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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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Structure and Variability of Iceland Scotland Overflow Water Transport in the Western Iceland Basin
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.
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- Award ID(s):
- 1948198
- PAR ID:
- 10558040
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 129
- Issue:
- 8
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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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.more » « less
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Abstract Because new observations have revealed that the Labrador Sea is not the primary source for waters in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) during the Overturning in the Subpolar North Atlantic Programme (OSNAP) period, it seems timely to re‐examine the traditional interpretation of pathways and property variability for the AMOC lower limb from the subpolar gyre to 26.5°N. In order to better understand these connections, Lagrangian experiments were conducted within an eddy‐rich ocean model to track upper North Atlantic Deep Water (uNADW), defined by density, between the OSNAP line and 26.5°N as well as within the Labrador Sea. The experiments reveal that 77% of uNADW at 26.5°N is directly advected from the OSNAP West section along the boundary current and interior pathways west of the Mid‐Atlantic Ridge. More precisely, the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea, while particles connecting OSNAP East to 26.5°N are exclusively advected from the Iceland Basin and Rockall Trough along the eastern flank of the Mid‐Atlantic Ridge. Although the pathways between OSNAP West and 26.5°N are only associated with a net formation of 1.1 Sv into the uNADW layer, they show large density changes within the layer. Similarly, as the particles transit through the Labrador Sea, they undergo substantial freshening and cooling that contributes to further densification within the uNADW layer.more » « less
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Abstract The export of the North Atlantic Deep Water (NADW) from the subpolar North Atlantic is known to affect the variability in the lower limb of the Atlantic meridional overturning circulation (AMOC). However, the respective impact from the transport in the upper NADW (UNADW) and lower NADW (LNADW) layers, and from the various transport branches through the boundary and interior flows, on the subpolar overturning variability remains elusive. To address this, the spatiotemporal characteristics of the circulation of NADW throughout the eastern subpolar basins are examined, mainly based on the 2014–20 observations from the transatlantic Overturning in the Subpolar North Atlantic Program (OSNAP) array. It reveals that the time-mean transport within the overturning’s lower limb across the eastern subpolar gyre [−13.0 ± 0.5 Sv (1 Sv ≡ 106m3s−1)] mostly occurs in the LNADW layer (−9.4 Sv or 72% of the mean), while the lower limb variability is mainly concentrated in the UNADW layer (57% of the total variance). This analysis further demonstrates a dominant role in the lower limb variability by coherent intraseasonal changes across the region that result from a basinwide barotropic response to changing wind fields. By comparison, there is just a weak seasonal cycle in the flows along the western boundary of the basins, in response to the surface buoyancy-induced water mass transformation.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2023JC020107
