Collaborative Research: Evolution and Fate of Eighteen Degree Water in the North Atlantic Subtropical Gyre

本项目将研究北大西洋副热带环流中与十八度水（EDW）的演变（形成、循环和破坏）相关的物理过程。EDW是所有副热带西边界流系统中典型的异常厚水和垂直均匀模态水的原型。EDW与向北输送热量的浅层翻转环流有关，是年际异常热量、营养物质和二氧化碳的储存库。了解EDW演变的年周期，特别是其相关的环流路径和破坏机制非常重要，因为尽管EDW在每年冬季结束时被隔离在分层的上层海洋之下，但它可能在随后的几年中重新出现，影响混合层的性质，从而影响海气相互作用和初级生产力。研究了EDW的循环途径、EDW的破坏过程以及EDW在调节海洋-大气热交换和光区的营养供应中的作用。该计划是综合广泛的观测数据，包括最近完成的CLIVAR模式水动力学实验（CLIMODE）收集的大量现场数据集。这一协调的现场工作包括大量剖面浮标、系泊浮标和若干水文测量。为了补充这个独特的数据集，研究人员还将研究当代和历史的水文、Argo剖面浮标、水面漂浮物、卫星测高和海洋颜色场，以及几个漩涡解析OGCM模拟的输出。知识价值：该项目将测试长期以来假设的EDW作为北大西洋浅层翻转环流的一个元素以及作为热量、营养物质和碳的短期储存库的气候重要性。重要的是，这项研究将促进对破坏EDW的各种过程的理解。此外，本研究还将首次对大尺度、低频环流和中尺度涡旋场在EDW扩散和破坏中的作用进行比较和对比。最后，由于模态水域存在于每个大洋盆地，因此本文的工作将为比较研究提供基础。更广泛的影响：澄清EDW在大气和海洋之间的热量交换中的作用，将支持正在进行的社区努力，以提高年际到十年的可预测性。例如，EDW及其相关的海气热通量可能直接影响墨西哥湾流附近北大西洋风暴路径上地表风暴振幅的年际变化。此外，了解EDW在养分储存和交换中的作用将促进我们对初级生产力年际变化的理解。使用独特的CLIMODE数据集进行严格的模型数据比较，也将通过提供对涡旋解析数值模型保真度的客观评估，帮助更广泛的社区。最后，该项目将支持培养两名博士后研究员和一名研究生。该项目是对美国CLIVAR（气候变率和可预测性）计划的贡献。

This project will examine the physical processes associated with the evolution (formation, circulation, and destruction) of Eighteen Degree Water (EDW) within the subtropical gyre of the North Atlantic Ocean. EDW is the archetype for the anomalously thick and vertically homogenous mode waters that are typical of all subtropical western boundary current systems. EDW is associated with a shallow overturning circulation that carries heat northward and is an interannual reservoir of anomalous heat, nutrients and CO2. Understanding the annual cycle of EDW evolution, and in particular its associated circulation pathways and destruction mechanisms, is important because though EDW is isolated beneath the stratified upper-ocean at the end of each winter, it may reemerge in subsequent years to influence mixed layer properties and consequently air-sea interaction and primary productivity.The pathways of EDW circulation, the processes of EDW destruction, and the role of EDW in modulating ocean-atmosphere heat exchange and nutrient supply to the euphotic zone will be investigated. The plan is to synthesize a broad spectrum of observations including a substantial in-situ dataset collected during the recently completed CLIVAR Mode Water Dynamics Experiment (CLIMODE). This coordinated field effort included a large array of profiling floats, a moored array, and several hydrographic surveys. To supplement this unique dataset the investigators will also examine contemporary and historical hydrography, Argo profiling floats, surface drifters, satellite altimetry and ocean color fields, and output from several eddy-resolving OGCM simulations.Intellectual Merit: This project will test the long-hypothesized climatic importance of EDW as an element of the North Atlantic's shallow overturning circulation and as a short-term reservoir of heat, nutrients, and carbon. Significantly, this study will advance understanding of the various processes that destroy EDW. In addition, this study will, for the first time, compare and contrast the roles of the large-scale, low-frequency circulation and the mesoscale eddy field in EDW dispersal and destruction. Finally, the proposed work will provide a foundation for comparative studies since mode waters are found in every ocean basin.Broader Impacts: Clarifying the role of EDW in the exchange of heat between the atmosphere and ocean will support ongoing community efforts to improve interannual-to-decadal predictability. For example, EDW and its associated air-sea heat flux may have a direct impact on the year-to-year variability of surface storm amplitude in the North Atlantic storm track near the Gulf Stream. In addition, understanding the role of EDW in the storage and exchange of nutrients will advance our understanding of interannual variability in primary productivity. Rigorous model-data comparisons using the unique CLIMODE dataset will also aid the broader community by providing an objective assessment of the fidelity of eddy-resolving numerical models. Finally, the project will support the training of two postdoctoral investigators and a graduate student.This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program.