Award: OCE-0726720

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 retirement of two of the original Line W PIs (Joyce and McCartney), we moved to broaden the research team by entraining WHOI Assistant Scientist Magdalena Andres into the effort. In addition, the Line...