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

北大西洋深层西边界流是大西洋纬向翻转环流（MOC）的下肢，因此是地球气候系统的重要组成部分。形成于格陵兰-苏格兰海脊北部的浓水通过海脊的缺口向南回流，并以溢流羽流的形式沿大陆坡下降。其中最大的是丹麦海峡溢出水（DSOW）羽流，它穿过格陵兰岛和冰岛之间，形成了MOC的丹麦部分。目前，人们认为DSOW的主要来源是东格陵兰海流，这是一股沿着格陵兰大陆坡向南流动的海流。然而，最近，在丹麦海峡附近的直接船上流速测量和详细的水文资料表明，溢出水的很大一部分来自冰岛一侧的海峡，除了格陵兰一侧。特别是，一个新发现的狭窄的电流，称为西北冰岛射流，似乎运输溢出水直接进入丹麦海峡从冰岛海。如果是这样的话，这意味着形成西部深层边界流的大部分的通风过程不是海洋学界目前所设想的。为了了解全球变暖和随之而来的地表淡水通量增加将如何影响MOC，必须准确了解DSOW的来源及其形成过程。本项目将使用系泊速度和系泊水文时间序列来确定和量化DSOW的来源，同时测量向北的热量和盐进入冰岛海南部的主要路径。这是通过与冰岛雷克雅未克海洋研究所（MRI）的科学家合作实现的，他们将为该项目提供系泊和船时间。首席研究员将在西北冰岛喷气式飞机上部署一系列剖面系泊装置，以确定其存在，并量化其结构，运输和动力学。同时，MRI将在东格陵兰海流、丹麦海峡的溢流羽流和冰岛的Irminger海流中保持系泊。与观测结果一致，将进行一项模拟研究，以了解西北冰岛急流的起源以及驱动它的水团转化和翻转。预期的结果是，将出现一种关于西部深层边界流起源的新范式，这将改变社区如何看待气候强迫和DSOW来源之间的联系。知识价值：拟议的项目将提高目前的水团改造，电流和路径，导致北大西洋西部边界流的形成的理解。更广泛的影响：通过澄清DSOW的起源，该项目的结果将揭示大气强迫和北大西洋MOC之间的联系，这反过来又影响全球海洋环流和地球？的气候。这将有助于更好地预测MOC将如何应对全球变暖。美国的实地工作与MRI的活动以及欧洲气候计划THOR（热盐翻转环流风险）密切协调。MRI在仪器和运输时间方面为该项目投入了大量资源。拟议的推广活动涉及来自四个不同国家（包括格陵兰）的学童参加该项目，一名研究生将接受该项目的教育。

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.