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Abstract A mooring array has been maintained across the West Greenland shelf and slope since 2014 as part of the Overturning in the Subpolar North Atlantic Program (OSNAP). Here, we use the first 8 years of data to investigate the interannual variability of the two overflow water components of the deep western boundary current (DWBC): the Denmark Strait Overflow Water (DSOW) and the Northeast Atlantic Deep Water (NEADW). While the velocity structure has remained similar throughout the record, both water masses have freshened considerably, especially the NEADW salinity core. Using revised density criteria to define these two components, their transports decreased significantly between 2014 and 2022: from 6.2 to 3.8 Sv (1 Sv ≡ 106m3s−1) (−0.33 Sv yr−1) for the DSOW and from 5.4 to 4.1 Sv (−0.19 Sv yr−1) for the NEADW. Since the overflows across the Denmark Strait and the Faroe Bank Channel have remained steady over this period, this points to decreased entrainment downstream of the sills as a possible mechanism for the observed transport reduction south of Greenland. Using shipboard and mooring data from the two sills, and a hydrographic database for the surrounding region, we predict the downstream transport of the two DWBC components via the framework of a streamtube model. The predicted transport explains 94% of the observed DSOW trend and 63% of the observed NEADW trend. This implies that further entrainment of the NEADW must occur during its long pathlength, which would also help explain the fresher-than-predicted NEADW salinity observed at the OSNAP array.
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The decreasing transport of the Deep Western Boundary Current in the subpolar Atlantic
Abstract The Deep Western Boundary Current (DWBC) – the primary component of the lower limb of the Atlantic Meridional Overturning Circulation – flows along the eastern flank of Greenland from a combination of Denmark Strait Overflow Water and Iceland Scotland Overflow Water. The Overturning in the Subpolar North Atlantic Program (OSNAP) has continuously measured the DWBC since 2014 using current meters, temperature/salinity sensors, and acoustic doppler current profilers. This mooring array located near Cape Farewell also incorporates data from the Ocean Observatories Initiative’s Global Irminger Sea Array to create the longest continuous observations of the DWBC closest to where Iceland Scotland Overflow Water and Denmark Strait Overflow water first merge. This study reveals that the DWBC has decreased by 26% over the first six years of OSNAP observations primarily due to a thinning of the traditionally defined DWBC layer (σθ > 27.8 kg m-3) due to a known freshening signal moving through the subpolar region. Despite this decrease, the Atlantic Meridional Overturning Circulation as calculated by OSNAP has remained relatively steady over the same period. Ultimately, the reason for this difference is due to the methods used to define these two circulations. Finding such notably different trends for two seemingly dependent circulations raises the question of how to best define these transports.
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- Award ID(s):
- 1948505
- PAR ID:
- 10567783
- Publisher / Repository:
- Research Square
- Date Published:
- Format(s):
- Medium: X
- Institution:
- Research Square
- Sponsoring Org:
- National Science Foundation
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Abstract The Iceland Scotland Overflow Water (ISOW) plume supplies approximately a third of the production of North Atlantic Deep Water and is a key component of the meridional overturning circulation (MOC). The Overturning in the Subpolar North Atlantic Program (OSNAP) mooring array in the Iceland Basin has provided high‐resolution observations of ISOW from 2014 to 2020. The ISOW plume forms a deep western boundary current along the eastern flank of Reykjanes Ridge, and its total transport varies by greater than a factor of two on intra‐seasonal timescales. EOF analysis of moored current meter records reveal two dominant modes of velocity variance. The first mode explains roughly 20% of the variance and shows a bottom intensified structure concentrated in the rift valley that runs parallel to the ridge axis. The transport anomaly reconstructed from the first mode explains nearly 80% of the total ISOW plume transport variance. The second mode accounts for 15% of velocity variance, but only 5% of the transport variance. The geostrophically estimated transport (2.9 Sv) recovers only 70% of the total ISOW transport along the ridge flank estimated from the direct current meter observations (4.2 Sv), implying a significant ageostrophic component of ISOW mean transport and variability. Ageostrophic flow is strongly linked to the leading mode of velocity variability within the rift valley. The ISOW transport variability along the upper and middle part of the ridge is further shown to correlate with changes in the strength of deep MOC limb across the basin‐wide OSNAP array.more » « less
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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.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Posted Content:
- https://doi.org/10.21203/rs.3.rs-3232952/v1
