Ventilation of Denmark Strait Overflow Water in the Iceland and Greenland Seas

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate system. Its upper limb consists of warm Atlantic-origin water that flows poleward across the Greenland-Scotland Ridge where it is transformed in the Nordic Seas and high Arctic Ocean. The dense water mass product then returns southward and enters the sub-polar North Atlantic through gaps in the Greenland-Scotland Ridge. The largest of these dense overflows is the Denmark Strait Overflow Water which feeds the bottom-most component of the Deep Western Boundary Current. Previous thinking has maintained that the dominant source of the Denmark Strait Overflow Water is the Atlantic-origin water transported equatorward in the East Greenland Current. In this scheme the warm-to-cold transformation of water takes place in the rim current system of the Nordic Seas. Recent observations, however, indicate that up to half of the Denmark Strait Overflow Water (in particular, the densest component) emanates from the North Icelandic Jet, which advects Arctic Intermediate Water along the north slope of Iceland. The question then arises, what is the source of the North Icelandic Jet and where does the warm-to-cold transformation occur that produces this water? To understand the workings of the AMOC and how it might respond to changes in climate, it is essential to determine all of the dense water sources to its lower limb and understand the atmospheric and oceanic processes that form the water. Herein a wintertime field program will be conducted in the Iceland and Greenland Seas together with a coordinated modeling study to fundamentally advance our knowledge of this newly identified source of dense water and its link to the AMOC. This project will, for the first time, document convection in the northwest Iceland Sea and southeast Greenland Sea in relation to the atmospheric forcing, determine the lateral extent of the transformation and the volume and water mass characteristics of the newly-ventilated water, and characterize the circulation in which the convection occurs. By doing so, the link between the water mass transformation process, the exit pathway of the North Icelandic Jet, and the deep limb of the AMOC will be quantified. An outreach program will engage secondary school children and the general public, and a graduate student will be trained by the project. The aim of this project is to fundamentally advance our knowledge of this newly identified source of dense water north of Iceland and its link to the AMOC. The field campaign consists of a six-week wintertime hydrographic cruise to the region that will coincide with the atmospheric field campaign from the United Kingdom. The modeling component will help interpret and tie together the extensive set of international observations that will be obtained. The winter cruise has four components: (1) a broad-scale survey of the northwest Iceland Sea and southeast Greenland Sea where the dense North Icelandic Jet water is believed to emanate from (a region essentially devoid of wintertime observations. This includes an excursion into the center of each gyre; (2) adaptive sampling in conjunction with the aircraft campaign to measure the ocean's response to cold-air outbreaks in targeted areas downstream of the ice-edge, where the mixed-layers should be deepest and densest; (3) focused sampling near the Jan Mayen Fracture Zone to assess the degree to which newly ventilated dense water is fluxed southward from the Greenland Sea to the Iceland Sea; and (4) a detailed survey of the region where the North Icelandic Jet is formed. This project is the U.S. contribution to a broader international program, known as the Iceland-Greenland Seas Project (IGP), to study the air-sea interaction in the northwest Iceland Sea and southeast Greenland Sea and the associated ocean circulation and water mass transformation. The IGP has contributors from the United Kingdom, Iceland, Canada, Norway, and the Netherlands and includes an aircraft campaign that is closely coupled to the oceanographic fieldwork, as well as atmospheric and oceanic modeling.