
NSF Org: |
OCE Division Of Ocean Sciences |
Recipient: |
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Initial Amendment Date: | February 20, 2008 |
Latest Amendment Date: | August 17, 2012 |
Award Number: | 0726720 |
Award Instrument: | Continuing Grant |
Program Manager: |
Eric C. Itsweire
OCE Division Of Ocean Sciences GEO Directorate for Geosciences |
Start Date: | March 1, 2008 |
End Date: | August 31, 2015 (Estimated) |
Total Intended Award Amount: | $5,549,995.00 |
Total Awarded Amount to Date: | $5,802,095.00 |
Funds Obligated to Date: |
FY 2010 = $1,345,404.00 FY 2011 = $1,963,078.00 FY 2012 = $612,888.00 |
History of Investigator: |
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Recipient Sponsored Research Office: |
266 WOODS HOLE RD WOODS HOLE MA US 02543-1535 (508)289-3542 |
Sponsor Congressional District: |
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Primary Place of Performance: |
266 WOODS HOLE RD WOODS HOLE MA US 02543-1535 |
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, BIOLOGICAL OCEANOGRAPHY, OCEAN TECH & INTERDISC COORDIN, OCE SPECIAL PROGRAMS, OCE-Ocean Sciences Research |
Primary Program Source: |
01001011DB NSF RESEARCH & RELATED ACTIVIT 01001112DB NSF RESEARCH & RELATED ACTIVIT 01001213DB 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
The meridional overturning circulation of the North Atlantic Ocean at mid-latitude principally involves poleward transport of warm water by the Gulf Stream and equatorward flow of colder intermediate and deep waters within a western-intensified boundary current. Comprehension of how these limbs of the global current system and their associated regional recirculations vary on decadal time scale is incomplete. In particular, very little is known on how inter-annual variations in air-sea exchange and water mass modification at high latitudes are transmitted equatorward, and what impacts or feedbacks such signals may have for the Atlantic-wide circulation. Limiting advance in understanding is the lack of long, well-resolved records to document inter-annual signals in water properties, stratification and transport of the Atlantic?s Deep Western Boundary Current (DWBC) system. Importantly, anomalies created at sub-polar latitudes may be profoundly altered or even blocked by the North Atlantic Current or Gulf Stream. Conversely, sub-polar anomalies may influence the position, strength, and/or stability of the Stream, and in turn affect patterns of air-sea exchange throughout the North Atlantic. These basic questions motivate the Line W program that is presently returning observations of the DWBC and Gulf Stream about 39 N between the Grand Banks and Cape Hatteras. Companion field programs are sampling the DWBC both north and south of our study region, and several complementary modeling programs are underway. Taken together, these studies will facilitate a comprehensive investigation of the mechanisms and rates of North Atlantic Deep Water export to lower latitudes and its relationships to the upper ocean circulation.
Intellectual Merit: The proposed study will build understanding of the mechanisms and rates of North Atlantic Deep Water export to lower latitudes and the relationships of varying deep-water flow and stratification changes to the upper ocean circulation including the Gulf Stream. This knowledge will help clarify the response of the ocean to variations in air-sea exchange and ultimately, the ocean's role in global climate change. This project will extend the on-going observations of temperature, salinity, tracer and velocity variations of the DWBC and Gulf Stream through 2013, yielding a full 10-year record synchronous with the planned U.K.-U.S. observational program at 26 N. The Line W program consists of a moored array over the continental slope south of Woods Hole, and repeated occupations of a hydrographic section along the array and extending south into the Sargasso Sea. The high spatial resolution sampling possible from ship is verifying that the array is spatially resolving the inter-annual signals as well as returning water samples for tracer analysis. Equally important, the investigators will explore whether a subset of current Line W program is sufficient to index water property and transport variations in this area, setting the stage for a long-term ocean observing system.
Broader Impacts: The observations proposed will explore and test theoretical ideas about the interaction of the Deep Western Boundary Current and Gulf Stream; this is the focus of a Ph.D. thesis under current Line W funding. MIT/WHOI Joint Program students enrolled in the introductory physical oceanography course will participate on the proposed cruises, introducing them to observational oceanography Moreover, the program will continue to support one Ph. D. student from an under-represented group whose dissertation research will focus on the acquired data. All observations and resulting data products will be made available to the community at the earliest possible time via the internet with the intention of fostering widespread use of these data so that any member of the community can utilize them in their own research program. As data are recovered and processed, the Line W observations are being made available for model validation and study of regional processes by any interested investigators. In addition to raw observations, value-added products such as time series of core properties and transport by water mass are being produced and distributed. Other researchers are also encouraged to build on the Line W infrastructure to augment the physical fields being sampled.
This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) Program.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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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.
Ocean currents are driven by winds and by the exchanges of heat and fresh water with the atmosphere. Generally speaking, the oceans are warmed at low latitudes and give heat back to the atmosphere at high latitudes. Ocean currents develop that carry the warmed waters poleward to maintain an approximate state of equilibrium. Thus the oceans, acting in concert with the atmosphere, may be seen as an integral part of earth's time-varying climate system. Because cold waters are dense and warm waters buoyant, these low- to high-latitude circulations have a vertical component as well, with the warm waters moving poleward near the surface and the cold waters returning equatorward at depth. These flow patterns have been termed the Meridional Overturning Circulation of the ocean. An international observational program is now underway to better understand the Meridional Overturning Circulation in the Atlantic Ocean (AMOC), and its relationships to climate variability. The Line W program, in part supported by this grant, focused on one element of the AMOC: an equatorward flow of cold water called the Deep Western Boundary Current (DWBC) that is concentrated on the North American continental slope. Results from the Line W program will build understanding of the AMOC and provide validation information for climate forecast models.
The Line W program specifically documented the evolving structure and strength of the Atlantic's DWBC southeast of Woods Hole through the 2004-2014 period, illuminating relationships between the DWBC, the Gulf Stream, and the AMOC near latitude 39°N. The field program consisted of a 5- (and later 6-) element moored array spanning the continental slope, and repeated ship-based sampling expeditions along the array and extending into the Sargasso Sea. Several scientific results have already been disseminated to the research community and well as the general public, and analysis of the Line W observations is ongoing with support from a companion NSF grant. A few highlights: We estimate that the 10-year-averaged equatorward DWBC transport of combined intermediate and deep water is approximately 26 million cubic meters per second (approximately equivalent to a flow of 26 Amazon Rivers) with a statistical uncertainty of less than 4%. The Line W shipboard measurements extended well beyond the mooring line and provided valuable information on the distribution of water masses within the DWBC as well as the Sargasso Sea. Owing to its close proximity to the energetic and strongly-eddying Gulf Stream, the DWBC flow is highly variable. One prominent mode of variability, documented by the shipboard data in conjunction with satellite altimeter data, is associated with southward meanders of the Stream, resulting in the generation of deep, counterclockwise-rotating eddies. These deep eddies are hypothesized to be responsible for significant horizontal stirring and mixing of DWBC waters with Sargasso Sea waters. Time series of the equatorward transport of specified DWBC layers derived from the 10-year mooring record shows equatorward DWBC flow ranging between about 80 million cubic meters per second and near zero. Despite this degree of variability, significant trends have emerged, the most notable being warming, increasing salinity, decreasing layer thickness and decreasing southward flow of the waters formed by air-sea interaction in the Labrador Sea that are carried south by the DWBC. Over the course of the 10-year Line W program, the southward flow of Labrador Sea Water fell by 30% of the time-mean value. Research now underway seeks to understand how these changes relate to the observed weakening of air-sea interaction and dense water formation in the Labrador Sea from the mid-1990s to 2012.
With the r...
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