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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Volume, heat and freshwater transports across the OSNAP array: 2014-2018
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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- Award ID(s):
- 1558742
- NSF-PAR ID:
- 10163042
- Date Published:
- Journal Name:
- Ocean science
- ISSN:
- 1812-0784
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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