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To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin. [ABSTRACT FROM AUTHOR]

The last deglacial was marked by tremendous changes in ocean temperature and circulation as well as atmospheric CO 2 and 14 C. We employed the “ 14 C plateau-tuning technique” to a centennial-scale planktic 14 C record of core MD08-3180 retrieved S.W. of the Azores Islands at ∼3060 m water depth to establish both a new standard of absolute age control and a record of past 14 C reservoir ages of ocean surface waters. Both δ 18 O minima of G. bulloides and high planktic reservoir ages of ∼1600 to 2170 yr suggest two major melt water incursions that reached from the Labrador Sea up to the subtropics over Heinrich Stadial 1 (HS-1). In parallel, we established a record of (apparent) benthic ventilation ages that add the planktic 14 C reservoir ages together with the benthic-planktic 14 C age difference at the site and time of deposition, a sum finally adjusted to past changes in atmospheric 14 C that occurred since the time of deep-water formation. Near the Azores apparent deep-water ages of the Last Glacial Maximum were as low as 340–740 yr, which suggests a lateral advection of young North Atlantic Deep Waters (NADW) from subpolar regions south of Iceland, in harmony with recent model simulation and in contrast to a widely assumed major shoaling of glacial deep-water formation. During HS-1, local benthic ventilation ages increased up to 2200–2550 yr, thus suggest an incursion of old southern-source deep waters, an unstable regime that was interrupted by brief pulses of NADW incursion near 16, 15.6 cal. ka, and most salient, near 14.9/14.7 ka. [ABSTRACT FROM AUTHOR]

ORA-S3 oceanological reanalysis data for 1959-2011 is applied to analyze the role different factors play in forming advective heat transfer anomalies on an interannual-decadal scale in the upper mixed layer of the North Atlantic. Regions are revealed in which horizontal heat advection anomalies are determined by variations in current intensity, temperature gradients, and their joint influence. It is demonstrated that the contribution of different mechanisms responsible for advective heat transfer anomalies in the upper mixed layer to the total anomalies of advective origin varies fundamentally from one current to another in the North Atlantic. In the Gulf Stream area (after it separates from the continental slope), horizontal heat advection anomalies in the upper mixed layer result mainly from fluctuations in current intensity, while in the Caribbean Current and the Gulf Stream area (until its separation), they result from variations in the horizontal temperature gradients in the upper mixed layer. In the Labrador Current, both of these mechanisms have the same sign and approximately the same absolute values. In the East Greenland Current, they compensate each other. The contribution of anomalies in horizontal temperature gradients transferred by anomalous currents to the formation of heat transfer anomalies in the upper layer of the North Atlantic are, on the whole, relatively small throughout the water area. The areas of the North Atlantic and West Greenland currents are exceptions. [ABSTRACT FROM AUTHOR]

Foraminifera abundance and stable isotope records from ODP Site 984 (61.25°N, 24.04°W, 1648 m) in the North Atlantic are used to reconstruct surface circulation variations and the relative strength of the North Atlantic Deep Water (NADW) formation over the period spanning the peak warmth of Marine Interglacial Stage (MIS) 9e (~324-336 ka). This interval includes the preceding deglaciation, Termination 4 (T4), and the subsequent glacial inception of MIS 9d. The records indicate a greatly reduced contribution of NADW during T4, as observed in more recent deglaciations. In contrast with the most recent deglaciation, the lack of a significant NADW signal extended from T4 well into the peak interglacial MIS 9e and persisted nearly until the transition to the subsequent glacial stage MIS 9d. Although NADW formation resumed during MIS 9e, only depths greater than 2000 m appear to have been ventilated. The poorly ventilated intermediate depth of Site 984 (<2000 m) may have resulted on one hand from a general reduction of deep water ventilation by NADW during the study interval or, on the other hand, from different pathways of the spread of newly formed NADW that bypassed the study location. The intermediate depths may have also been invaded by southern-sourced waters as the formation of intermediate depth NADW weakened. The absence of any significant NADW signal at the water depth of Site 984 during the climatic optimum contrasts sharply with subsequent interglacial peaks (MIS 5e and the Holocene). Despite the perturbed intermediate depth circulation, oceanic heat transport northeastward was not interrupted and may have contributed to the relatively mild interglacial conditions of MIS 9e. [ABSTRACT FROM AUTHOR]

The Holocene is characterized by the late Holocene cooling trend as well as by internal short-term centennial fluctuations. Because Arctic sea ice acts as a significant component (amplifier) within the climate system, investigating its past long- and short-term variability and controlling processes is beneficial for future climate predictions. This study presents the first biomarker-based (IP and PIP) sea ice reconstruction from the Kara Sea (core BP00-07/7), covering the last 8 ka. These biomarker proxies reflect conspicuous short-term sea ice variability during the last 6.5 ka that is identified unprecedentedly in the source region of Arctic sea ice by means of a direct sea ice indicator. Prominent peaks of extensive sea ice cover occurred at ~3, ~2, ~1.3 and ~0.3 ka. Spectral analysis of the IP record revealed ~400- and ~950-year cycles. These periodicities may be related to the Arctic/North Atlantic Oscillation, but probably also to internal climate system fluctuations. This demonstrates that sea ice belongs to a complex system that more likely depends on multiple internal forcing. [ABSTRACT FROM AUTHOR]

Recent observations indicate that ice-ocean interaction drives much of the recent increase in mass loss from the Greenland Ice Sheet; however, the role of ocean forcing in driving past glacier change is poorly understood. To extend the observational record and our understanding of the ocean-cryosphere link, we used a multi-proxy approach that combines new data from proglacial lake sediments, 14C-dated in situ moss that recently emerged from beneath cold-based ice caps, and 10Be ages to reconstruct centennial-scale records of mountain glacier activity for the past ~10 k.y. in West Greenland. Proglacial lake sediment records and 14C dating of moss indicate the onset of Neoglaciation in West Greenland at ca. 5 ka with substantial snowline lowering and glacier expansion at ca. 3.7 ka followed by additional ice expansion phases at ca. 2.9, ca. 1.7, and ca. 1.4 ka and during the Little Ice Age. We find that widespread glacier growth at ca. 3.7 ka in West Greenland coincides with marked cooling and reduced strength of the West Greenland Current in Disko Bugt. The transition to cooler ocean conditions at ca. 3.7 ka identified in Disko Bugt is registered by marine proxy data farther afield in East Greenland and on the northwestern Icelandic shelf, implying large-scale paleoceanographic changes across the North Atlantic during this interval. The similarity between glacier change on West Greenland and multiple marine and terrestrial records across the North Atlantic suggests that glaciers are strongly influenced by changes in ocean circulation and consequently implies that the ocean-cryosphere teleconnection is a persistent feature of the Arctic system. [ABSTRACT FROM AUTHOR]

The stable carbon isotope composition of dissolved inorganic carbon (δ13CDIC) in seawater was measured in samples collected during June"July 2014 in the subpolar North Atlantic. Sample collection was carried out on the RRS James Clark Ross cruise JR302, part of the 'Radiatively Active Gases from the North Atlantic Region and Climate Change' (RAGNARoCC) research programme. The observed δ13CDIC values for cruise JR302 fall in a range from 0.07 ° to +1.95 °, relative to the Vienna Peedee Belemnite standard. From duplicate samples collected during the cruise, the 1σ precision for the 341 results is 0.08 °, which is similar to our previous work and other studies of this kind. We also performed a cross-over analysis using nearby historical δ13CDIC data, which indicated that there were no significant systematic offsets between our measurements and previously published results. We also included seawater reference material (RM) produced by A. G. Dickson (Scripps Institution of Oceanography, USA) in every batch of analysis, enabling us to improve upon the calibration and quality-control procedures from a previous study. The δ13CDIC is consistent within each RM batch, although its value is not certified. We report δ13CDIC values of 1.15 ± 0.03 ° and 1.27 ± 0.05 ° for batches 141 and 144 respectively. Our JR302 δ13CDIC data can be used " along with measurements of other biogeochemical variables " to constrain the processes that control DIC in the interior ocean, in particular the oceanic uptake of anthropogenic carbon dioxide and the biological carbon pump. Our δ13CDIC results are available from the British Oceanographic Data Centre - doi:10.5285/22235f1a-b7f3-687f-e053-6c86abc0c8a6. [ABSTRACT FROM AUTHOR]

The stable carbon isotope composition of dissolved inorganic carbon (δ13C-DIC) can be used toquantify fluxes within the carbon system. For example, knowing the δ13C-DIC signature of the inorganic carbon pool can help to describe the exchange between ocean and atmosphere as well as the amount of anthropogenic carbon in the water column. The measurements can also be used for evaluating modeled carbon fluxes, for making basin wide estimates, studying seasonal and interannual variability or decadal trends in interior ocean biogeochemistry. For all these purposes, it is not only important to have a sufficient amount of data, but these data must also be internally consistent and of high quality. In this study, we present a δ13C-DIC dataset for the North Atlantic, which has undergone secondary quality control. The data originate from oceanographic research cruises between 1981 and 2012. During a primary quality control step based on simple range tests obviously bad data were flagged. In a second quality control step, biases between measurements from different cruises were quantified through a crossover analysis using nearby data of the respective cruises and absolute values of biased cruises were adjusted in the data product. the crossover analysis was possible for 22 of the 29 cruises in our dataset and adjustments were applied to 10 of these. The internal accuracy of this dataset is 0.017 °. The dataset is available via CDIAC at 10.3334/CDIAC/OTG.NAC13v1. [ABSTRACT FROM AUTHOR]

The spring bloom – a massive growth of phytoplankton that occurs annually during the spring season in mid and high latitudes – plays an important role in carbon export to the deep ocean. The onset of this event has been explained from bottom-up and top-down perspectives, exemplified by the “critical-depth” and the “dilution-recoupling” hypotheses, respectively. Both approaches differ in their key expectations about how seasonal fluctuations of the mixed layer affect the plankton community. Here we assess whether the assumptions inherent to these hypotheses are met inside a typical onedimensional Nutrient–Phytoplankton–Zooplankton–Detritus (NPZD) model, optimized to best represent climatological annual cycles of satellite-based phytoplankton biomass in the Subpolar North Atlantic. The optimized model is used in idealized experiments that isolate the effects of mixed layer fluctuations and zooplankton grazing, in order to elucidate their significance. We analyzed the model sensitivity qualitatively and using a second-order Taylor series decomposition of the model equations. Our results show that the conceptual bases of both bottom-up and top-down approaches are required to explain the process of blooming; however, neither of their bloom initiation mechanisms fully applies in the experiments. We find that a spring bloom can develop in the absence of mixed layer fluctuations, and both its magnitude and timing seem to strongly depend on nutrient and light availability. Furthermore, although zooplankton populations modulate the phytoplankton concentrations throughout the year, directly prescribed and physically driven changes in zooplankton grazing do not produce significant time shifts in bloom initiation, as hypothesized. While recognizing its limitations, our study emphasizes the processes that require further testing in order to discern among competing hypotheses. [ABSTRACT FROM AUTHOR]

The Atlantic meridional overturning circulation (AMOC) and the subpolar gyre (SPG) are important elements in mechanisms for multidecadal variability in models in the North Atlantic Ocean. In this study, a 2000-year long global ocean model integration forced with the atmospheric patterns associated with a white noise North Atlantic Oscillation (NAO) index is shown to have three distinct timescales of North Atlantic Ocean variability. First, an interannual timescale with variability shorter than 15 years, that can be related to Ekman dynamics. Second, a multidecadal timescale, on the 15- to 65-year range, that is mainly concentrated in the SPG region and is controlled by constructive interference between density anomalies around the gyre and the changing NAO forcing. Finally, the centennial timescales, with variability longer than 65 years, that can be attributed to the ocean being in a series of quasi-equilibrium states. The relationship between the ocean's response and the NAO index differs for each timescale; the 15-year and shorter timescales are directly related to the NAO of the same year, 15- to 65-year timescales are dependent on the NAO index in the last 25-30 years in a sinusoidal sense while the 65-year and longer timescales relate to a sum of the last 50-80 years of the NAO index. [ABSTRACT FROM AUTHOR]

The size partitioning of dissolved iron and organic iron-binding ligands into soluble and colloidal phases was investigated in the upper 150 m of two stations along the GA03 U.S. GEOTRACES North Atlantic transect. The size fractionation was completed using cross-flow filtration methods, followed by analysis by isotope dilution inductively-coupled plasma mass spectrometry (ID-ICP-MS) for iron and competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) for iron-binding ligands. On average, 80% of the 0.1–0.65 nM dissolved iron (< 0.2 μm) was partitioned into the colloidal iron (cFe) size fraction (10 kDa < cFe < 0.2 μm), as expected for areas of the ocean underlying a dust plume. The 1.3–2.0 nM strong organic iron-binding ligands, however, overwhelmingly (75–77%) fell into the soluble size fraction (< 10 kDa). As a result, modeling the dissolved iron size fractionation at equilibrium using the observed ligand partitioning did not accurately predict the iron partitioning into colloidal and soluble pools. This suggests that either a portion of colloidal ligands is missed by current electrochemical methods because they react with iron more slowly than the equilibration time of our CLE-ACSV method, or part of the observed colloidal iron is actually inorganic in composition and thus cannot be predicted by our model of unbound iron-binding ligands. This potentially contradicts the prevailing view that greater than > 99% of dissolved iron in the ocean is organically complexed. Disentangling the chemical form of iron in the upper ocean has important implications for surface ocean biogeochemistry and may affect iron uptake by phytoplankton. [ABSTRACT FROM AUTHOR]

Coccolithophore assemblages recovered from Integrated Ocean Drilling Program (IODP) Site U1313 are investigated to reconstruct the palaeoceanographic evolution of a sector of the North Atlantic during Marine Isotope Stage (MIS) 19 (~ 790–760 ka) at orbital, suborbital and millennial time-scales. The end of the glacial MIS 20 is marked by the arrival of cold waters, deriving from ice melting at higher latitudes, as recorded by Coccolithus pelagicus pelagicus . The MIS 19c is characterised by an extension of warmer North Atlantic Transitional Waters (NATW) reflecting the intensification of subtropical gyre influence on Site U1313, identified by an increase in warm species Umbilicosphaera sibogae . The influence on the site of cooler NATW is inferred from higher percentages of Gephyrocapsa margereli , which starts increasing from ~ 779 ka. Finally, the transition from MIS 19a to MIS 18 is characterised by a southward shift of the palaeoceanographic system due to a southward extension of the subpolar front, as indicated by an increase of cold subspecies C. pelagicus pelagicus . The evolution of the palaeoproductivity proxy records (number of coccoliths/g of sediment and Nannofossil Accumulation Rate — NAR) and the abundances of small Gephyrocapsa , G. margereli , U. sibogae and C. pelagicus pelagicus show that fluctuations in sea-surface productivity and alternations between cooling and warming phases occurred on the precessional timescale. Spectral analyses of U. sibogae , number of coccoliths/g of sediment and NAR indicate a significant concentration of variance close to half precession (~ 10 kyr); this reflects a nonlinear response of the mid-latitude North Atlantic surface ocean to low-latitude insolation forcing. Furthermore, millennial to multi-centennial instability in sea-surface dynamics is highlighted by spectral and wavelet analyses of U. sibogae , G. margereli and C. pelagicus pelagicus percentages, number of coccoliths/g of sediment and NAR, which reveal the occurrence of rapid events, related to stadial and interstadial-type conditions. [ABSTRACT FROM AUTHOR]

Debate is ongoing about the source(s) and paths of meltwater that drained into the North Atlantic Ocean at the time of the Younger Dryas (YD), especially the eastern route from glacial Lake Agassiz and predecessor lakes of the Laurentian Great Lakes located along the southeastern edge of the Laurentide Ice Sheet. Here, evidence is presented for meltwater drainage via the St. Lawrence eastern route from five new sediment cores from Cabot Strait, Laurentian Channel and Scotian Shelf at the time of the YD. Palynological analyses are used to reconstruct sea surface conditions based on dinoflagellate cyst records, and pollen data are used for additional correlation. The reconstructions show distinct drops in salinity and temperature and increased sea ice cover duration within the YD period. In addition to these new records, we present a re-examination of original data and paleoceanographic interpretation of surface waters based on a new analysis of dinoflagellate cyst zonation in combination with an updated chronology supported by new radiocarbon dates and refined calibrations. The results clearly define the YD core intervals which contain strong evidence of lowered salinity, thereby re-establishing the St. Lawrence drainage system as a significant route for inflow of YD meltwater to the North Atlantic. This inflow does not exclude the possibility for another source of freshwater, as suggested by geographical differences in the duration of cold, low salinity conditions west and east of Laurentian Channel associated with the YD climatic event. [ABSTRACT FROM AUTHOR]

The long-lived uranium decay products 230 Th and 231 Pa are widely used as quantitative tracers of adsorption to sinking particles (scavenging) in the ocean by exploiting the principles of radioactive disequilibria. Because of their preservation in the Pleistocene sediment record and through largely untested assumptions about their chemical behavior in the water column, the two radionuclides have also been used as proxies for a variety of chemical fluxes in the past ocean. This includes the vertical flux of particulate matter to the seafloor, the lateral flux of insoluble elements to continental margins (boundary scavenging), and the southward flux of water out of the deep North Atlantic. In a section of unprecedented vertical and zonal resolution, the distributions of 230 Th and 231 Pa across the North Atlantic shed light on the marine cycling of these radionuclides and further inform their use as tracers of chemical flux. Enhanced scavenging intensities are observed in benthic layers of resuspended sediments on the eastern and western margins and in a hydrothermal plume emanating from the Mid-Atlantic Ridge. Boundary scavenging is clearly expressed in the water column along a transect between Mauritania and Cape Verde which is used to quantify a bias in sediment fluxes calculated using 230 Th-normalization and to demonstrate enhanced 231 Pa removal from the deep North Atlantic by this mechanism. The influence of deep ocean ventilation that leads to the southward export of 231 Pa is apparent. The 231 Pa/ 230 Th ratio, however, predominantly reflects spatial variability in scavenging intensity, complicating its applicability as a proxy for the Atlantic meridional overturning circulation. [ABSTRACT FROM AUTHOR]

The PO 4 , δ 13 C and Cd depth distributions measured in the N. Atlantic Ocean during recent GEOTRACES and CLIVAR cruises are a result of biological and circulation processes, with in-situ uptake and remineralization of organic matter dominating in the upper ~300 m and mixing of water masses dominating in the thermocline and deep sea. The observed slopes of the NO 3 –PO 4 , preindustrial δ 13 C–PO 4 and (by analogy) Cd–PO 4 along isopycnals deviate significantly from that expected from Redfield in the thermocline and deep sea and in each case the observed slope is similar to the slopes of the end-member water masses being mixed. The depth increase in the Cd–PO 4 slope resulting from end-member mixing explains the observed kink at ~1 μM PO 4 in the Cd–PO 4 relationship in the N. Atlantic. The observation that the PO 4 - preindustrial δ 13 C–Cd relationship in the modern N. Atlantic is heavily influenced by end-member mixing throughout most of the water column complicates the use of δ 13 C and Cd as proxies of the paleocean’s nutrient distribution because changes in the PO 4 –δ 13 C–Cd composition of locally and remotely produced end-members water masses would change the PO 4 , δ 13 C and Cd depth distributions in the paleo N. Atlantic Ocean. [ABSTRACT FROM AUTHOR]

The size partitioning of dissolved Fe (dFe<0.2 µm) into soluble (sFe<0.02 µm) and colloidal (0.02 µm

As part of the CONCEPTS (Canadian Operational Network of Coupled Environmental PredicTion Systems) initiative, a high-resolution (1/12°) ice-ocean regional model is developed covering the North Atlantic and the Arctic oceans. The long-term objective is to provide Canada with short-term ice-ocean predictions and hazard warnings in ice-infested regions. To evaluate the modelling component (as opposed to the analysis - or data-assimilation - component, which is not covered in this contribution), a series of hindcasts for the period 2003-2009 is carried out, forced at the surface by the Canadian GDPS reforecasts (Smith et al., 2014). These hindcasts test how the model represents upper ocean characteristics and ice cover. Each hindcast implements a new aspect of the modelling or the ice-ocean coupling. Notably, the coupling to the multi-category ice model CICE is tested. The hindcast solutions are then assessed using a verification package under development, including in situ and satellite ice and ocean observations. The conclusions are as follows: (1) the model reproduces reasonably well the time mean, variance and skewness of sea surface height; (2) the model biases in temperature and salinity show that while the mean properties follow expectations, the Pacific Water signature in the Beaufort Sea is weaker than observed; (3) the modelled freshwater content of the Arctic agrees well with observational estimates; (4) the distribution and volume of the sea ice are shown to be improved in the latest hindcast due to modifications to the drag coefficients and to some degree to the ice thickness distribution available in CICE; (5) nonetheless, the model still overestimates the ice drift and ice thickness in the Beaufort Gyre. [ABSTRACT FROM AUTHOR]

Size-fractionated aerosol impactor samples (n = 3) representing North American, North African, and marine air mass influences were collected over the North Atlantic Ocean as part of the 2011 US GEOTRACES cruise (GEOTRACES GA03), and a molecular-level chemical characterization of the water soluble organic matter (WSOM) was obtained using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Remarkable differences were found between the molecular characteristics of WSOM from the fine (< 0.49 μm) and coarse size fractions demonstrating the importance of collecting size-fractionated aerosols in order to obtain a full molecular characterization of aerosol WSOM. The greatest molecular complexity, as defined by the number of formulas in each fraction, was found in the < 0.49 μm fraction of North American and North African-influenced aerosols, and in the > 7.2 μm fraction of marine-influenced aerosols. Principal component analysis (PCA) showed the < 0.49 μm fraction of the North American-influenced WSOM to be differentiated from all other size fractions by the prevalence of highly oxidized compounds, an indicator of atmospheric aging. The coarse fractions of the North American-influenced sample showed evidence of important contributions from low O/C (< 0.2) and condensed aromatic compounds indicative of primary combustion-influenced organic matter (OM). The fine fractions of the marine and North African-influenced samples showed characteristics of a primary marine biological source including peptide-like, phospholipid-like, and carbohydrate-like molecular formulas. The coarse fractions of the marine sample and the 0.49–3.0 μm fraction of the North African-influenced sample were defined by molecular formulas at intermediate O/C (0.2 to 0.6) and high H/C (> 1.5) ratios including many heteroatom (N, S, P) containing formulas. Condensed aromatic compounds, which originate from anthropogenic sources and can affect the global climate by absorbing solar radiation, as well as compounds that can be attributed to secondary organic aerosols (SOA), were found in all 17 WSOM fractions analyzed. This indicates the ubiquitous nature of these types of compounds across aerosol size fractions and air mass influences, even in aerosols collected over the middle of the ocean. These results provide an indication of size-fractionated aerosol composition, and future molecular characterization work should be paired with physical characteristics to inform size-fractionated and source-resolved models of aerosol OM impacts in the atmosphere. [ABSTRACT FROM AUTHOR]

We review some historical aspects of the major observational programs in the North Atlantic and adjacent regions that contributed to establishing and maintaining the global ocean climate monitoring network. The paper also presents the oceanic perspectives of climate change and touches the important issues of ocean climate variability on time scales from years to decades. Some elements of the improved understanding of the causes and mechanisms of variability in the subpolar North Atlantic and adjacent seas are discussed in detail. The sophistication of current oceanographic analysis, especially in connection with the most recent technological breakthroughs – notably the launch of the global array of profiling Argo floats – allows us to approach new challenges in ocean research. We demonstrate how the ocean-climate changes in the subpolar basins and polar seas correlate with variations in the major climate indices such as the North Atlantic Oscillation and Atlantic Multidecadal Oscillation , and discuss possible connections between the unprecedented changes in the Arctic and Greenland ice-melt rates observed over the past decade and variability of hydrographic conditions in the Labrador Sea. Furthermore, a synthesis of shipboard and Argo measurements in the Labrador Sea reveals the effects of the regional climate trends such as freshening of the upper layer – possible causes of which are also discussed – on the winter convection in the Labrador Sea including its strength, duration and spatial extent. These changes could have a profound impact on the regional and planetary climates. A section with the highlights of all papers comprising the Special Issue concludes the Preface . [ABSTRACT FROM AUTHOR]

The Deep Western Boundary Current (DWBC) along the western margin of the subpolar North Atlantic is an important component of the deep limb of the Meridional Overturning near its northern origins. A network of moored arrays from Denmark Strait to the tail of the Grand Banks has been installed for almost two decades to observe the boundary currents and transports of North Atlantic Deep Water as part of an internationally coordinated observatory for the Atlantic Meridional Overturning Circulation. The dominant variability in all of the moored velocity time series is in the week-to-month period range. While the temporal characteristics of this variability change only gradually between Denmark Strait and Flemish Cap, a broad band of longer term variability is present farther along the path of the DWBC at the Grand Banks and in the interior basins (Labrador and Irminger Seas). The vigorous intra-seasonal variability may well mask possible interannual to decadal variability that is typically an order of magnitude smaller than the high-frequency fluctuations. Here, the intra-seasonal variability is quantified at key positions along the DWBC path using both, observations and high resolution model data. The results are used to evaluate the model circulation, and in turn the model is used to relate the discrete measurements to the overall pattern of the subpolar circulation. Topographic waves are found to be trapped by the steep topography all around the western basins, the Labrador and Irminger Seas. In the Labrador Sea, the high intra-seasonal variability of the boundary current regime is separated by a region of extremely low variability in narrow recirculation cells from the basin interior. There, the variability is also on intra-seasonal time scales, but at much longer periods around 50 days. [ABSTRACT FROM AUTHOR]

The meridional overturning circulation (MOC) in the North Atlantic transports heat from the subtropics to high latitudes and hence plays an important role in the Earth’s climate. A region crucial for the MOC is the northern North Atlantic and the adjacent Nordic Seas, where waters transported northward in the MOC upper limb progressively cool, gain density and eventually sink. Here we discuss the variability of the gyre circulation, the MOC and heat flux as quantified from a joint analysis of hydrographic and velocity data from six repeats of the Greenland to Portugal OVIDE section (1997–2010), satellite altimetry and Argo float measurements. For each repeat of the OVIDE section, the full-depth absolute circulation and transports were assessed using an inverse model constrained by ship-mounted Acoustic Doppler Current Profiler data and by an overall mass balance. The obtained circulation patterns revealed remarkable transport changes in the whole water column and evidenced large variations (up to 50% of the lowest value) in the magnitude of the MOC computed in density coordinates (MOC σ ). The extent and time scales of the MOC σ variability in 1993–2010 were then evaluated using a monthly MOC σ index built upon altimetry and Argo. The MOC σ index, validated by the good agreement with the estimates from repeat hydrographic surveys, shows a large variability of the MOC σ at OVIDE on monthly to decadal time scales. The intra-annual variability is dominated by the seasonal component with peak-to-peak amplitude of 4.3 Sv (1 Sv = 10 6 m 3 s –1 ). On longer time scales, the MOC σ index varies from less than 15 Sv to about 25 Sv. It averages to 18.1 ± 1.4 Sv and shows an overall decline of 2.5 ± 1.4 Sv (95% confidence interval) between 1993 and 2010. The heat flux estimates from repeat hydrographic surveys, which vary between 0.29 and 0.70 ± 0.05 PW, indicate that the heat flux across the OVIDE section is linearly related to the MOC σ intensity (0.054 PW/Sv). [ABSTRACT FROM AUTHOR]

We examine the variability of near-surface salinity in a 10° x 10° region of the eastern North Atlantic, the location of the first part of the Salinity Processes in the Upper-ocean Regional Study (SPURS-1). The data used were collected over a two-year period, largely by a group of two types of profiling floats equipped with sensors that record high-resolution temperature and salinity measurements in the upper few meters of the water column. By comparing the SPURS-1 measurements to observations in the area from previous decades, we examine variability at time scales ranging from a few hours (mostly consisting of rainfall-driven decreases in salinity) to diurnal cycles in temperature and salinity, seasonal variability and the annual cycle, and finally to decadal-scale changes. The relationship of near-surface salinity to the hydrological cycle suggests a continuous spectrum of variability in this cycle from hours to decades. [ABSTRACT FROM AUTHOR]

Within the North Atlantic subtropical gyre lies a salinity maximum region, relatively constant in time, yet forced by a seasonally varying strong evaporation zone located several degrees south and advected by wind-driven Ekman currents and geostrophic gyre currents and eddies. Large-scale calculations using in situ salinities to quantify salt divergence in the mixed layer, together with freshwater flux products, cannot account for the observed surface salinity signature. Small-scale and vertical processes must complete the budget. The Aquarius satellite system, launched in June 2011, now provides sea surface salinity observations every seven days at approximately 100 km spacing. Here, we reexamine the surface freshwater balance in the salinity maximum region, the location of the Salinity Processes in the Upper-ocean Regional Study (SPURS) campaign, from a satellite-sensed perspective. Advection of surface salinity by Ocean Surface Current Analyses Real-time (OSCAR) satellite-based surface currents is investigated for the whole region as well as within two boxes that isolate the salinity maximum and the maximum evaporation regions. Locations of imbalance, variability of surface salinity forcing terms, and areas of potential transport and redistribution are explored using satellite observations. A discussion then considers the vertical pathways by which surface waters reach and exchange salinity with the deep ocean, thus contributing to the signal seen at the surface. [ABSTRACT FROM AUTHOR]

Surface salinity variations and processes affecting surface salinity in the high-salinity region of the subtropical North Atlantic (the SPURS-1 area) are investigated by combining data from in situ observations and satellite remote-sensing measurements. On temporal average, the surface freshwater flux term (evaporation minus precipitation) in the SPURS-1 region increases mixed-layer salinity. Oceanic advection plays the largest role in compensating this salinity increase. On seasonal time scales, mixed-layer salinity increases from April to August and decreases from September to March. This seasonal evolution of the mixed-layer salinity is largely controlled by the freshwater flux term, with vertical entrainment playing a secondary role. The domain-averaged oceanic advection and diffusion terms do not show significant seasonal cycles. The sum of all estimated salinity budget terms largely captures salinity variations on interannual time scales. Unlike the seasonal cycle, variations in freshwater flux, oceanic advection, and vertical entrainment all contribute to interannual variations in surface salinity. Oceanic advection plays a larger role in salinity changes during 2008-2012, whereas the surface freshwater flux term dominates surface salinity evolution during 2004-2007 and in 2013. Although evaporation in the SPURS-1 region dominates the mean freshwater flux, precipitation plays a larger role in interannual variations of the freshwater flux. Separating the advection term into geostrophic and Ekman components indicates that the Ekman component dominates the total advection term. The effect of Ekman advection on salinity changes in the SPURS-1 region is closely linked to the spatial distribution of salinity anomalies. Therefore, it is important to understand large-scale forcing changes. [ABSTRACT FROM AUTHOR]

The Aquarius satellite mission and Salinity Processes in the Upper-ocean Regional Study (SPURS) are providing the scientific community with new insights into the role seawater salinity plays in the Earth system. Aquarius and SPURS scientists and engineers, working with the University of Maine-based Salinity Public Engagement and Communications team, developed webinars that focused on how these programs' findings increase knowledge about topics such as the water cycle, ocean circulation, and climate. Direct involvement of research scientists and engineers was key to the success of these efforts. These experts learned how to use interactive concept maps to "deconstruct" scientific content into simpler graphical formats for their presentations. A benefit to webinar participants, presenters, and facilitators was that they honed their critical thinking skills. In addition, the webinars allow people traditionally not represented in science, technology, engineering and math to gain better access to high-quality NASA materials. Post-event audience evaluation data provide valuable feedback on the impacts of sharing the results of ocean salinity research beyond the scientific community. [ABSTRACT FROM AUTHOR]

Knowledge of the intensity and spatial-temporal distribution of rainfall over the ocean is critical in understanding the global hydrological cycle. However, rain has proven difficult to measure over the ocean due to problems associated with platform motion and flow distortion combined with the spatial and temporal variability of rainfall itself. Underwater acoustical rain gauges avoid these issues by using the loud and distinctive underwater sound generated by raindrops on the ocean surface to detect and quantify rainfall. Here, the physics and operation of and results from an instrument that uses underwater ambient sound to measure rainfall rate and wind speed are presented. Passive Aquatic Listener (PAL) instruments were mounted on a buoy deployed at Ocean Station P and on 13 Argo profilers that were deployed as part of the US National Aeronautics and Space Administration-sponsored Salinity Processes in the Upper-ocean Regional Study (SPURS) field experiment in the North Atlantic Ocean. The PALs provide near-continuous measurements of rain rate and wind speed during the two-year period over the SPURS study region defined by the Argo profilers. Comparisons of PAL data with rain and wind measured by other techniques, including direct in situ observations and satellite measurements, show good agreement for both rain rate and wind speed. [ABSTRACT FROM AUTHOR]

We investigated a 100 x 100 km high-salinity region of the North Atlantic subtropical gyre during the Sub-Tropical Atlantic Surface Salinity Experiment/Salinity Processes in the Upper-ocean Regional Study (STRASSE/SPURS) cruise from August 21, 2012, to September 9, 2012. Results showed great variability in sea surface salinity (SSS; over 0.3 psu) in the mesoscale, over 7 cm of total evaporation, and little diapycnal mixing below 36 m depth, the deepest mixed layers encountered. Strong currents in the southwestern part of the domain, and the penetration of freshwater, suggest that advection contributed greatly to salinity evolution. However, it was further observed that a smaller cyclonic structure tucked between the high SSS band and the strongest currents contributed to the transport of high SSS water along a narrow front. Cross-frontal transport by mixing is also a possible cause of summertime reduction of SSS. The observed structure was also responsible for significant southward salt transport over more than 200 km. [ABSTRACT FROM AUTHOR]

The North Atlantic subtropical salinity maximum harbors the saltiest surface waters of the open world ocean. Subduction of these waters gives rise to Subtropical Underwater, spreading the high-salinity signature over the entire basin. The Salinity Processes in the Upper-ocean Regional Study (SPURS) is aimed at understanding the physics controlling the thermohaline structure in the salinity maximum region. A combination of moored and autonomous float observations is used here to describe the vertical water mass interleaving in the area. Seasonal intensification of interleaving in late spring and the abundance of small-scale thermohaline intrusions point to an important role for submesoscale processes in the initial subduction and subsequent evolution of Subtropical Underwater. [ABSTRACT FROM AUTHOR]

The Global Drifter Program deployed a total of 144 Lagrangian drifters drogued at 15 m depth, including 88 equipped with salinity sensors, in support of the first Salinity Processes in the Upper-ocean Regional Study (SPURS-1) in the subtropical North Atlantic Ocean with the goal of measuring salt fluxes associated with surface currents. The quality-controlled data set consists of 996,583 salinity observations collected between August 2012 and April 2014. A comparison of the drifter salinities with Aquarius satellite sea surface salinity (SSS) data shows that the lifespan of the salinity sensor fitted to the drifters is of the order of one year. The salinity and velocity data from the drifters were used to validate salt transport divergence computed with satellite products, with satellite salinity taken from the standard Aquarius v3.0 data set. The results indicate good agreement between the two independent methods, and also demonstrate that the effect of the eddy field combined with SSS variability at the surface dominates the signal. SSS variability within spatial bins as compared to Aquarius-beam footprints measured by drifters can be in excess of 0.1 psu. This result suggests that careful evaluation of the representation error is required when single-point in situ measurements, such as those collected by Argo floats, are used to validate spatially averaged Aquarius salinity data. [ABSTRACT FROM AUTHOR]

We developed the data management system for the US National Aeronautics and Space Administration-sponsored Salinity Processes in the Upper-ocean Regional Study (SPURS) Atlantic field campaign (SPURS-1). Data management support means more than simply collecting and archiving static data sets. It involves a complex mixture of data visualization, interaction with principal investigators, Web development, public outreach, quality assurance, and archiving for posterity. [ABSTRACT FROM AUTHOR]

A characterization of extreme wave parameters during extratropical cyclones in the Northern hemisphere is made from WAM wave model hindcasts. In February 2007 two extratropical storms were observed in the North Atlantic and the wave fields associated with them are modeled in this paper. Wave buoy and satellite altimetry data were used to validate the WAM hindcast results. The distribution of the Benjamin–Feir index (BFI), kurtosis and the ratio of maximum wave height to significant wave height (abnormality index) around the eye of the two extratropical cyclones is studied. It is found that under these conditions the BFI and kurtosis are significantly larger mainly in the fourth quadrant and also when the wind direction is aligned with the wave propagation direction. In these regions the probability of occurrence of abnormal waves is higher. [ABSTRACT FROM AUTHOR]

Ocean Drilling Program (ODP) Holes 980 B and C, Feni Drift at the eastern slope of the Rockall Plateau, eastern North Atlantic, were examined for late Quaternary deep-sea ostracode taxonomy. Nineteen genera and 32 species were examined and (re-)illustrated with high-resolution scanning electron microscopy images. One new species Cytheropteron paramassoni n. sp. is described and one new name Eucytherura zehali is proposed for Eucytherura hazeli Yasuhara et al., 2009. This study provides updated taxonomic information for deep-sea ostracode genera and species from the eastern North Atlantic, which is an important baseline for application of deep-sea ostracodes to paleoceanographical reconstructions and paleoecological studies in this region. [ABSTRACT FROM AUTHOR]

The article investigates the changes in ocean subsurface salinity in 2013. Topics discussed include the average gridded salinity anomalies at standard depths from the surface to 2,000 meters, the zonally-averaged difference between salinities in the Pacific Ocean and the long-term averages, and the increased salinity reported in the subtropical and tropical North Atlantic and the in the subpolar North Atlantic.

The article highlights trends in meridional overturning circulation (MOC) in the North Atlantic Ocean in 2013. Topics discussed include the mean variability of the MOC, the variability of transported properties, and the strength of the MOC. The lack of consistent trend in the MOC during the year is noted.

The article offers information on the quality control (QC) strategy for SeaDataNet II (SDN), a project that collects marine datasets. It says that the QC strategy was developed with the MyOcean InSitu Thematic Assemble Centre (INS-TAC) and the Ocean Data View (ODV) software is used as QC analysis tool to ensure quality of datasets. It mentions the SDN datasets for North Atlantic Ocean and cites its several releases and insertion of new data to improve quality.

The article reports on the first retrieval of data from the 53 moorings in an array called the Overturning in the Subpolar North Atlantic Program (OSNAP) which stretches from Labrador to Greenland to Scotland. Topics discussed include the array's measurements of ocean temperature, salinity, and current velocity, the efforts to understand the Atlantic Meridional Overturning Circulation (AMOC), and the scientists' attempt to study the pull in the Southern Ocean that grids Antarctica.