Line W: Sustained measurements of the North Atlantic Meridional Overturning Circulation
The oceans play a fundamental role in the earth's climate system by exchanging heat, fresh water, carbon, and other substances across the air-sea interface at some locations, then transporting and releasing them back to the atmosphere in other places. A major agent in this transport process is the ocean Meridional Overturning Circulation, manifested in the North Atlantic by the poleward flow of warm surface waters (principally within the Gulf Stream) and an equatorward return flow of colder, denser subsurface waters. The latter is concentrated in a feature termed the Deep Western Boundary Current, pressed up against the continental slope of the Americas. The Line W program is dedicated to obtaining a 10-year record of meridional transport variability by the DWBC at a logistically-accessible site southeast of Woods Hole. (Line W is named in memory of Val Worthington, a physical oceanographer at Woods Hole who devoted a considerable part of his career to measuring and understanding the properties and flows in the Gulf Stream and DWBC). Our research goals include characterizing the nature of the water property transport anomalies and relating the transport anomalies seen at Line W to fluctuations at other latitudes and ultimately to variations in air-sea exchange at subpolar latitude where the deep waters are exposed to the atmosphere. We hope that greater fidelity of climate models and ultimately, improved understanding of the physical processes responsible for MOC variability and its impact on earth's climate system will result.
Operationally, we are observing the DWBC and Gulf Stream at Line W utilizing a combination of moored instrumentation and periodic shipboard sampling. Building on a significant archive of historical observations from the region, the modern measurement program was initiated in 2001 with seed funding from the G. Unger Vetlesen Foundation. That support allowed us to deploy one mooring near the 3000 m isobath for two sequential one-year periods (2001-2 and 2003-4) and conduct some shipboard sampling of the DWBC water properties. In turn, we were able to parlay that seed money into now two substantial grants from the U.S. National Science Foundation. The first of these supported an initial 4-year sampling effort (spring 2004-spring 2008); the second is providing partial funding for 6 more years of measurements. In these times of tight science funding, we were required to trim significantly our initial funding request to the NSF for the current phase 2 of the Line W program. Given the more-or-less fixed costs to conduct the actual fieldwork, the only available budget area to cut was PI time to analyze the acquired data. We are therefore very grateful that the Comer Foundation through WHOI's Ocean and Climate Change Institute stepped up to provide supplemental support for the Line W science team. With this funding the Line W Pis will be able to oversee the collection, processing and distribution of the basic observations as well as carry out scientific analysis of those data.