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The northwestern Tropical Atlantic Ocean is a turbulent region, filled with mesoscale eddies and regional currents. In this intense dynamical context, several water masses with thermohaline characteristics of different origins are advected, mixed, and stirred at the surface and at depth. The EUREC 4 A-OA/ATOMIC experiment that took place in January and February 2020 was dedicated to assessing the processes at play in this region, especially the interaction between the ocean and the atmosphere. For that reason, four oceanographic vessels and different autonomous platforms measured properties near the air–sea interface and acquired thousands of upper-ocean (up to 400–2000 m depth) profiles. However, each device had its own observing capability, varying from deep measurements acquired during vessel stations to shipboard underway near-surface observations and measurements from autonomous and uncrewed systems (such as Saildrones). These observations were undertaken with a specific sampling strategy guided by near-real-time satellite maps and adapted every half day, based on the process that was investigated. These processes were characterized by different spatiotemporal scales, from mesoscale eddies, with diameters exceeding 100 km, to submesoscale filaments of 1 km width. This article describes the datasets gathered from the different devices and how the data were calibrated and validated. In order to ensure an overall consistency, the platforms' datasets are cross-validated using a hierarchy of instruments defined by their own specificity and calibration procedures. This has enabled the quantification of the uncertainty in the measured parameters when different datasets are used together, e.g., 10.17882/92071. [ABSTRACT FROM AUTHOR]

The northwestern Tropical Atlantic Ocean is a turbulent region, filled with mesoscale eddies and large-scale currents. In this intense dynamical context, several water masses with thermohaline characteristics of different origins are advected, mixed, and stirred, at the surface and at depth. The EUREC4A-OA/ATOMIC experiment that took place in January and February 2020 was dedicated to assess the processes at play in this region, especially the interaction between the ocean and the atmosphere. For that, four oceanographic vessels and different autonomous platforms measured properties near the air-sea interface and acquired thousands of upper-ocean (400-2000 m) profiles. However, each device had its own observing capability, varying from deep measurements acquired during vessel stations to shipboard underway near-surface observations and measurements from autonomous and uncrewed systems (such as Saildrones). These observations were undertaken with a specific sampling strategy guided by near-real time satellite images and adapted every half day based on the process that was investigated. These processes were characterized by different spatio-temporal scales: from mesoscale eddies, with diameters exceeding 100 km, to submesoscale filaments of 1 km width. This article describes the data sets gathered from the different devices and how the data were calibrated and validated, in order to ensure an overall consistency, the platforms’ datasets are cross-validated using a hierarchy of instruments defined by their own specificity and calibration procedures. This has enabled the quantification of the uncertainty of the measured parameters when the different datasets are used together. [ABSTRACT FROM AUTHOR]

We investigate the Chukchi and the Beaufort seas in the Arctic Ocean, where salty and warm Pacific Water flows in through the Bering Strait and interacts with the sea ice, contributing to its summer melt. Thanks to in situ measurements recorded by two saildrones deployed during summer 2019 and to refined sea ice filtering in satellite L‐band radiometric data, we demonstrate the ability of satellite sea surface salinity (SSS) observed by Soil Moisture and Ocean Salinity and Soil Moisture Active and Passive to capture SSS freshening induced by sea ice melt. We refer to these freshening events as meltwater lenses (MWL). The largest MWL observed by the saildrones during this period occupied a large part of the Chukchi shelf, with a SSS freshening reaching −5 practical salinity scale, persisting for up to 1 month. This MWL restricted the transfer of air‐sea momentum to the upper ocean, as illustrated by measured wind speed and vertical profiles of currents. With satellite‐based sea surface temperature, satellite SSS provides a monitoring of the different water masses encountered in the region during summer 2019. Using sea ice concentration and estimated Ekman transport, we analyze the spatial variability of sea surface properties after the sea ice edge retreat over the Chukchi and the Beaufort seas. The two MWL captured by the saildrones and the satellite measurements resulted from different dynamics. Over the Beaufort Sea, the MWL evolution followed the meridional sea ice retreat whereas, in the Chukchi Sea, a large persisting MWL was generated by advection and subsequent melting of a sea ice filament. Plain Language Summary: The Arctic Ocean is an area of large variations in salinity. Salinity is a main driver of ocean circulation as it determines (with seawater temperature) the seawater density. However, very little is known about salinity variations there, due to the paucity of measurements near ice and in river plumes where surface water is freshest. Here, we use surface salinity measurements from two autonomous vehicles, named saildrones, to show that satellite measurements can identify the evolution of freshwater lenses that result from sea ice melt. Over the Chukchi and the Beaufort seas, sea surface salinity exhibits large seasonal changes, partly because of the sea ice melting. In this region, water from the North Pacific Ocean enters the Arctic Ocean, resulting in large gradients of salinity and temperature. During summer 2019, the saildrones measured the surface salinity and temperature variability as the sea ice retreated northwards. By comparing these data with the measurements from satellites, we showed that satellites can detect these pools of fresh surface water in the Arctic Ocean, increasing the field of application of satellites to understand changes in conditions that determine the Arctic's role in climate change. Key Points: Saildrones and L‐band radiometers detect large sea surface salinity variability induced by sea ice over the Chukchi and the Beaufort SeaLow surface salinity due to sea ice melting decreases the vertical extent of momentum transfer, inhibiting it beyond 10 m depthMeltwater lenses may persist more than 1 month and reach a surface salinity 5 pss fresher than surrounding waters [ABSTRACT FROM AUTHOR]

Ten years of L-band radiometric measurements have proven the capability of satellite sea surface salinity (SSS) to resolve large-scale-to-mesoscale SSS features in tropical to subtropical ocean. In mid-to-high latitudes, L-band measurements still suffer from large-scale and time-varying errors. Here, a simple method is proposed to mitigate the large-scale and time-varying errors. First, an optimal interpolation using a large correlation scale (;500 km) is used to map independently Soil Moisture Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) level-3 (L3) data. The mapping is compared with the equivalent mapping of in situ observations to estimate the large-scale and seasonal biases. A second mapping is performed on adjusted SSS at the scale of SMOS/SMAP spatial resolution (;45 km). This procedure merges both products and increases the signal-to-noise ratio of the absolute SSS estimates, reducing the root-mean-square difference of in situ satellite products by about 26%-32% from mid- to high latitudes, respectively, in comparison with the existing SMOS and SMAP L3 products. However, in the Arctic Ocean, some issues on satellite retrieved SSS related to, for example, radio frequency interferences, land-sea contamination, and ice-sea contamination remain challenging to reduce given the low sensitivity of L-band radiometric measurements to SSS in cold water. Using the International Thermodynamic Equation Of Seawater--2010 (TEOS-10), the resulting level-4 SSS satellite product is combined with satellite-microwave SST products to estimate sea surface density, spiciness, and haline contraction and thermal expansion coefficients. For the first time, we illustrate how useful these satellite-derived parameters are to fully characterize the surface ocean water masses at large mesoscale. [ABSTRACT FROM AUTHOR]

Variability of surface water masses of the Laptev and the East Siberian seas in August–September 2018 is studied using in situ and satellite data. In situ data were collected during the ARKTIKA-2018 expedition and then complemented with satellite-derived sea surface temperature (SST), salinity (SSS), sea surface height, wind speed, and sea ice concentration. The estimation of SSS fields is challenging in high-latitude regions, and the precision of soil moisture and ocean salinity (SMOS) SSS retrieval is improved by applying a threshold on SSS weekly error. For the first time in this region, the validity of DMI (Danish Meteorological Institute) SST and SMOS SSS products is thoroughly studied using ARKTIKA-2018 expedition continuous thermosalinograph measurements and conductivity–temperature–depth (CTD) casts. They are found to be adequate to describe large surface gradients in this region. Surface gradients and mixing of the river and the sea water in the ice-free and ice-covered areas are described with a special attention to the marginal ice zone at a synoptic scale. We suggest that the freshwater is pushed northward, close to the marginal ice zone (MIZ) and under the sea ice, which is confirmed by the oxygen isotope analysis. The SST-SSS diagram based on satellite estimates shows the possibility of investigating the surface water mass transformation at a synoptic scale and reveals the presence of river water on the shelf of the East Siberian Sea. The Ekman transport is calculated to better understand the pathway of surface water displacement on the shelf and beyond. [ABSTRACT FROM AUTHOR]

We use new observations of stable water isotopes from a research cruise in early 2017 to highlight ocean‐ice interactions occurring under Filchner‐Ronne Ice Shelf, the largest Antarctic ice shelf. In particular, we investigate the properties of Ice Shelf Water with temperature lower than the surface freezing point, in the Filchner Depression. We identify two main flavors of Ice Shelf Water emerging from beneath the Filchner Ice Shelf, which originate from High Salinity Shelf Water end members with distinct characteristics. Furthermore, these two High Salinity Shelf Water end members interact with areas of the ice shelves with different basal properties to produce different versions of Ice Shelf Water. These water masses are associated with different rates of basal melting and refreezing, which we quantify. Ice Shelf Water types that flow out of the ice shelf cavity are composed of 0.4% of mass from ice shelf (melt minus freeze) when sampled at the front of Filchner Ice Shelf. The slightly lighter Ice Shelf Water version mixes directly with ambient water masses as it flows northward on the continental shelf. The resulting Ice Shelf Water mixture with temperature below the surface freezing point ultimately sinks along the continental slope into the deep ocean, as a precursor of the Weddell Sea bottom waters. Key Points: Water masses outflowing from beneath Filchner Ice Shelf in the Weddell Sea are identified and linked to two distinct inflow sourcesThe two inflow sources interact with different ice shelf characteristics, with meltwater, being ∼0.4% of the outflowing water massesThe freshest outflow mixes with ambient waters to ultimately sink along the continental slope and entrains overlying warmer waters [ABSTRACT FROM AUTHOR]

Abstract: We combine 12-year long hydrographic data from a ship of opportunity with altimetric sea level data to infer the distribution and genesis of surface meso scale frontal structures in the Irminger Sea, detected on a salinity criterion. We find a complex spatial pattern in the occurrence of fronts: preferential locations are found both in the interior of the basin, and in the vicinity of the Irminger Current, a major current flowing along the western slope of the Mid-Atlantic Ridge, that separates contrasted surface water masses. These regions of high density of fronts do not systematically match regions of high eddy kinetic energy, but the occurrence of increased horizontal gradients is usually associated with confluence. We were able to reproduce reasonably well the spatial variability of fronts, by a simple model that passively advects the large scale salinity climatology with the altimetry-derived surface velocities. We find that stirring induced by meso scale turbulence as well as displacements of water masses are important contributors to the genesis of fronts. Thus, these fronts are often the result of a non-local process. [Copyright &y& Elsevier]

Dissolved neodymium (Nd) isotopic composition (expressed as εNd) has been analysed for 82 seawater samples collected from 13 stations stretching from the Alboran Sea to the Iceland Basin. The distribution of the εNd values of water masses was thus investigated for the first time along the western European margin in order to explore whether the water masses flowing in the eastern subpolar and subtropical Atlantic reveal distinct isotopic patterns. The Modified Atlantic Water (MAW) in the Alboran Sea displays εNd values (between −9.2 ± 0.2 and −8.9 ± 0.2) that are significantly more radiogenic than those reported in previous studies (−10.8 ± 0.2 to −9.7 ± 0.2), suggesting temporal variations in the Nd isotopic composition of the water that enters the Mediterranean Sea from the Strait of Gibraltar. The εNd value of the underlying modified Winter Intermediate Water (WIW) has been established for the first time (−9.8 ± 0.3) and is compatible with a Nd signature acquired from the sinking of MAW in the northwestern Mediterranean Sea. Within the Gulf of Cádiz, southern Mediterranean Sea Water (MSW) (−10.6 ± 0.2) differs slightly from the northern MSW (−9.9 ± 0.4) owing to a significant contribution of modified East Antarctic Intermediate Water (EAAIW) (−10.9 ± 0.2). In the northeast Atlantic, the North Atlantic Current surface water located in the inter-gyre region (north of 46°N) displays εNd values of between −14.0 ± 0.3 and −15.1 ± 0.3, reflecting the subpolar gyre signature. Along the western European margin, εNd values of surface water decrease toward the north (from −10.4 ± 1.6 to −13.7 ± 1.0) in agreement with the gradual mixing between subtropical and subpolar water. At intermediate depth, εNd values decrease from −9.9 ± 0.4 within the Gulf of Cádiz to −12.1 ± 0.2 within the Porcupine Seabight, indicating a strong dilution of the MSW with subpolar water. Within the Rockall Trough and the Iceland Basin, the more negative εNd values at mid-depth (<−13.5 ± 0.3) indicate that the MSW has no influence, even during periods of low NAO index. Water masses deeper than 1200 m in the northeast Atlantic are clearly influenced by the less radiogenic Labrador Sea Water (LSW) (εNd between −13.4 ± 0.3 and −14.0 ± 0.3) that mixes locally in the Iceland basin with the Iceland-Scotland Overflow Water (ISOW) (between −10.3 ± 0.2 and −11.3 ± 0.3). [ABSTRACT FROM AUTHOR]

Abstract: The lithium/magnesium (Li/Mg) molar ratios, radiocarbon measurements (Δ14C) and Nd-isotopic composition (εNd) of the aragonite skeleton of a branching cold-water coral (CWC) species Madrepora oculata collected alive in the Bay of Biscay at ∼691m water depth were investigated to reconstruct a robust record of the mid-depth water mass dynamics between 1950 and 1990 AD. Temperature estimates based on the skeletons Li/Mg molar ratios reveal small decadal changes of about 1°C at thermocline depth synchronous to and of similar amplitude as surface temperature anomalies. Δ14C measurements shows quasi-decadal oscillations of 15‰ around pre-bomb Δ14C average value of −59±6‰ and post-bomb Δ14C of −12±6‰, which most likely reflect decadal changes of water mass exchange across the thermocline. The coral εNd values remain in narrow ranges of −11.9 to −10.2, similar to the isotopic composition of East North Atlantic Central Water, but show highest values in the late 1950s, and early 1960s, 1970s, and 1980s. The punctuated changes of the coral Nd-isotopic composition may thus reflect periods of particular enhanced advection of temperate intermediate water (mid-depth Subpolar Gyre/Mediterranean Sea Water). Altogether, our robust multi-proxy record provides new evidence that Northern Hemisphere atmospheric variability (such as, North Atlantic Oscillation and East Atlantic pattern) drives changes not only in the thermocline but also in the mid-depth water-mass advection patterns in the Northeastern Atlantic margin. However, the interannual variability of our record remains to be tested. [Copyright &y& Elsevier]