Denmark Strait Overflow Water: A New Paradigm for the Origin of the Deep Western Boundary Current

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

The North Atlantic Deep Western Boundary Current constitutes the lower limb of the Atlantic meridional overturning circulation (MOC), and, as such, is a crucial component of the Earth's climate system. Dense water formed north of the Greenland-Scotland Ridge returns southward by flowing through the gaps in the ridge and descending the continental slope as overflow plumes. The largest of these is the Denmark Strait Overflow Water (DSOW) plume which passes between Greenland and Iceland and forms the densest component of the MOC. Presently, it is believed that the primary source of the DSOW is the East Greenland Current, a southward-flowing current along the continental slope of Greenland. Recently, however, direct shipboard velocity measurements and detailed hydrographic information in the vicinity of Denmark Strait suggest that a substantial fraction of the overflow water originates from the Iceland side of the strait, in addition to the Greenland side. In particular, a newly discovered narrow current, called the Northwest Icelandic Jet, appears to transport overflow water directly into Denmark Strait from the Iceland Sea. If this is the case, it means that the ventilation process forming a large part of the Deep Western Boundary Current is not what the oceanographic community presently envisions. In order to understand how global warming and the consequent enhanced surface freshwater fluxes will impact the MOC, it is imperative to have an accurate understanding of where the DSOW emanates from and the processes that form it. This project will use moored velocity and moored hydrographic time series to determine and quantify the sources of the DSOW, while simultaneously measuring the main northward pathway of heat and salt to the southern Iceland Sea. This is made possible through collaborations with scientists at the Marine Research Institute (MRI) in Reykjavik, Iceland, who will contribute both moorings and ship time to the project. The principal investigator will deploy an array of profiling moorings across the Northwest Icelandic Jet to establish its existence, and quantify its structure, transport, and dynamics. At the same time MRI will maintain moorings in the East Greenland Current, the overflow plume at Denmark Strait, and in the Icelandic Irminger Current. In concert with the observations a modeling study will be conducted to understand the origin of the Northwest Icelandic Jet and the water mass transformation and overturning that drives it. The expected outcome is that a new paradigm will emerge regarding the origin of the Deep Western Boundary Current, which will modify how the community views the links between climatic forcing and the sources of the DSOW. Intellectual Merit: The proposed project will enhance the current understanding of the water mass transformation, currents, and pathways that lead to the formation of the North Atlantic Deep Western Boundary Current. Broader Impacts: By clarifying the origin of the DSOW, the results of the project will shed light on the links between the atmospheric forcing and the North Atlantic MOC, which in turn impacts the global ocean circulation and Earth?s climate. This will allow to better anticipate how the MOC will respond to global warming. The U.S. fieldwork is closely coordinated with the activities of MRI as well as with the European climate program THOR (Thermohaline Overturning Circulation at Risk). MRI has committed significant resources to the project in the way of instrumentation and ship time. The proposed outreach involves participation in the project by school children from four different countries (including Greenland), and a graduate student will be educated by the project.