Collaborative Research: Export Pathways from the Subpolar North Atlantic: Phase Two

智力上的优点：1990年代在北大西洋西部次极地进行的浮标观测，似乎与目前对深西部边界流(DWBC)的理解相矛盾，后者是拉布拉多海水从亚极地到亚热带纬度的主要输出途径。所有离开DWBC的拉布拉多海的剖面浮标都离开了大浅滩尾部以北的边界，沿着北大西洋洋流(NAC)在北纬50°的路径向东进入海洋内陆。这一意外的观测促使了一项长期的观测努力，以通过新的浮标观测和历史水文数据分析来更好地了解LSW路径及其变异性。该项目始于2003年，外业部分已接近完成。在三年的时间里，每三个月在DWBC中名义上每三个月释放一组六个声学跟踪的RAFOS浮标，这些浮标在高LSW和经典LSW级别的50°N附近被释放。当最后一个浮标在2008年11月浮出水面时，大约62个RAFOS浮标的两年高分辨率轨迹将可供分析。到目前为止，对已经浮出水面的RAFOS浮标和由海洋涡旋解析环流模型产生的“电子浮标”的研究出现了两个重要结果。首先，几乎没有证据表明沿DWBC从亚极地到亚热带盆地存在“快速通道”。其次，相当数量的RAFOS和e-Floats沿着L西南从纽芬兰盆地向南到东部副热带环流的内部路径追踪，这一路径根本没有被廓线浮标观测到。这些结果对DWBC作为新通风水域输出路径的重要性提出了关键问题，因此它在大西洋子午线颠覆环流(AMOC)中的作用是海洋研究优先计划(ORPP)的近期优先事项。随着RAFOS野外工作的结束，以及产生电子浮标轨迹的能力在手，研究人员现在准备在理解LSW的输出路径及其对AMOC的影响方面取得重大进展。这项分析的具体目标是：1.完成对整个RAFOS浮标数据集的统计分析，并与使用海洋环流模式产生的电子浮标进行的类似研究进行比较。调和纽芬兰盆地DWBC的浮游物迅速丧失与将DWBC作为从亚极区到亚热带地区的LSW直接输出途径这一概念之间的明显矛盾。阐明中尺度、年际和年代际变化对低纬向涛动路径的影响。评估LSW相对于DWBC的内部路径对AMOC影响的数量和性质输送的重要性：通过提供对亚极地到亚热带海洋路径的更好理解，这项拟议的研究将有助于揭示海洋在全球气候中的作用。拉格朗日观点将提供关于AMOC下肢连续性的宝贵信息，因此将有助于监测方案的设计。该项目还将产生更广泛的影响，因为它将有助于培训和教育杜克大学的一名研究生和伍兹霍尔海洋研究所的一名博士后研究员。

Intellectual Merit: Profiling float observations in the western subpolar North Atlantic made in the 1990s appeared to contradict the current understanding of the Deep Western Boundary Current (DWBC) as the dominant export pathway for Labrador Sea Water (LSW) from subpolar to subtropical latitudes. All of the profiling floats that exited the Labrador Sea in the DWBC left the boundary well north of the Tail of the Grand Banks, turning eastward into the ocean interior along the path of the North Atlantic Current (NAC) at about 50°N. This unexpected observation motivated a long-term observational effort to obtain a better understanding of LSW pathways and their variability with new float observations and historical hydrographic data analyses.The field component of this project, which began in 2003, is nearly complete. For three years, groups of six acoustically tracked RAFOS floats were released nominally every three months in the DWBC near 50ºN at Upper LSW and Classical LSW levels. When the last floats surface in November 2008, two-year high-resolution trajectories from about 62 RAFOS floats will be available for analysis. Two important results have emerged to date from a study of RAFOS floats that have already surfaced and "e-floats" generated from an ocean eddy-resolving general circulation model. First, there is little evidence for a "fast-track" from the subpolar to the subtropical basin along the DWBC. Second, a significant number of RAFOS and e-floats trace an interior pathway for LSW southward from the Newfoundland Basin to the eastern subtropical gyre, a pathway that was not observed at all by the profiling floats. These results raise critical questions about the importance of the DWBC as an export pathway for newly-ventilated waters, and hence its role in the Atlantic Meridional Overturning Circulation (AMOC) which is a near-term priority of the Ocean Research Priorities Plan (ORPP).With the conclusion of the RAFOS field work in sight, and the capability to generate e-float trajectories in hand, the investigators are now poised to make significant advances in the understanding of the export pathways of LSW and their impact on the AMOC. The specific objectives of this analysis are to:1. Complete the statistical analysis of the full RAFOS float data set and compare to a similar study using e-floats generated from an ocean general circulation model.2. Reconcile the apparent contradiction between the rapid loss of floats from the DWBC in the Newfoundland Basin and the concept of the DWBC as a direct LSW export pathway from the subpolar to subtropical region.3. Elucidate the impact of mesoscale, interannual and decadal variability on LSW pathways.4. Assess the importance of the interior pathway of LSW, relative to the DWBC, to the volume and property transport of the AMOC.Broader Impacts: By providing an improved understanding of subpolar-to-subtropical ocean pathways, this proposed research will contribute to an unraveling of the ocean's role in global climate. The Lagrangian perspective will provide valuable information on the continuity of the lower limb of the AMOC and as such will aid in the design of monitoring programs. This project will also have a broader impact in that it will contribute to the training and education of a graduate student at Duke University and a postdoctoral investigator at Woods Hole Oceanographic Institution.