Labrador Sea Water Research Articles

Observation-Based Estimates of Water Mass Transformation and Formation in the Labrador Sea

Abstract The water mass produced during wintertime convection in the Labrador Sea [i.e., the Labrador Sea Water (LSW)] is characterized by distinct thermohaline properties. It has been shown to exert a critical impact on the property and circulation fields of the North Atlantic. However, a quantitative understanding of the transformation and formation processes that produce LSW is still incomplete. Here, we evaluate the mean water mass transformation (WMT) and formation rates in the Labrador Sea, along with their forcing attributions, in both density and thermohaline coordinates using observation-based datasets during 2014–19. We find that while surface buoyancy loss results in an expected densification of the basin and thus LSW formation, interior mixing has an indispensable and more complex impact. In particular, mixing across density surfaces is estimated to account for 63% of the mean formation rate in the LSW layer [4.9 Sv (1 Sv ≡ 106 m3 s−1)] and does so by converting both upper-layer and overflow layer waters into the LSW layer. In addition, mixing along density surfaces is shown to be responsible for the pronounced diathermohaline transformation (∼10 Sv) west of Greenland. This is the primary process through which the cold and fresh LSW in the basin interior is exchanged with the warm and salty Irminger Water in the boundary current. Results from this study underline the critical role of mixing (both across and along density surfaces) in determining the volume and properties of the LSW, with implications for better understanding and simulating deep-water evolution under climate change.

Tracing Ocean Circulation and Mixing From the Arctic to the Subpolar North Atlantic Using the 129I–236U Dual Tracer

AbstractThis study represents the first use of the artificial radionuclides 129I and 236U, released into the ocean mainly from Nuclear Reprocessing Plants, as a dual tracer in the vicinity of Iceland with novel estimation of ocean circulatory pathways and mixing in the region. Iceland lies at the gateway to the Arctic where warm, saline Atlantic waters interact with waters of Arctic origin in ways that have critical consequences for the strength and stability of the Atlantic Meridional Overturning Circulation. Many of these interactions are not yet fully understood, such as how Atlantic water circulates around the Arctic Ocean and Nordic Seas and the composition and fate of the major overflows of the Greenland‐Scotland Ridge. Using new and previous measurements of 129I and 236U in seawater, we present a new method of appraising water mass provenance and mixing in the form of the 129I–236U dual mixing plot. With this method, we estimate that at least half the Atlantic‐origin water entering the Arctic Ocean circulates around the Canada Basin before exiting at Fram Strait and that this outflow is increased by about 40% by mixing with Return Atlantic Water “short‐circuiting” the Arctic Ocean at Fram Strait. We present tracer‐based evidence that water carried by the East Greenland Current has an unbroken pathway to the Faroe‐Shetland Channel and that Iceland‐Scotland Overflow Water (ISOW) entrains 60% Labrador Sea Water during transit past southeast Iceland. We present an unambiguous way to differentiate ISOW from DSOW after they partially merge in the Irminger Sea.

Multidisciplinary characterisation of the biodiversity, geomorphology, oceanography and glacial history of Bowditch Seamount in the Sargasso Sea

The first multidisciplinary characterisation of Bowditch Seamount in the Sargasso Sea was conducted to provide new baseline knowledge of the biodiversity, geomorphology, oceanography and glacial history of this seamount. A dropframe camera transect 1483–1562 m deep on the seamount documented 77 megafaunal taxa including Vulnerable Marine Ecosystem indicator taxa such as sponges, cold-water corals, and stalked crinoids. Seabed terrain analysis of multibeam echosounder data showed species varied significantly along this transect in response to local geomorphological variability (R2adj = 31%, p < 0.0001), with changes in seafloor relief and substrata driving species composition over the seamount. 14C-calibrated and 230Th-ages of fossil corals (Desmophyllum dianthus) collected by Van Veen grabs 1517 m deep showed corals thrived on the seamount ∼24 ka BP and ∼17 ka BP. Abrupt excursions between higher and lower radiogenic εNd-composition values of the skeletons suggested that D. dianthus persisted on the seamount over times of southern source water input and detrital sediments from the melting Laurentide Ice Sheet, respectively. In agreement with other studies from the western North Atlantic, living D. dianthus were absent in the contemporary setting at these depths, and suggest a significant re-organisation of the seamount community since the deglacial when ice-rafted debris of carbonates likely resulted in a lower aragonite compensation depth allowing D. dianthus to proliferate at deeper depths. New conductivity-depth-temperature profiling revealed the seamount at these depths is now bathed by highly oxygenated Labrador Sea Water (LSW) formed at high latitudes. Co-analysis of a newly constructed 70-year long time series of temperature and salinity for the Labrador Sea and Bermuda regions revealed a 10-year transit time from high latitudes to Bowditch Seamount. This multidisciplinary approach shows how geomorphology drives local biodiversity patterns, but also how upstream climatic forcing in subpolar regions may influence Bermuda's subtropical seamount ecosystem.

Surface factors controlling the volume of accumulated Labrador Sea Water

Abstract. We explore historical variability in the volume of Labrador Sea Water (LSW) using ECCO, an ocean state estimate configuration of the Massachusetts Institute of Technology general circulation model (MITgcm). The model's adjoint, a linearization of the MITgcm, is set up to output the lagged sensitivity of the water mass volume to surface boundary conditions. This allows us to reconstruct the evolution of LSW volume over recent decades using historical surface wind stress, heat, and freshwater fluxes. Each of these boundary conditions contributes significantly to the LSW variability that we recover, but these impacts are associated with different geographical fingerprints and arise over a range of time lags. We show that the volume of LSW accumulated in the Labrador Sea exhibits a delayed response to surface wind stress and buoyancy forcing outside the convective interior of the Labrador Sea at important locations in the North Atlantic Ocean. In particular, patterns of wind and surface density anomalies can act as a “traffic controller” and regulate the North Atlantic Current's (NAC's) transport of warm and saline subtropical water masses that are precursors for the formation of LSW. This propensity for a delayed response of LSW to remote forcing allows us to predict a limited yet substantial and significant fraction of LSW variability at least 1 year into the future. Our analysis also enables us to attribute LSW variability to different boundary conditions and to gain insight into the major mechanisms that contribute to volume anomalies in this deep water mass. We point out the important role of key processes that promote the formation of LSW in both the Irminger and Labrador seas: buoyancy loss and preconditioning along the NAC pathway and in the Iceland Basin, the Irminger Sea, and the Nordic Seas.

Intensification and shutdown of deep convection in the Labrador Sea were caused by changes in atmospheric and freshwater dynamics

Labrador Sea winter convection forms a cold, fresh and dense water mass, Labrador Sea Water, that sinks to the intermediate and deep layers and spreads across the ocean. Convective mixing undergoes multi-year cycles of intensification (deepening) and relaxation (shoaling), which have been also shown to modulate long-term changes in the atmospheric gas uptake by the sea. Here I analyze Argo float and ship-based observations to document the 2012-2023 convective cycle. I find that the highest winter cooling for the 1994-2023 period was in 2015, while the deepest convection for the 1996-2023 period was in 2018. Convective mixing continued to deepen after 2015 because the 2012-2015 winter mixing events preconditioned the water column to be susceptible to deep convection in three more years. The progressively intensified 2012-2018 winter convections generated the largest and densest class of Labrador Sea Water since 1995. Convection weakened afterwards, rapidly shoaling by 800 m per year in the winters of 2021 and 2023. Distinct processes were responsible for these two convective shutdowns. In 2021, a collapse and an eastward shift of the stratospheric polar vortex, and a weakening and a southwestward shift of the Icelandic Low resulted in extremely low surface cooling and convection depth. In 2023, by contrast, convective shutdown was caused by extensive upper layer freshening originated from extreme Arctic sea-ice melt due to Arctic Amplification of Global Warming.

The Impacts of Air-Sea Fluxes and Model Resolution on Seasonal and Inter-Annual Variability of the Atlantic Meridional Overturning Circulation across the OSNAP West Section

The present study investigates the Atlantic Meridional Overturning Circulation (AMOC) in the sub-polar North Atlantic. Despite the identification of a weaker AMOC across the OSNAP (Overturning in the Subpolar North Atlantic Program) West section, low-resolution models have continuously overestimated the subpolar AMOC link to the Labrador Sea Water mass formation, underscoring the need for a deeper understanding of this process. Nevertheless, strong air-sea gas exchange from the surface to the deep ocean, associated with the ventilation of the subpolar North Atlantic by Labrador Sea Water means the Labrador Sea remains a crucial region to study. To explore the AMOC across OSNAP, we use three configurations of the NEMO (Nucleus for European Modelling of the Ocean) ocean model with varying horizontal and vertical resolutions, including a 1/60th degree Adaptive Grid Refinement In Fortran (AGRIF) nest over the Labrador Sea. The high-resolution configuration best represents the OSNAP estimates of mean AMOC. The modeled mean and seasonal AMOC using the high-resolution configuration are improved by switching to an atmospheric forcing with stronger air-sea heat loss, while showing minimal sensitivity to the Greenland melt freshwater input. The results highlight the impact of the model resolution and representation of atmospheric forcing on the seasonal and interannual variability of subpolar AMOC link to the water mass formation in the Labrador basin.

Impact of Water Mass Mixing on the Composition and Mineralization of Dissolved Organic Matter in the Deep Labrador Sea

AbstractThe Labrador Sea is a region of the North Atlantic known for its strong ocean currents and deep water formation, which contribute to the transport and mixing of water masses throughout the region. The absorbing and fluorescing properties of dissolved organic matter (DOM) were assessed to track the water masses and the in situ production in the Labrador Sea (&gt;200 m). No significant differences in DOM composition were found in the mesopelagic waters (200–1,000 m). In the bathypelagic waters (1,000–4,000 m), the estimated dissolved organic carbon (DOC) and dissolved lignin concentrations, as well as humic‐like fluorescence intensities, allowed the discrimination of North East Atlantic Deep Water versus Denmark Strait overflow water. The humic‐like intensities were significantly different between upper Labrador Sea water (uLSW) and deep LSW (dLSW) suggesting their applications as tracers of deep winter mixing. The significant correlations with apparent oxygen utilization support the in situ production of humic‐like fluorescence and the net microbial consumption of DOC and lignin in the dark Labrador Sea. We also demonstrated that microbial activities play a role in the production of humic‐like compounds in the dLSW that experiences deep convection mixing.

Interplay between autotrophic and heterotrophic prokaryotic metabolism in the bathypelagic realm revealed by metatranscriptomic analyses

BackgroundHeterotrophic microbes inhabiting the dark ocean largely depend on the settling of organic matter from the sunlit ocean. However, this sinking of organic materials is insufficient to cover their demand for energy and alternative sources such as chemoautotrophy have been proposed. Reduced sulfur compounds, such as thiosulfate, are a potential energy source for both auto- and heterotrophic marine prokaryotes.MethodsSeawater samples were collected from Labrador Sea Water (LSW, ~ 2000 m depth) in the North Atlantic and incubated in the dark at in situ temperature unamended, amended with 1 µM thiosulfate, or with 1 µM thiosulfate plus 10 µM glucose and 10 µM acetate (thiosulfate plus dissolved organic matter, DOM). Inorganic carbon fixation was measured in the different treatments and samples for metatranscriptomic analyses were collected after 1 h and 72 h of incubation.ResultsAmendment of LSW with thiosulfate and thiosulfate plus DOM enhanced prokaryotic inorganic carbon fixation. The energy generated via chemoautotrophy and heterotrophy in the amended prokaryotic communities was used for the biosynthesis of glycogen and phospholipids as storage molecules. The addition of thiosulfate stimulated unclassified bacteria, sulfur-oxidizing Deltaproteobacteria (SAR324 cluster bacteria), Epsilonproteobacteria (Sulfurimonas sp.), and Gammaproteobacteria (SUP05 cluster bacteria), whereas, the amendment with thiosulfate plus DOM stimulated typically copiotrophic Gammaproteobacteria (closely related to Vibrio sp. and Pseudoalteromonas sp.).ConclusionsThe gene expression pattern of thiosulfate utilizing microbes specifically of genes involved in energy production via sulfur oxidation and coupled to CO2 fixation pathways coincided with the change in the transcriptional profile of the heterotrophic prokaryotic community (genes involved in promoting energy storage), suggesting a fine-tuned metabolic interplay between chemoautotrophic and heterotrophic microbes in the dark ocean.13R5usaxM1iwstSWbHDM6jVideo

The interior spreading story of Labrador Sea Water

The unique convective anomalies of Labrador Sea Water (LSW) can be used as advective tracers when assessing equatorward spreading pathways and timescales of LSW. In this study, we explore advective pathways of two LSW classes formed in the 1990s and early 2000s, respectively, along constant neutral density planes. Hydrographic observations showcase the prevalence of both LSW classes within the Atlantic interior, supporting a recirculation feature that branches from the Deep Western Boundary Current (DWBC) at 36°N among other pathways. Spreading characteristics of both LSW classes from the Labrador Sea to the subtropics are reinforced through a spatial pattern analysis of salinity anomalies and geostrophic velocities along the characteristic neutral density planes of each respective LSW class. We observe both classes to advect out of the Labrador Sea to (i) the eastern subpolar region and down the eastern boundary towards the Atlantic interior, (ii) directly into the Atlantic interior likely from an injection by recirculations from the subpolar gyre and DWBC leakage, and (iii) equatorward along the western boundary via the DWBC. Findings highlight the abundance of LSW within the Atlantic interior, not just along the western boundary, suggesting that interior pathways play an influential role on the export of these subpolar climate signals.

Decadal variability of oxygen uptake, export, and storage in the Labrador Sea from observations and CMIP6 models

The uptake of dissolved oxygen from the atmosphere via air-sea gas exchange and its physical transport away from the region of uptake are crucial for supplying oxygen to the deep ocean. This process takes place in a few key regions that feature intense oxygen uptake, deep water formation, and physical oxygen export. In this study we analyze one such region, the Labrador Sea, utilizing the World Ocean Database (WOD) to construct a 65–year oxygen content time series in the Labrador Sea Water (LSW) layer (0–2200 m). The data reveal decadal variability associated with the strength of deep convection, with a maximum anomaly of 27 mol m–2 in 1992. There is no long-term trend in the time series, suggesting that the mean oxygen uptake is balanced by oxygen export out of the region. We compared the time series with output from nine models of the Ocean Model Intercomparison Project phase 1 in the Climate Model Intercomparison Project phase 6, (CMIP6-OMIP1), and constructed a “model score” to evaluate how well they match oxygen observations. Most CMIP6-OMIP1 models score around 50/100 points and the highest score is 57/100 for the ensemble mean, suggesting that improvements are needed. All of the models underestimate the maximum oxygen content anomaly in the 1990s. One possible cause for this is the representation of air-sea gas exchange for oxygen, with all models underestimating the mean uptake by a factor of two or more. Unrealistically deep convection and biased mean oxygen profiles may also contribute to the mismatch. Refining the representation of these processes in climate models could be vital for enhanced predictions of deoxygenation. In the CMIP6-OMIP1 multi-model mean, oxygen uptake has its maximum in 1980–1992, followed by a decrease in 1994–2006. There is a concurrent decrease in export, but oxygen storage also changes between the two periods, with oxygen accumulated in the first period and drained out in the second. Consequently, the change in oxygen export (5%) is much less than that in uptake (28%), suggesting that newly ventilated LSW which remains in the formation region acts to buffer the linkage between air-sea gas exchange and oxygen export.

Labrador sea water spreading and the Atlantic meridional overturning circulation.

In 1982, Talley and McCartney used the low potential vorticity signature of Labrador Sea Water (LSW) to make the first North Atlantic maps of its properties. Forty years later, our understanding of LSW variability, spreading time scales and importance has deepened. In this review and synthesis article, I showcase recent observational advances in our understanding of how LSW spreads from its formation regions into the Deep Western Boundary Current and southward into the subtropical North Atlantic. I reconcile the fact that decadal variability in LSW formation is reflected in the Deep Western Boundary Current with the fact that LSW formation does not control subpolar overturning strength and discuss hypothesized connections between LSW spreading and decadal Atlantic Meridional Overturning Circulation variability. Ultimately, LSW spreading is of fundamental interest because it is a significant pathway for dissolved gasses such as oxygen and carbon dioxide into the deep ocean. We should hence prioritize adding dissolved gas measurements to standard hydrographic and circulation observations, particularly at targeted western boundary locations. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

The spatial and temporal distribution of neodymium isotopic composition within the Rockall Trough

Rare Earth Element (REE) concentrations and Nd isotopic compositions (εNd) of five new seawater profiles scattered across the Rockall Trough (North-East Atlantic) were investigated in order: (1) to establish the seawater εNd distribution in the Rockall Trough, which is poorly known so far; (2) to identify the southward-flowing Wyville-Thompson Ridge Overflow Water (WTOW) originating from the Nordic Seas; (3) to evaluate the potential impact of lithogenic sources on the seawater εNd in the Rockall Trough. Combined with two previously published seawater stations, the results reveal a spatial εNd gradient in the upper layer (roughly the first 900 m) of the Rockall Trough with εNd values ranging from −14.9 ± 0.3 to −11.6 ± 0.2. The influence of Nd lithogenic inputs through seawater-sediment interactions have been found to be likely negligible, so the spatial gradient has been attributed to the relative influence of unradiogenic Subpolar Gyre (SPG) water carried by the North Atlantic Current (NAC) and more radiogenic water derived from the Subtropical Gyre (STG) flowing into the Shelf Edge Current (SEC) along the European margin. Freshwater from the interior seas (Northern UK) is an additional source of radiogenic εNd, although it seems restricted to the shallow continental shelf. Below the upper layer, WTOW has been characterized for the first time with εNd (mean value of −13.8 ± 0.5; n = 9) as a homogeneous intermediate water body between 600 and 1200 m. The results reveal a strong modification of the outflowing water from the Norwegian Sea that cascades through the Wyville-Thompson ridge, as WTOW in the Rockall Trough is composed of more than 80 % Atlantic water. On the western flank of the Rockall Trough, a deep WTOW component has been identified that is responsible for a modification of the Labrador Sea Water (LSW) εNd values (-13.0 ± 0.3 to −12.1 ± 0.3). Two stations separated by only a few kilometers from each other but sampled four years apart (2012 and 2016) show a multi-annual variation of water properties and εNd values (∼1 ε-unit) for the first 800 m, pointing to a change in the relative contribution of both SPG and STG to the composition of water flowing in the Rockall Trough to the Nordic Seas. Overall, these results confirm that εNd can be used to assess the water mass composition in the Rockall Trough, allowing to reconstruct North Atlantic gyre dynamics in paleoceanographic studies.

Holocene sedimentary sequences from Nachvak and Saglek Fjords (Northern Labrador) as a record of deglaciation of the Torngat Mountains and Hudson Bay

Constraining the final retreat of the Laurentide Ice Sheet offers the best opportunities to understand the impacts of glacial retreat on landscapes and sedimentary processes, and their relation to oceanography and climate variations in Eastern North America. In Northern Labrador, an area that lacks chronological constraints on terrestrial data, investigating deglacial dynamics is also important as meltwater pulses may affect the deepwater formation that takes place in the Labrador Sea. In this paper, we investigate the deglacial sedimentary sequences from Nachvak and Saglek Fjords located in the Torngat Mountains of Northern Labrador. Sub-bottom profiles reveal that both fjords contain a deglacial sequence thicker than 100 m, including important mass-movement deposits reaching thicknesses of 40 m. These events were possibly triggered by glacio-isostatically induced earthquakes during the Younger Dryas and the Early Holocene. Estimations based on well-dated gravity cores sampled in both fjords suggest a deglaciation before the Younger Dryas for the Nachvak Outer Basin, but only 2000 yr later for the equivalent basin in Saglek Fjord. Such asynchronous retreat is explained by the presence of Saglek Bay acting as a buffer zone against oceanic forcing at the entrance of Saglek Fjord. In the sediment cores, high concentrations of detrital carbonates and an equivalent of the Hudson-Strait red bed are correlated with major deglacial events occurring between 9.2 and 8.1 cal ka BP in the Hudson Bay area. A progressive postglacial transition began around 8.0 cal ka BP with a decrease of detrital carbonate inputs and an increasing influence of North Atlantic waters. This transition ended abruptly with a regional major environmental change around 6.3 cal ka BP after the end of the Laurentide Ice Sheet melting. Magnetic properties reflecting enhanced bottom oxygen consumption reveal a high primary productivity from 6.3 to 3.8 cal ka BP in Saglek and Nachvak main basins, and probably a stronger water stratification. Overall, these results indicate that the deglacial and postglacial sedimentary dynamics of Nachvak and Saglek Fjords was influenced by (1) massive mass-movement deposits possibly generated by earthquakes, (2) sediment export by deglacial processes from both the local and the Hudson Bay areas and (3) the influence of Labrador Sea waters.

Comparing the Contributions of Temperature and Salinity Changes to the AMOC Decline at 26.5°N

Abstract At 26.5°N in the North Atlantic, a continuous transbasin observational array has been established since 2004 to detect the strength of the Atlantic meridional overturning circulation. The observational record shows that the subtropical Atlantic meridional overturning circulation has weakened by 2.5 ± 1.5 Sv (as mean ± 95% interval; 1 Sv ≡ 106 m3 s−1) since 2008 compared to the initial 4-yr average. Strengthening of the upper southward geostrophic transport (with a 2.6 ± 1.6 Sv southward increase) derived from thermal wind dominates this Atlantic meridional overturning circulation decline. We decompose the geostrophic transport into its temperature and salinity components to compare their contributions to the transport variability. The contributions of temperature and salinity components to the southward geostrophic transport strengthening are 1.0 ± 2.5 and 1.6 ± 1.3 Sv, respectively. The variation of salinity component is significant at the 95% confidence level, while the temperature component’s variation is not. This result highlights the vital role that salinity plays in the subtropical Atlantic meridional overturning circulation variability, which has been overlooked in previous studies. We further analyze the geostrophic transport variations and their temperature and salinity components arising from different water masses, which shows that a warming signal in Labrador Sea Water and a freshening signal in Nordic Sea Water are two prominent sources of the geostrophic transport increase. Comparison of the temperature and salinity records of the 26.5°N array with the upstream records from repeated hydrographic sections across the Labrador Sea suggests that these thermohaline signals may be exported from the subpolar Atlantic via the deep western boundary current.

Wind‐Forced Upwelling Along the West Greenland Shelfbreak: Implications for Labrador Sea Water Formation

AbstractArctic‐origin and Greenland meltwaters circulate cyclonically in the boundary current system encircling the Labrador Sea. The ability of this freshwater to penetrate the interior basin has important consequences for dense water formation and the lower limb of the Atlantic Meridional Overturning Circulation. However, the precise mechanisms by which the freshwater is transported offshore, and the magnitude of this flux, remain uncertain. Here, we investigate wind‐driven upwelling northwest of Cape Farewell using 4 years of high‐resolution data from the Overturning in the Subpolar North Atlantic Program west Greenland mooring array, deployed from September 2014–2018, along with Argo, shipboard, and atmospheric reanalysis data. A total of 49 upwelling events were identified corresponding to enhanced northwesterly winds, followed by reduced along‐stream flow of the boundary current and anomalously dense water present on the outer shelf. The events occur during the development stage of forward Greenland tip jets. During the storms, a cross‐stream Ekman cell develops that transports freshwater offshore in the surface layer and warm, saline, Atlantic‐origin waters onshore at depth. The net fluxes of heat and freshwater for a representative storm are computed. Using a one‐dimensional mixing model, it is shown that the freshwater input resulting from the locus of winter storms could significantly limit the wintertime development of the mixed layer and hence the production of Labrador Sea Water in the southeastern part of the basin.

Inferring Advective Timescales and Overturning Pathways of the Deep Western Boundary Current in the North Atlantic Through Labrador Sea Water Advection

AbstractThe Subpolar North Atlantic plays a critical role in the formation of the deep water masses which drive Atlantic Meridional Overturning Circulation (AMOC). Labrador Sea Water (LSW) is formed in the Labrador Sea and exported predominantly via the Deep Western Boundary Current (DWBC). The DWBC is an essential component of the AMOC advecting deep waters southward, flowing at depth along the continental slope of the western Atlantic. By combining sustained hydrographic observations from the Labrador Sea to 26.5°N, we investigate the signal propagation and advective timescales of LSW via the DWBC from its source region to the Tropical Atlantic through various approaches using robust neutral density classifications. Two individually defined LSW classes are observed to advect on timescales that support a new plausible hydrographically observed advective pathway. We find each LSW class to advect on independent timescales, and validate a hypothesized alternative‐interior advection pathway branching from the DWBC by observing the arrival of LSW outside of the DWBC in the Bermuda basin on timescales similar to arriving at 26.5°N, 10–15 yr after leaving the source region. Advective timescales estimated herein indicate that this interior pathway is likely the main advective pathway; it remains uncertain whether a direct pathway plays a significant advective role. Using LSW convective signals as advective tracers along the DWBC permits the estimation of advective timescales from the subpolar to tropical latitudes, illuminating deep water advection pathways across the North Atlantic and the lower‐limb of AMOC as a whole.

Subpolar gyre decadal variability explains the recent oxygenation in the Irminger Sea

Accurate monitoring of the long-term trend of oxygen content at global scale requires a better knowledge of the regional oxygen variability at interannual to decadal time scale. Here, we combined the Argo dataset and repeated ship-based sections to investigate the drivers of the oxygen variability in the North Atlantic Ocean, a key region for the oxygen supply into the deep ocean. We focus on the Labrador Sea Water in the Irminger Sea over the period 1991–2018 and we show that the oxygen solubility explains less than a third of the oxygen variability. In turn, the main drivers of the oxygen variability are due to changes in vertical mixing, advection, and other processes as revealed by Apparent Oxygen Utilization computation. Our findings revealed the key role of physical processes on the changes in oxygen variability and highlight the need of keeping a sustained monitoring of those processes to disentangle human-induced changes in oxygen from decadal natural variability.

The first appearance of Glacial North Atlantic Intermediate Water (GNAIW) during the Mid-Pleistocene transition

The Mid-Pleistocene Transition (MPT) is arguably the most prominent shift in the Earth's climate system during the entire Pleistocene. It is characterized by a shift from a ∼ 40 kyr to a ∼ 100 kyr glacial cyclicity, going in hand with considerable growth of glacial continental ice-sheets. In search for potential drivers of these changes in the cryosphere, a slow-down of deep ocean circulation has been proposed as an important factor that might have reduced the atmospheric pCO2 level and thus created boundary conditions favourable for pertaining large ice-sheets. Additionally, as argued for the Last Glacial Maximum, enhanced production of intermediate waters might enhance intermediate to deep oceanic stratification and thus suppress CO2 release from the deep ocean and thereby reduce atmospheric greenhouse gas concentrations. To investigate potential changes in the North Atlantic intermediate water-mass configuration across the MPT we utilize δ18O and δ13C records obtained on the deep thermocline-dwelling foraminifera Globorotalia crassaformis from mid-latitude International Ocean Discovery Program (IODP) Site U1313. In conjunction with benthic and planktic δ18O and δ13C data from reference sites for Labrador Sea Water (IODP Site U1305) and intermediate water formation in the north-eastern North Atlantic (Ocean Drilling Program Site 982) our data show that major changes in the production and dispersal of intermediate water masses took place across the MPT. In particular, we note a shoaling of glacial intermediate waters commencing with Marine Isotope Stage (MIS) 22, which we interpret as the first occurrence of Glacial North Atlantic Intermediate Water, a prominent constituent of Late Pleistocene glacial water masses in the Atlantic Ocean. We further infer that the amount and vertical position of the warm and saline Mediterranean Outflow Water (MOW) had a major impact on the spatial configuration of North Atlantic intermediate water masses during the MPT, due to its capacity to modulate subsurface heat transport and isopycnal configuration. Our new findings thus provide evidence for a so far underestimated role of intermediate water circulation in shaping oceanic and potentially atmospheric changes during the MPT.

Two Sources of Deep Decadal Variability in the Central Labrador Sea Open‐Ocean Convection Region

AbstractThe ventilation of the central Labrador Sea is important for the uptake of ocean tracers and carbon. Using historical ocean observations, we construct a simple multiple linear regression model that successfully reconstructs the decadal variability of the upper ∼2,000 m of the central Labrador Sea water properties based on observed indices that represent two different open‐ocean ventilation mechanisms. The first mechanism is the modification of deep ocean properties through local decadal variability of the Labrador Sea deep convective mixing. The second, more novel, mechanism is the climatological convective vertical redistribution of upper central Labrador Sea temperature and salinity anomalies associated with the nonlocal large‐scale subpolar Atlantic Multidecadal Variability and the Atlantic Meridional Overturning Circulation. The ventilated decadal central Labrador Sea signal subsequently spreads into the western subpolar North Atlantic. The results have important implications for predicting decadal ventilated signals in the Labrador Sea that are associated with the large‐scale climate variability.

Changing Hydrographic, Biogeochemical, and Acidification Properties in the Gulf of Maine as Measured by the Gulf of Maine North Atlantic Time Series, GNATS, Between 1998 and 2018

The Gulf of Maine North Atlantic Time Series (GNATS) has been run since 1998, across the Gulf of Maine (GoM), between Maine and Nova Scotia. GNATS goals are to provide ocean color satellite validation and to examine change in this coastal ecosystem. We have sampled hydrographical, biological, chemical, biogeochemical, and bio‐optical variables. After 2008, warm water intrusions (likely North Atlantic Slope Water [NASW]) were observed in the eastern GoM at 50–180 m depths. Shallow waters (<50 m) significantly warmed in winter, summer, and fall but cooled during spring. Surface salinity and density of the GoM also significantly increased over the 20 years. Phytoplankton standing stock and primary production showed highly‐significant decreases during the period. Concentrations of phosphate increased, silicate decreased, residual nitrate [N*; nitrate‐silicate] increased, and the ratio of dissolved inorganic nitrogen:phosphate decreased, suggesting increasing nitrogen limitation. Dissolved organic carbon (DOC) and its optical indices generally increased over two decades, suggesting changes to the DOC cycle. Surface seawater carbonate chemistry showed winter periods where the aragonite saturation (Ωar) dropped below 1.6 gulf‐wide due to upward winter mixing of cool, corrosive water. However, associated with increased average GoM temperatures, Ωar has significantly increased. These results reinforce the hypothesis that the observed decrease in surface GoM primary production resulted from a switch from Labrador Sea Water to NASW entering the GoM. A multifactor analysis shows that decreasing GoM primary production is most significantly correlated to decreases in chlorophyll and particulate organic carbon plus increases in N* and temperature.