanges are important for the pathways and the properties of water masses arriving at 53°N. It reveals a complex three-dimensional view of the convected water export, with implications for the arrival time of signals of variability therein at 53°N and thus for our understanding of the AMOC. [ABSTRACT FROM AUTHOR]

Spatial and temporal trends of remotely sensed sea-ice cover, sea surface temperatures, chlorophyll-a concentration and primary production in the Baffin Bay, Davis Strait and Labrador Sea were analyzed for the 1998–2017 period. We found spatial variability in the trends of these cryospheric, biologic and oceanographic phenomena. For example, in the northern Baffin Bay, we observed decreases in annual sea-ice persistence, yet increases along the Labrador Sea-ice edge during winter, with the latter having significant correlations with broader atmospheric patterns. In general, we observed increases in summer sea surface temperatures across the study region, except a small area of cooling along the southern Greenlandic coast. We also found significant negative trends in April chlorophyll-a and primary production followed by significant positive trends for both biological phenomena in May, owing to anomalously high values in 2014 and 2015. Notably, we found a significant positive correlation between days of monthly sea ice presence in April with May primary production quantities. Finally, we found a significant positive trend in total annual primary production over the study period. This novel finding suggests an important relationship between the timing of breakup along the sea-ice edge and peaks in biological production. [ABSTRACT FROM AUTHOR]

We have analyzed marine palynomorphs (mainly dinocysts and acritarchs) from the Integrated Ocean Drilling Program Site U1307 in the Labrador Sea in order to establish a detailed biostratigraphy for the Late Pliocene to Early Pleistocene. We have defined three magnetostratigraphically calibrated dinocyst and acritarch biozones in the Late Pliocene to Early Pleistocene. Zone LS1 is defined based on the highest occurrence of Barssidinium graminosum and covers the later Pliocene from 3.21 to 2.75 Ma. Zone LS2 is marked by the acme of Pyxidinopsis braboi which occurs between 2.75 and 2.57 Ma, thus encompassing the Plio–Pleistocene transition. Finally, zone LS3 extends from 2.57 to 2.23 Ma in the Early Pleistocene. The palynostratigraphic record of IODP Site U1307 is difficult to correlate to other North Atlantic and Nordic Seas sites mainly because of a different temporal resolution and a lack of well-defined biostratigraphic marker species at the basin scale. The low abundance, discontinuous occurrence and asynchronous events of warm-water Pliocene taxa such as Invertocysta lacrymosa, Impagidinium solidum, Ataxiodinium confusum, Melitasphaeridium choanophorum and Operculodinium ? eirikianum suggest cooler conditions in the Labrador Sea than elsewhere in the North Atlantic, reflecting a strong regionalism. Nevertheless, as recorded at other locations in the North Atlantic, the disappearance of many dinocyst and acritarch taxa around 2.75 Ma at Site U1307 reflects a strong ecological response accompanying the intensification of the Northern Hemisphere glaciation. [ABSTRACT FROM AUTHOR]

Provides background and analysis for a paper that reports on unexpected developments from the convective center of the Labrador Sea in the North Atlantic Ocean. The overall freshening of the Labrador Sea Water (LSW) from 1966 to 1992; The abrupt increase of the density of the LSW between 1990 and 1995; The recurrent atmospheric circulation mode known as the North Atlantic Oscillation (NAO); The paper by Sy and others in the April 17, 1997 issue of `Nature.'

As decadal predictions become operational, the need to use, understand, and extract information from them becomes essential. A climate index is a simple diagnostic quantity that can be used to characterize integral aspects of a geophysical system such as circulation patterns, and thus can be used to evaluate decadal forecasts. One of the most studied and well documented regions of the World Ocean is the North Atlantic. The North Atlantic subpolar gyre is an important region for the modulation of European climate and where skillful predictions of up to a decade can be obtained. Ocean re‐analysis (ORA‐S4) data from 1959 to 2017 are used to introduce a new methodology to compute a climate index of the North Atlantic subpolar gyre that captures both the variability in its strength and shape, the latter was to our knowledge never investigated previously as part of the variability of the gyre on interannual to decadal time scales. The methodology reveals two states of the gyre (before and after 2000), the former is mainly driven by temperature and the latter by a combination of mechanisms that interact to sustain a relatively stable subpolar gyre in terms of strength. Plain Language Summary: Decadal predictions is a relatively new topic in climate science and is driven by the needs of industry and society to adapt to future climate states. To that end, elaborate earth system models provide forecasts that nevertheless need to be validated. Some regions of the world ocean are easier to predict such as the North Atlantic subpolar gyre. One way to validate the forecasts is through climate indeces. To that extent, we follow a new approach to define a climate index for the subpolar gyre that captures many features of the circulation that relate to the strength and the size of the gyre. Key Points: A climate index that is a measure of the changes in strength and in the shape of the North Atlantic subpolar gyre systemThe suggested methodology can be applied to various data sets and can be improved with data sets with higher spatial resolutionThe climate index distinguishes periods with different dynamical characteristics before and after 2000 [ABSTRACT FROM AUTHOR]

Coastal waters in the Labrador Sea are influenced by the seasonal input of meltwater from the Greenland ice sheet, which is predicted to more than double by the end of the century. Mechanisms controlling the offshore export of meltwater can have a significant effect on stratification and vertical stability in the Labrador Sea, being particularly important if the meltwater is transported toward the interior of the basin where winter convection occurs. Here we use a high‐resolution ocean model to show that coastal upwelling winds play a critical role transporting the meltwater offshore to about 150 km from the coast, where increased eddy activity and mean circulation can then transport the meltwater farther offshore. While meltwater discharged from West Greenland is either transported to Baffin Bay or circumnavigates the basin flowing mostly along isobaths, meltwater from East Greenland can reach the interior of the basin where it may influence stratification and winter convection whenever winds are anomalously upwelling favorable in late summer and early fall. Plain Language Summary: Every summer, coastal waters off Greenland are influenced by the input of freshwater from ice sheet melting. Based on analyses of ocean model simulations, we show that meltwater transport in the Labrador Sea is controlled by multiple processes. Winds play an important role, transporting the freshwater to about 150 km from the coast, where the large‐scale circulation and eddies can transport the meltwater farther offshore. Most of the meltwater is transported either northward into Baffin Bay or around the basin. When winds are anomalously upwelling favorable in late summer or early fall, however, meltwater from East Greenland can be transported into the interior of the basin where it may influence vertical stratification and winter convection. Key Points: Winds play a major role transporting meltwater within 150 km from the West Greenland coastWestward veering of large‐scale circulation and eddies transport meltwater farther offshore into the Labrador SeaMeltwater from East Greenland can reach the Labrador Sea interior when winds are anomalously upwelling favorable in late summer/early fall [ABSTRACT FROM AUTHOR]

The meridional overturning circulation (MOC) decreases rapidly in subpolar and Nordic regions where the warm upper layer loses its buoyancy due to intense heat loss, sinks, and flows south. The major volume loss of the upper limb of the MOC, ~9.6 Sv out of 18.4 ± 3.4 Sv, occurs as subduction across the Iceland Basin and Irminger Sea while the major heat loss, 273 TW out of 395 ± 74 TW is associated with the MOC branch that continues into the Nordic Seas where North Atlantic deep overflow water is produced. The 122 ± 79 TW heat flux convergence in the subpolar gyre appears to be significantly larger than various estimates of heat loss to the atmosphere. Much of the 0.09 ± 0.02 Sv freshwater divergence is presumably balanced by runoff from the Greenland shelf. These estimates suggest that the Nordic Seas, not the Labrador Sea, are key to the state of the MOC. Plain language summary: The meridional overturning circulation is a two‐dimensional view of the flow north of upper‐ocean warm water and its return south as cold deep and intermediate water. But the actual pathways of warm‐to‐cold conversion are several and remarkably diverse: One branch continues into the Nordic Seas where very dense water is produced and eventually spills back into the deep North Atlantic, another branch weaves its way around the entire subpolar basin and the southern tip of Greenland to the Labrador Sea where intermediate water is formed, and the third branch is an overturning that takes place within the subpolar waters between Greenland and Scotland. Volumetrically, this is the largest branch, but in terms of heat loss the Nordic Seas, branch surrenders far more heat to the atmosphere than the other two combined. It thus plays the key role in maintaining a strong meridional overturning circulation. Key Points: Great heat loss and production of dense water overflow water in Nordic Seas key to the state of the MOC not the Labrador SeaHeat flux convergence greater than climatological estimates of subpolar gyre heat lossSubpolar fresh water divergence balanced by freshwater loss from Greenland shelf [ABSTRACT FROM AUTHOR]

Labrador Sea Water (LSW) is one of the main contributors to the lower limb of the Atlantic Meridional Overturning Circulation. In this study, we explore the sensitivity of LSW formation to model resolution, Greenland melt, absence of high‐frequency atmospheric phenomena, and changes in precipitation. We use five numerical model simulations at both (1/4)° and (1/12)° resolutions. A kinematic subduction approach is used to obtain the LSW formation rate over the period 2004 to 2016. The control simulation, with (1/4)° resolution, showed a mean annual production rate of 1.9 Sv (1 Sv = 106 m3/s) in the density range of 27.68–27.80 kg/m3 for the period 2004–2016. Deep convection events that occurred during 2008, 2012, and 2014–2016 were captured. We found that with (1/4)° resolution the LSW formation rate is 19% larger compared with its counterpart at (1/12)° resolution. The presence of Greenland melt and an increase in the precipitation impact the denser LSW layer replenishment but do not decrease the overall LSW formation rate nor the maximum convection depth. A dramatic response was found when filtering the atmospheric forcing, which induced a decrease of 44% in heat loss over the Labrador Sea, strong enough to halt the deep convection and decrease the LSW formation rate by 89%. Even if our experiment was extreme, a decrease in the storms crossing the Labrador Sea with a consequent reduction in the winter heat loss might be a bigger threat to deep convection and LSW formation in the future than the expected increases in the freshwater input. Plain Language Summary: The Labrador Sea, located between Greenland and Canada, is where strong winter cooling makes the surface waters lose heat to the atmosphere, get denser, and sink to depths between 500 and 2,500 m. This sinking brings heat and dissolved gases like oxygen and carbon dioxide into the deep ocean. The resulting water mass is known as Labrador Sea Water (LSW). The process through which LSW is formed is sensitive to freshwater inflow into the formation region and storms passing over the Labrador Sea. While an increase in freshwater would inhibit the densification and sinking of surface water, fewer storms would reduce heat loss also reducing the ability of the surface waters to gain in density and sink to greater depths. These are all changes projected to occur due to the ongoing anthropogenic climate change. By using a numerical model, we explore how the increase in freshwater from Greenland melt and precipitation, and the absence of storms could impact LSW formation. We found that by having more freshwater going into the formation region the overall LSW formation rate does not decrease, however, the water mass becomes lighter. In the absence of the storms, the formation rate drastically decreased. Key Points: Simulated convection events in years 2008, 2012, and 2014–2016 resulted in LSW formation rates of 3.5, 5.1, 1.25, 2, and 7 Sv, respectivelyIncreased Greenland melt and precipitation impact denser LSW replenishment; the overall LSW formation rate and maximum MLD did not decreasePotential decreases in Labrador Sea winter heat loss due to global warming may be a bigger threat to LSW formation than freshwater increase [ABSTRACT FROM AUTHOR]

A multi-category numerical sea ice model CICE was used along with data assimilation to derive sea ice parameters in the region of Baffin Bay and Labrador Sea. The assimilation of ice concentration was performed using the data derived from the Advanced Microwave Scanning Radiometer (AMSR-E and AMSR2). The model uses a mixed-layer slab ocean parameterization to compute the sea surface temperature (SST) and thereby to compute the freezing and melting potential of ice. The data from Advanced Very High Resolution Radiometer (AVHRR-only optimum interpolation analysis) were used to assimilate SST. The modelled ice parameters including concentration, ice thickness, freeboard and keel depth were compared with parameters estimated from remote-sensing data. The ice thickness estimated from the model was compared with the measurements derived from Soil Moisture Ocean Salinity – Microwave Imaging Radiometer using Aperture Synthesis (SMOS–MIRAS). The model freeboard estimates were compared with the freeboard measurements derived from CryoSat2. The ice concentration, thickness and freeboard estimates from the model assimilated with both ice concentration and SST were found to be within the uncertainty in the observation except during March. The model-estimated draft was compared with the measurements from an upward-looking sonar (ULS) deployed in the Labrador Sea (near Makkovik Bank). The difference between modelled draft and ULS measurements estimated from the model was found to be within 10 cm. The keel depth measurements from the ULS instruments were compared to the estimates from the model to retrieve a relationship between the ridge height and keel depth. [ABSTRACT FROM AUTHOR]

Atmospheric reanalyses are commonly used to force numerical ocean models, but despite large discrepancies reported between different products, the impact of reanalysis uncertainty on the simulated ocean state is rarely assessed. In this study, the impact of uncertainty in surface fluxes of buoyancy and momentum on the modeled Atlantic meridional overturning at 25°N is quantified for the period January 1994–December 2011. By using an ocean-only climate model and its adjoint, the space and time origins of overturning uncertainty resulting from air–sea flux uncertainty are fully explored. Uncertainty in overturning induced by prior air–sea flux uncertainty can exceed 4 Sv (where 1 Sv ≡ 106 m3 s−1) within 15 yr, at times exceeding the amplitude of the ensemble-mean overturning anomaly. A key result is that, on average, uncertainty in the overturning at 25°N is dominated by uncertainty in the zonal wind at lags of up to 6.5 yr and by uncertainty in surface heat fluxes thereafter, with winter heat flux uncertainty over the Labrador Sea appearing to play a critically important role. [ABSTRACT FROM AUTHOR]

The stable isotope composition of dissolved silicon in seawater (δ30SiDSi) was examined at 10 stations along the GEOVIDE section (GEOTRACES GA-01), spanning the North Atlantic Ocean (40-60° N) and Labrador Sea. Variations in δ30SiDSi below 500m were closely tied to the distribution of water masses. Higher δ30SiDSi values are associated with intermediate and deep water masses of northern Atlantic or Arctic Ocean origin, whilst lower δ30SiDSi values are associated with DSi-rich waters sourced ultimately from the Southern Ocean. Correspondingly, the lowest δ30SiDSi values were observed in the deep and abyssal eastern North Atlantic, where dense southern-sourced waters dominate. The extent to which the spreading of water masses influences the δ30SiDSi distribution is marked clearly by Labrador Sea Water (LSW), whose high δ30SiDSi signature is visible not only within its region of formation within the Labrador and Irminger seas, but also throughout the mid-depth western and eastern North Atlantic Ocean. Both δ30SiDSi and hydrographic parameters document the circulation of LSW into the eastern North Atlantic, where it overlies southern-sourced Lower DeepWater. The GEOVIDE δ30SiDSi distribution thus provides a clear view of the direct interaction between subpolar/ polar water masses of northern and southern origin, and allow examination of the extent to which these far-field signals influence the local δ30SiDSi distribution. [ABSTRACT FROM AUTHOR]

Although proxies have generally been used to study deep ocean convection and overturning circulation in the Labrador Sea, their efficacy has not been explicitly evaluated because observations that directly measure those variables are scarce. In this study, the volume of newly formed Labrador Sea Water (LSW) and the overturning circulation in the Labrador Sea are estimated using observational data and output from a high-resolution ocean model and then compared to proxies used to represent those variables. The comparisons reveal the limitations of proxies, highlighting the desirability of robust estimates derived from direct monitoring in the region [i.e., from Argo and Overturning in the Subpolar North Atlantic Program (OSNAP)]. A linkage among LSW formation, overturning circulation in the Labrador Sea, and the export of LSW from the basin on interannual time scales is found in the model. [ABSTRACT FROM AUTHOR]

This study examines the relative roles of the Arctic freshwater exported via different pathways on deep convection in the North Atlantic and the Atlantic meridional overturning circulation (AMOC). Deep water feeding the lower branch of the AMOC is formed in several North Atlantic marginal seas, including the Labrador Sea, Irminger Sea, and the Nordic seas, where deep convection can potentially be inhibited by surface freshwater exported from the Arctic. The sensitivity of the AMOC and North Atlantic to two major freshwater pathways on either side of Greenland is studied using numerical experiments. Freshwater export is rerouted in global coupled climate models by blocking and expanding the channels along the two routes. The sensitivity experiments are performed in two sets of models (CM2G and CM2M) with different control simulation climatology for comparison. Freshwater via the route east of Greenland is found to have a larger direct impact on Labrador Sea convection. In response to the changes of freshwater route, North Atlantic convection outside of the Labrador Sea changes in the opposite sense to the Labrador Sea. The response of the AMOC is found to be sensitive to both the model formulation and mean-state climate. [ABSTRACT FROM AUTHOR]

Abstract: Deep water formation supplies oxygen‐rich water to the deep sea, spreading throughout the ocean by means of the global thermohaline circulation. Models suggest that dissolved gases in newly formed deep water do not come to equilibrium with the atmosphere. However, direct measurements during wintertime convection are scarce, and the controls over the extent of these disequilibria are poorly quantified. Here we show that, when convection reached deeper than 800 m, oxygen in the Labrador Sea was consistently undersaturated at −6.1% to −7.6% at the end of convection. Deeper convection resulted in greater undersaturation, while convection ending later in the year resulted in values closer to equilibrium, from which we produce a predictive relationship. We use dissolved oxygen data from six profiling Argo floats in the Labrador Sea between 2003 and 2016, allowing direct observations of wintertime convection. Three of the six optode oxygen sensors displayed substantial average in situ drift of −3.03 μmol O2 kg−1 yr−1 (−0.94% O2 yr−1), which we corrected to stable deepwater oxygen values from repeat ship surveys. Observations of low oxygen intrusions during restratification and a simple mixing calculation demonstrate that lateral processes act to lower the oxygen inventory of the central Labrador Sea. This suggests that the Labrador Sea is a net sink for atmospheric oxygen, but uncertainties in parameterizing gas exchange limit our ability to quantify the net uptake. Our results constrain the oxygen concentration of newly formed Labrador Sea Water and allow more precise estimates of oxygen utilization and nutrient regeneration in this water mass. [ABSTRACT FROM AUTHOR]

Wintertime convective mixing plays a pivotal role in the sub-polar North Atlantic spring phytoplankton blooms by favoring phytoplankton survival in the competition between light-dependent production and losses due to grazing and gravitational settling. We use satellite and ocean reanalyses to show that the area-averaged maximum winter mixed layer depth is positively correlated with April chlorophyll concentration in the northern Labrador Sea. A simple theoretical framework is developed to understand the relative roles of winter/spring convection and gravitational sedimentation in spring blooms in this region. Combining climate model simulations that project a weakening of wintertime Labrador Sea convection from Arctic sea ice melt with our framework suggests a potentially significant reduction in the initial fall phytoplankton population that survive the winter to seed the region’s spring bloom by the end of the 21st century. [ABSTRACT FROM AUTHOR]

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Argo floats are used to investigate Labrador Sea overturning and its variability on seasonal time scales. This is the first application of Argo floats to estimate overturning in a deep-water formation region in the North Atlantic. Unlike hydrographic measurements, which are typically confined to the summer season, floats offer the advantage of collecting data in all seasons. Seasonal composite potential density and absolute geostrophic velocity sections across the mouth of the Labrador Sea assembled from float profiles and trajectories at 1000 m are used to calculate the horizontal and overturning circulations. The overturning exhibits a pronounced seasonal cycle; in depth space the overturning doubles throughout the course of the year, and in density space it triples. The largest overturning [1.2 Sv (1 Sv ≡ 106 m3 s−1) in depth space and 3.9 Sv in density space] occurs in spring and corresponds to the outflow of recently formed Labrador Sea Water. The overturning decreases through summer and reaches a minimum in winter (0.6 Sv in depth space and 1.2 Sv in density space). The robustness of the Argo seasonal overturning is supported by a comparison to an overturning estimate based on hydrographic data from the AR7W line. [ABSTRACT FROM AUTHOR]

A necessary step before assessing the performance of decadal predictions is the evaluation of the processes that bring memory to the climate system, both in climate models and observations. These mechanisms are particularly relevant in the North Atlantic, where the ocean circulation, related to both the Subpolar Gyre and the Meridional Overturning Circulation (AMOC), is thought to be important for driving significant heat content anomalies. Recently, a rapid decline in observed densities in the deep Labrador Sea has pointed to an ongoing slowdown of the AMOC strength taking place since the mid 90s, a decline also hinted by in-situ observations from the RAPID array. This study explores the use of Labrador Sea densities as a precursor of the ocean circulation changes, by analysing a 300-year long simulation with the state-of-the-art coupled model HadGEM3-GC2. The major drivers of Labrador Sea density variability are investigated, and are characterised by three major contributions. First, the integrated effect of local surface heat fluxes, mainly driven by year-to-year changes in the North Atlantic Oscillation, which accounts for 62% of the total variance. Additionally, two multidecadal-to-centennial contributions from the Greenland-Scotland Ridge outflows are quantified; the first associated with freshwater exports via the East Greenland Current, and the second with density changes in the Denmark Strait Overflow. Finally, evidence is shown that decadal trends in Labrador Sea densities are followed by important atmospheric impacts. In particular, a positive winter NAO response appears to follow the negative Labrador Sea density trends, and provides a phase reversal mechanism. [ABSTRACT FROM AUTHOR]

Ventilation of Labrador Sea Water (LSW) receives ample attention because of its potential relation to the strength of the Atlantic Meridional Overturning Circulation (AMOC). Here, we provide an overview of the changes of LSW from observations in the Labrador Sea and from the southern boundary of the subpolar gyre at 47° N. A strong winter-time atmospheric cooling over the Labrador Sea led to intense and deep convection, producing a thick and dense LSW layer as, for instance, in the early to mid-1990s. The weaker convection in the following years mostly ventilated less dense LSW vintages and also reduced the supply of oxygen. As a further consequence, the rate of uptake of anthropogenic carbon by LSW decreased between the two time periods 1996-1999 and 2007-2010 in the western subpolar North Atlantic. In the eastern basins, the rate of increase in anthropogenic carbon became greater due to the delayed advection of LSW that was ventilated in previous years. Starting in winter 2013/2014 and prevailing at least into winter 2015/2016, production of denser and more voluminous LSW resumed. Increasing oxygen signals have already been found in the western boundary current at 47° N. On decadal and shorter time scales, anomalous cold atmospheric conditions over the Labrador Sea lead to an intensification of convection. On multi-decadal time scales, the 'cold blob' in the subpolar North Atlantic projected by climate models in the next 100 years is linked to a weaker AMOC and weaker convection (and thus deoxygenation) in the Labrador Sea. [ABSTRACT FROM AUTHOR]

The authors identify an interdecadal oscillatory mode of the North Atlantic atmosphere-ocean system in a general circulation model (GFDL CM2.1) via a linear inverse model (LIM). The oscillation mechanism is mostly embedded in the subpolar gyre: anomalous advection generates density anomalies in the eastern subpolar gyre, which propagate along the mean gyre circulation and reach the subpolar gyre center around 10 years later, when associated anomalous advection of the opposite sign starts the other half cycle. As density anomalies reach the Labrador Sea deep convection region, Atlantic meridional overturning circulation (AMOC) anomalies are also induced. Both the gyre and AMOC anomalies then propagate equatorward slowly, following the advection of density anomalies. The oscillation is further demonstrated to be more likely an ocean-only mode while excited by the atmospheric forcing; in particular, it can be approximated as a linearly driven damped oscillator that is partly excited by the North Atlantic Oscillation (NAO). The slowly evolving interdecadal oscillation significantly improves and prolongs the LIM's prediction skill of sea surface temperature (SST) evolution. [ABSTRACT FROM AUTHOR]

This article investigates the dynamics and temporal evolution of the Atlantic multidecadal variability (AMV) in a coupled climate model. The model contains a correction to the North Atlantic flow field to improve the path of the North Atlantic Current, thereby alleviating the surface cold bias, a common problem with climate models, and offering a unique opportunity to study the AMV in a model. Changes in greenhouse gas forcing or aerosol loading are not considered. A striking feature of the results is the contrast between the western and eastern sides of the subpolar gyre in the model. On the western side, anomalous heat supply by the ocean plays a major role, with most of this heat being given up to the atmosphere in the warm phase, largely symmetrically about the time of the AMV maximum. By contrast, on the eastern side, the ocean anomalously gains heat from the atmosphere, with relatively little role for ocean heat supply in the years before the AMV maximum. Thereafter, the balance changes with heat now being anomalously removed from the eastern side by the ocean, leading to a reduced ocean heat content, behavior associated with the establishment of an intergyre gyre at the time of the AMV maximum. In the warm phase, melting sea ice leads to a freshening of surface waters northeast of Greenland that travel southward into the Irminger and Labrador Seas, shutting down convection and terminating the AMV warm phase. [ABSTRACT FROM AUTHOR]

Egg production rates (EPRs) for freshly caught female Calanus finmarchicus increase with increasing in situ chlorophyll concentration to upper limits, but with considerable scatter for individual points around fitted curves. Here, using time course experiments, we investigated whether females exhibit synchronous diel egg-laying behaviour, leading to variations in measured EPRs for different experimental start times. Also, we compared 24 h EPRs from these experiments with results obtained using two other standard 24 h incubation methods. We found (i) female C. finmarchicus from the Labrador Sea did not exhibit diel egg-laying behaviour in spring, (ii) egg-laying was sometimes more frequent during the first 6h of incubation than thereafter and (iii) standard 24h incubations underestimated EPRs in 20-36% of experiments due to egg loss. Using results from this and a previous study Head et al. [(2013a) Characteristics of egg production of the planktonic copepod, Calanus finmarchicus, in the Labrador Sea: 1997-2010. J. Plankton Res. 35, 281-298] we developed a new expression relating in situ EPRs to chlorophyll concentration, temperature, female size and season. Season is represented by empirically derived coefficients, which vary regionally reflecting differences in spring bloom dynamics. For 100 experimental stations predicted and measured EPRs were highly correlated (r² = 0.56, P < 0.001). Additional sources of variability are discussed and recommendations are made regarding EPR measurement methods. [ABSTRACT FROM AUTHOR]

To better understand coastal sea level variability and changes, a theory that predicts sea levels along a curved western boundary using interior ocean sea level information is proposed. The western boundary sea level at a particular latitude is expressed by the sum of contributions from interior sea levels propagating onto the western boundary by long Rossby waves between that latitude and a higher latitude, and from the western boundary sea level at the higher latitude. This theory is examined by using a linear, reduced gravity model. A comparison between the theory and the model shows good agreement. A simple scaling law (or rule of thumb) derived from the theory provides a measure of the higher-latitude sea level and ocean interior sea level contributions. The theory is then tested using data from 34 climate models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) for dynamic sea level changes between the end of the twentieth and twenty-first centuries. The theory captures the nearly uniform sea level rise from the Labrador Sea to New York City (NYC), with a reduction in the increase of sea level farther south toward the equator, qualitatively consistent with the CMIP5 multimodel ensemble, even though the theory underestimates the equatorward reduction rate. Along the South American east coast, the theory successfully reproduced the spatial pattern of the sea level change. The theory suggests a strong link between a sea level rise hot spot along the northeastern coast of North America and the sea level increase in the Labrador Sea, consistent with the result that rates of NYC sea level rise are highly correlated to those in the Labrador Sea in CMIP5 models. [ABSTRACT FROM AUTHOR]

Arctic freshwater discharges to the Labrador Sea from melting glaciers and sea ice can have a large impact on ocean circulation dynamics in the North Atlantic, modifying climate and deep water formation in this region. In this study, we present for the first time a high resolution record of ice rafting in the Labrador Sea over the last millennium to assess the effects of freshwater discharges in this region on ocean circulation and climate. The occurrence of ice-rafted debris (IRD) in the Labrador Sea was studied using sediments from Site GS06-144-03 (57.29° N, 48.37°W; 3432m water depth). IRD from the fraction 63-150 µm shows particularly high concentrations during the intervals AD1000-1100, ~1150-1250, ~1400-1450, ~1650-1700 and ~1750-1800. The first two intervals occurred during the Medieval Climate Anomaly (MCA), whereas the others took place within the Little Ice Age (LIA). Mineralogical identification indicates that the main IRD source during the MCA was SE Greenland. In contrast, the concentration and relative abundance of hematite-stained grains reflects an increase in the contribution of Arctic ice during the LIA. The comparison of our Labrador Sea IRD records with other climate proxies from the subpolar North Atlantic allowed us to propose a sequence of processes that led to the cooling that occurred during the LIA, particularly in the Northern Hemisphere. This study reveals that the warm climate of the MCA may have enhanced iceberg calving along the SE Greenland coast and, as a result, freshened the subpolar gyre (SPG). Consequently, SPG circulation switched to a weaker mode and reduced convection in the Labrador Sea, decreasing its contribution to the North Atlantic deep water formation and, thus, reducing the amount of heat transported to high latitudes. This situation of weak SPG circulation may have made the North Atlantic climate more unstable, inducing a state in which external forcings (e.g. reduced solar irradiance and volcanic eruptions) could easily drive periods of severe cold conditions in Europe and the North Atlantic like the LIA. This analysis indicates that a freshening of the SPG may play a crucial role in the development of cold events during the Holocene, which may be of key importance for predictions about future climate. [ABSTRACT FROM AUTHOR]