Line W: A Sustained Measurement Program Sampling the North Atlantic Deep Western Boundary Current and Gulf Stream at 39oN

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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.