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1. Bottle-calibrated dissolved oxygen (DO) profiles from US Overturning in the Subpolar North Atlantic Program (OSNAP) cruises in 2020 and 2022 (AR45 and AR69-03)

   https://doi.org/10.26008/1912/bco-dmo.933743.1  

   Fogaren, Kristen E; Palevsky, Hilary I; Palevsky, Hilary I (January 2024, Biological and Chemical Oceanography Data Management Office (BCO-DMO))

   This dataset contains bottle-calibrated dissolved oxygen (DO) profiles collected from Conductivity Temperature Depth (CTD) casts during cruises in 2020 (AR45) and 2022 (AR69-03) to recover and redeploy Overturning in the Subpolar North Atlantic Program (OSNAP) moorings in the Labrador Sea and western Irminger Sea. DO profiles were used in conjunction with oxygen bottle measurements (Winklers) to produce a post-cruise oxygen-calibrated CTD product for scientific use as part of Gases in the Overturning and Horizontal circulation of the Subpolar North Atlantic Program (GOHSNAP), which has added moored oxygen sensors to the OSNAP mooring array, beginning in 2020. This documentation contains overviews of CTD data collection and processing and of the oxygen sensor calibration method. For each cruise, we provide a summary of relevant cruise events, oxygen sensor calibration results, and issues/problems associated with oxygen data collected. 

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2. Mixing and air–sea buoyancy fluxes set the time-mean overturning circulation in the subpolar North Atlantic and Nordic Seas

   https://doi.org/10.5194/os-19-745-2023  

   Evans, Dafydd Gwyn; Holliday, N. Penny; Bacon, Sheldon; Le Bras, Isabela (January 2023, Ocean Science)

   Abstract. The overturning streamfunction as measured at the OSNAP (Overturning in the Subpolar North Atlantic Program) mooring array represents the transformation of warm, salty Atlantic Water into cold, fresh North Atlantic Deep Water (NADW). The magnitude of the overturning at the OSNAP array can therefore be linked to the transformation by air–sea buoyancy fluxes and mixing in the region north of the OSNAP array. Here, we estimate these water mass transformations using observational-based, reanalysis-based and model-based datasets. Our results highlight that air–sea fluxes alone cannot account for the time-mean magnitude of the overturning at OSNAP, and therefore a residual mixing-driven transformation is required to explain the difference. A cooling by air–sea heat fluxes and a mixing-driven freshening in the Nordic Seas, Iceland Basin and Irminger Sea precondition the warm, salty Atlantic Water, forming subpolar mode water classes in the subpolar North Atlantic. Mixing in the interior of the Nordic Seas, over the Greenland–Scotland Ridge and along the boundaries of the Irminger Sea and Iceland Basin drive a water mass transformation that leads to the convergence of volume in the water mass classes associated with NADW. Air–sea buoyancy fluxes and mixing therefore play key and complementary roles in setting the magnitude of the overturning within the subpolar North Atlantic and Nordic Seas. This study highlights that, for ocean and climate models to realistically simulate the overturning circulation in the North Atlantic, the small-scale processes that lead to the mixing-driven formation of NADW must be adequately represented within the model's parameterisation scheme. 

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3. Overturning in the subpolar North Atlantic: a review

   https://doi.org/10.1098/rsta.2022.0191  

   Lozier, M Susan (December 2023, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   The Overturning in the Subpolar North Atlantic Program (OSNAP) was initiated in the spring of 2010 through a collaborative effort involving the USA, the UK, Germany, the Netherlands and Canada. A key feature of OSNAP is a trans-basin observing system deployed in the summer of 2014 for the continuous measure of volume, heat and freshwater fluxes in the subpolar North Atlantic. This review focuses on advancements made possible by the collective OSNAP observations. Chief among those advancements is the quantification of the dominant role of the eastern subpolar North Atlantic in the production of dense waters that reside in the lower limb of the overturning: the Irminger and Iceland basins contributed approximately three times as much dense water compared with the Labrador Sea over the observational period. Other advancements include elucidation of the relationship between convective activity in the basin interior and boundary current anomalies; the spread of overflow waters in the subpolar region; the seasonality of the meridional volume, heat and freshwater fluxes; and the challenges involved in designing a simpler, less costly observing system. Collectively, OSNAP measurements are laying a framework on which to assess the overturning circulation's vulnerability to continued warming and freshening as climate change continues apace. This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’. 

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4. Observation-based estimates of heat and freshwater exchanges from the subtropical North Atlantic to the Arctic

   Li, F.; Lozier, M. S.; Holliday, N. P.; Johns, W. E.; Le Bras, I. A.; Moat, B. I.; Cunningham, S. A.; de Jong, M. F. (July 2021, Progress in Oceanography)

   Continuous measurements from the OSNAP (Overturning in the Subpolar North Atlantic Program) array yield the first estimates of trans-basin heat and salinity transports in the subpolar latitudes. For the period from August 2014 to May 2018, there is a poleward heat transport of 0.50 ± 0.05 PW and a poleward salinity transport of 12.5 ± 1.0 Sv across the OSNAP section. Based on the mass and salt budget analyses, we estimate that a surface freshwater input of 0.36 ± 0.05 Sv over the broad subpolar-Arctic region is needed to balance the ocean salinity change created by the OSNAP transports. The overturning circulation is largely responsible for setting these heat and salinity transports (and the derived surface freshwater input) derived from the OSNAP array, while the gyre (isopycnal) circulation contributes to a lesser, but still significant, extent. Despite its relatively weak overturning and heat transport, the Labrador Sea is a strong contributor to salinity and freshwater changes in the subpolar region. Combined with trans-basin transport estimates at other locations, we provide new estimates for the time-mean surface heat and freshwater divergences over a wide domain of the Arctic-North Atlantic region to the north and south of the OSNAP line. Furthermore, we estimate the total heat and freshwater exchanges across the surface area of the extratropical North Atlantic between the OSNAP and the RAPID-MOCHA (RAPID Meridional Overturning Circulation and Heat-flux Array) arrays, by combining the cross-sectional transports with vertically-integrated ocean heat and salinity content. Comparisons with the air-sea heat and freshwater fluxes from atmospheric reanalysis products show an overall consistency, yet with notable differences in the magnitudes during the observation time period. 

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5. Water temperature, salinity, and others taken by CTD and Niskin bottles from the research vessel Neil Armstrong in the North Atlantic Ocean during 2020 Ocean Observations Initiative (OOI) Irminger Sea 7 and Overturning in the Subpolar North Atlantic Program – Greenland Deep Western Boundary Current, ONSAP-GDWBC, cruise AR46, from 2020-08-08 to 2020-08-26 (NCEI Accession 0252117)

   https://doi.org/10.25921/51b4-ac30  

   McRaven, Leah (January 2022, NOAA National Centers for Environmental Information)

   This dataset contains salinity-calibrated Conductivity Temperature Depth (CTD) data from the 2020 Ocean Observations Initiative (OOI) Irminger Sea 7 and Overturning in the Subpolar North Atlantic Program – Greenland Deep Western Boundary Current (ONSAP GDWBC) cruise (AR46). Data quality control methods have been used to assess performance of the CTD instrument. Resulting high-quality profiles were then used together with salinity bottle data analyzed at sea to create a post-cruise salinity-calibrated CTD product. This dataset has been produced as part of an ongoing effort to more fully utilize CTD data collected by OOI Irminger cruises, which have been taking place annually since 2014. The hydrographic data collection facilitated by OOI in the Irminger Sea currently supports science for not only OOI end users, but also international oceanographic research projects, including the Overturning in the Subpolar North Atlantic Program (https://www.o-snap.org/), Atlantic Meridional Overturning Circulation Program (https://usclivar.org/amoc) and BioGeoChemical Array for Real-time Geostrophic Oceanography program (https://biogeochemical-argo.org/index.php). Such programs require a higher-level data product than what OOI provides through its standard data dissemination, and hence a quality controlled, salinity-calibrated data product has been produced. Data are in text formats. 

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