| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | March 11, 2020 |
| Latest Amendment Date: | October 20, 2020 |
| Award Number: | 2017522 |
| Award Instrument: | Continuing Grant |
| Program Manager: |
Baris Uz
bmuz@nsf.gov (703)292-4557 OCE Division Of Ocean Sciences GEO Directorate for Geosciences |
| Start Date: | October 1, 2019 |
| End Date: | February 28, 2023 (Estimated) |
| Total Intended Award Amount: | $508,701.00 |
| Total Awarded Amount to Date: | $508,701.00 |
| Funds Obligated to Date: |
FY 2019 = $218,140.00 FY 2020 = $220,907.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
926 DALNEY ST NW ATLANTA GA US 30318-6395 (404)894-4819 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
GA US 30332-0420 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | PHYSICAL OCEANOGRAPHY |
| Primary Program Source: |
01001920DB NSF RESEARCH & RELATED ACTIVIT 01002021DB NSF RESEARCH & RELATED ACTIVIT |
| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Variability in the transport of heat, freshwater and mass in the North Atlantic has been linked to a number of societally relevant factors including sea level change, the melting of sea and glacial ice, and anthropogenic carbon storage in the deep ocean. However, the current understanding of the drivers of such changes is limited, making an investigation of overturning in the Sub-polar region a key ocean research priority. This field program explicitly addresses this gap. Furthermore, by engaging the larger international community studying the North Atlantic, Overturning in the Sub-polar North Atlantic Program (OSNAP) is playing a key role in catalyzing scientific progress by promoting international collaborations, synthesis, and the sharing of resources and by training future generations. Key to the latter is a network of early career scientists whose professional growth will continue to be facilitated through targeted activities that have been tested and improved during OSNAP's initial phase. Graduate students and post-docs will be trained in seagoing operations and in the processing and analysis of observational data.
The overall goal of this field project, in conjunction with other measurements in the North Atlantic, is to establish determination of the linkage between intermediate and deep water formation and the overturning circulation--a connection that is present in climate models, but has yet to be observed. This project will also provide the first comprehensive view of spreading pathways for the deep waters in the Sub-polar basin. Starting in 2014, the US-led international project, Overturning in the Sub-polar North Atlantic Program (OSNAP), provides a continuous record of the full-water column, trans-basin fluxes of heat, mass and freshwater in the Sub-polar North Atlantic, in partnership with the UK, Netherlands, Canada, Germany and China. Data from the first 21 months of the full OSNAP observing system has been used to produce an initial time series of the meridional overturning, heat and freshwater fluxes for the Sub-polar basin. These preliminary results reveal a highly variable overturning in the basin, interior pathways for overflow waters, surprisingly energetic boundary current systems on timescales from days to months, and a strong overflow plume in the Iceland Basin. The additional two years of OSNAP observations will cover a time frame sufficient to make comparisons with other overturning time series and optimize the OSNAP measurement system--all necessary tasks prior to a request for longer term measurements. This component of OSNAP (OSNAP West) is for the western array across the Labrador Sea between Greenland and Canada in coordination with Germany and Canada. The sub-goal for OSNAP is to quantify the structure and transport of the West Greenland boundary current system and determine the nature of its variability and shelf-slope exchange. In addition, the overflow water pathways throughout the subpolar region and the mechanisms responsible for their variability will be studied by analyzing the subsurface float data collected since 2014 and the output of high-resolution ocean circulation models.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
The ocean's meridional overturning circulation (MOC) is a key component of the global climate system. The MOC, characterized in the Atlantic by a northward flow of warm upper-ocean waters and a compensating southward flow of cool deep waters, plays a fundamental role in establishing the mean climate and its variability on interannual to longer time scales. The heat carried northward as part of the upper MOC limb keeps the Northern Hemisphere generally, and western Europe in particular, warmer than they would be otherwise. Variations in the strength of the MOC are believed to influence North Atlantic sea surface temperatures, leading to impacts on rainfall over the African Sahel, India, and Brazil; Atlantic hurricane activity; and summer climate over Europe and North America. Finally, variability of the inflow of warm Atlantic waters into high latitudes has been linked to the decline of Arctic sea ice and mass loss from the Greenland Ice Sheet, both of which have profound consequences for climate variability.
Though less studied than its impact on climate, the MOC?s role in the ocean carbon cycle has emerged as a recent concern. The North Atlantic is a strong sink for atmospheric carbon dioxide, accounting for ~40% of the annual mean uptake of carbon dioxide across the air?sea interface, with nearly half of that uptake occurring north of 50?N. Furthermore, modeling and observational studies show that the North Atlantic plays a crucial role in the uptake of anthropogenic carbon.
Given the imperative of understanding MOC variability and based on recommendations of the ocean science community, an international team of oceanographer, led by NSF-funded US PIs, developed an observing system for sustained transbasin measurements in the subpolar North Atlantic, called Overturning in the Subpolar North Atlantic Program (OSNAP). Deployed in the summer of 2014, OSNAP is measuring the full-depth transport of water associated with the AMOC as well as meridional heat and freshwater transport. The first 21-month record from this time series produced a ?sea change? in our understanding of the MOC. In a departure from the prevailing view that changes in the deep water formation in the Labrador Sea dominate MOC variability, the first OSNAP results suggest that the conversion of warm, salty and shallow Atlantic waters into colder, fresher and deeper waters that move southward in the Irminger and Iceland basins is largely responsible for the overturning and its variability in the subpolar basin.
This result is important to oceanographers since we now understand where to focus our observational efforts and how to interpret the signals of downstream MOC variability. This result is important to climate scientists since we now have a better understanding of what drives MOC variability and where to look for surface property changes, such as warming and freshening, that will impact local convection and MOC variability. The overall goal is to understand MOC variability in order to improve model projections of our future climate.
The Overturning in the Subpolar North Atlantic Program (OSNAP) also deployed 135 acoustically-tracked deep floats to track the spreading pathways of Iceland-Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) from 2014 to 2018. These water masses, which originate in the Nordic Seas, are transported by the deepest branch of the Atlantic Meridional Overturning Circulation (AMOC). The OSNAP floats provide the first directly-observed, comprehensive view of ISOW and DSOW spreading pathways throughout the subpolar North Atlantic. The collection of OSNAP float trajectories, complemented by model simulations, reveals that their pathways are not restricted to western boundary currents, and they are remarkably different from each other in character. The spread of DSOW from the Irminger Sea is primarily via the swift deep boundary currents of the Irminger and Labrador Seas, whereas the spread of ISOW out of the Iceland Basin is slower and along multiple export pathways. The characterization of these Overflow Water pathways has important implications for our understanding of the AMOC and its variability. Finally, reconstructions of AMOC variability from proxy data, involving either the strength of boundary currents and/or the property variability of deep waters, should account for the myriad pathways of DSOW and ISOW, but particularly so for the latter.
Last Modified: 08/03/2023
Modified by: Susan Lozier
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