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1. A sea change in our view of overturning in the subpolar North Atlantic

   https://doi.org/10.1126/science.aau6592  

   Lozier, M. S.; Li, F.; Bacon, S.; Bahr, F.; Bower, A. S.; Cunningham, S. A.; de Jong, M. F.; de Steur, L.; deYoung, B.; Fischer, J.; et al (January 2019, Science)

   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. 

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2. Water Mass Transformation and Its Relationship With the Overturning Circulation in the Eastern Subpolar North Atlantic

   https://doi.org/10.1029/2024JC021222  

   Fu, Yao; Lozier, M_Susan; Majumder, Sudip; Petit, Tillys (December 2024, Journal of Geophysical Research: Oceans)

   Abstract A recent study using the first 21 months of the OSNAP time series revealed that the export of dense waters in the eastern subpolar North Atlantic―as part of the Atlantic Meridional Overturning Circulation (MOC)―can be almost wholly attributed to surface‐forced water mass transformation (SFWMT) in the Irminger and Iceland basins, thus suggesting a minor role for other means of transformation, such as diapycnal mixing. To understand whether this result is valid over a period that exceeds the current observational record, we use four different ocean reanalysis products to investigate the relationship between surface buoyancy forcing and dense water production in this region. We also reexplore this relationship with the now available 6‐year OSNAP time series. Our analysis finds that although surface transformation in the eastern subpolar gyre dominates the production of deep waters, mixing processes downstream of the Greenland Scotland Ridge are also responsible for the production of waters carried within the AMOC's lower limb both in the observations and reanalyses. Further analysis of the reanalyses shows that SFWMT partly explains MOC interannual variability, the remaining portion can be attributed to basin storage and mixing. Compared to the observations, the reanalyses exhibit stronger MOC variance but comparable SFWMT variance on interannual timescales. 

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3. Volume, heat and freshwater transports across the OSNAP array: 2014-2018

   Li, Feili (January 2020, Ocean science)

   With contributions from the US, UK, Germany, the Netherlands, Canada and China, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was installed in the summer of 2014, which aims at measuring and understanding what drives the Atlantic Meridional Overturning Circulation (AMOC) and its variability. This coast-to-coast array of high-resolution moorings provides a continuous record of the full water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic. Data from observing system between August 2014 – June 2018 have been used to estimate those key variables for the full array as well as two sub-sections: OSNAP West, in the Labrador Sea, and OSNAP East, between Greenland and the Scottish shelf. We show notable differences in the magnitude and variability of the MOC across the full array between 2014-2016 and 2016-2018, and discuss the associated changes in the heat and freshwater transports. Differences between the fluxes across the OSNAP West and OSNAP East subsections will also be presented, along with a discussion of how this relates to the formation and transport of deep waters in the region. 

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4. Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation

   https://doi.org/10.1038/s41467-021-23350-2  

   Li, F.; Lozier, M. S.; Bacon, S.; Bower, A. S.; Cunningham, S. A.; de Jong, M. F.; deYoung, B.; Fraser, N.; Fried, N.; Han, G.; et al (December 2021, Nature Communications)

   Abstract Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region. 

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5. Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation

   Li, F.; Lozier, M. S.; Bacon, S.; Bower, A. S.; Cunningham, S. A.; De Jong, M. F.; DeYoung, B.; Fraser, N.; Fried, N.; Han, G.; et al (February 2021, Nature communications)

   Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region. 

   more » « less