Subpolar-Subtropical Connectivity of the North Atlantic Circulation

海洋通过其巨大的储热能力，在地球气候和气候变异性中发挥着关键作用。自1955年以来，全球海洋变暖约占地球系统变暖的93%。然而，这种升温在空间上既不是均匀的，在时间上也不是恒定的。在全球长期趋势上叠加的是年际到年代际时间尺度和区域到流域尺度上的气候变化。卫星高度计和水文观测表明，自1990年代初以来，北大西洋，包括亚极地地区，迅速变暖和变咸。一个新的图景是，北大西洋最近20年左右的变暖在一定程度上代表了大西洋数十年变化模式从冷阶段向暖阶段的转变。这些十年气候转变既涉及北大西洋副热带和亚极地涡旋的横向变化，也涉及大西洋子午线翻转环流(AMOC)的垂直变化，大西洋子午线翻转环流是全球热量和淡水循环系统的关键组成部分。这项对北大西洋环流的研究集中在纽芬兰大浅滩周围的一个过渡区，在那里边界流和急流、再环流和中尺度涡旋(长度尺度通常小于100公里)的影响占主导地位。强烈的相互作用发生在这个过渡区，横向上发生在副极地和副热带涡旋之间，垂直方向上发生在大西洋子午线环流(AMOC)的冷分支和暖分支之间。有证据表明，这一相对紧凑的区域在更大范围内改变甚至调节AMOC方面发挥了关键作用，因此对大西洋的长期、年代际变化很重要。然而，尽管有许多观测现场计划，但对该地区的动态和影响还没有很好的了解。该项目将通过解决副极地和副热带环流的连通性问题，有助于了解大气环流的可变性。这种模型-数据合成的结果将对耦合气候模型具有特别重要的意义，而耦合气候模型是理解和预测全球气候变化的核心。该项目的教育/推广部分将侧重于通过教育工作者的各种在线资源/互动工具、佛罗里达州立大学高中生青年学者计划和“学校中的科学家”计划，在K-12学校、大学一级和当地社区培养与海洋气候研究有关的科学素养。最后，申请的资金将用于支持一名初级教员和一名研究生，他们将接受海洋建模、数据分析和解释方面的培训。通过在亚极地和亚热带北大西洋正在进行的主要观测计划，海洋学家们正在朝着更好地了解这些次盆地内大气层有机化合物的结构和变异性方面取得长足进步。这里提出的工作是对这些观测的补充，重点是关于什么控制之间的环流以及亚极地到亚热带的连通性在多大程度上调制更大尺度的AMOC的关键问题。该项目旨在阐明控制过渡区环流的物理动力，特别是涡旋和西部深层边界流的相对重要性，并记录过渡区在更大尺度环流中的作用和影响，特别是亚极区和副热带北大西洋大气环流和水性质从年际到年代际和更长时间尺度的变化。涡旋和时间平均环流的相互作用是预测全球气候变率的最大挑战之一，可以用本项目中包含的精细网格分辨率模式进行研究。这些目标将通过开展详细的模式数据综合研究来实现，该研究将使用混合坐标海洋模式(HYCOM)的一套高分辨率大西洋模拟的数值结果与现有观测(卫星测高、漂浮物/浮游物、水文学、示踪器和系泊阵列)相结合。三维大西洋环流将通过对水质量传输和转化、被动示踪剂以及位涡和动量通量的分析来量化。结果表明，涡旋分辨率HYCOM代表了过渡区和大尺度北大西洋的基本环流特征，既包括时间平均结构，也包括时间变率。

The ocean, through its large capacity to store heat, plays a critical role in Earth's climate and climate variability. Warming of the world's oceans since 1955 accounts for approximately 93% of the warming of the Earth system. However, this warming is neither spatially uniform nor temporally constant. Superimposed on the global long-term trend is climate variability on inter-annual to inter-decadal time scales and regional to basin scales. Satellite altimeters and hydrographic observations show that the North Atlantic, including the sub-polar region, has rapidly become warmer and saltier since the early 1990s. An emerging picture is that the most recent 20 years or so of warming in the North Atlantic represents, in part, a transition of the Atlantic multi-decadal variability pattern from a cold to a warm phase. These decadal climate transitions involve changes both laterally in the sub-tropical and sub-polar gyres of the North Atlantic and vertically in the Atlantic Meridional Overturning Circulation (AMOC), a key component of the global heat and freshwater circulation system. This study of the North Atlantic circulation concentrates on a transition region around the Grand Banks of Newfoundland, where the effects of boundary currents and jets, recirculations, and mesoscale eddies (length scales typically less than 100 km) are dominant. Strong interactions occur in this transition region, laterally between the subpolar and subtropical gyres and vertically between the cold and warm limbs of the Atlantic Meridional Circulation (AMOC). There is evidence that this relatively compact region plays a key role in altering and even modulating the AMOC over a much larger scale and thus is important for the long-term, decadal variability of the Atlantic Ocean. Yet, despite many observational field programs, the dynamics and impacts of this region are not well understood. The project will contribute to understanding the variability of the AMOC by addressing the connectivity of the sub-polar and the sub-tropical gyres. The results of this model-data synthesis will be of particular significance to coupled climate models, which are central to understanding and predicting global climate change. The educational/outreach components of this project will be focused on cultivating scientific literacy with regards to ocean climate research in K-12 schools, at the university level, and in the local community through a variety of online resources/interactive tools for educators, the Florida State University Young Scholars program for high school students, and the "Scientists in the Schools" program. Finally, the requested funding will support a junior faculty member and a graduate student who will be trained in ocean modeling, data analysis and interpretation.Through ongoing major observation programs in the sub-polar and sub-tropical North Atlantic Ocean, oceanographers are making great strides toward a better understanding of the structure and variability of the AMOC within these sub-basins. The work proposed here complements these observations by focusing on key questions pertaining to what controls the circulation in between and how much the sub-polar to sub-tropical connectivity modulates the larger scale AMOC. This project aims to elucidate the physical dynamics that controls circulation in the transition region, especially the relative importance of the eddies and the deep western boundary current, and document the role and impact of the transition region on the larger scale circulation, especially the variability of the AMOC and water properties in the sub-polar and sub-tropical North Atlantic from inter-annual to decadal and longer time scales. The interaction of eddies and time mean circulations represents one of the greatest challenges to prediction of global climate variability, and it can be studied with the fine-grid resolution model included in this project. These objectives will be met by performing a detailed model-data synthesis study, combining numerical results from a suite of high-resolution Atlantic simulations using the HYbrid Coordinate Ocean Model (HYCOM) and existing observations (satellite altimetry, drifters/floats, hydrography, tracers, and mooring arrays). The three-dimensional Atlantic circulation will be quantified by performing analysis of water mass transport and transformation, passive tracers, and potential vorticity and momentum fluxes. It has been demonstrated that the eddy-resolving HYCOM represents the basic circulation features in the transition region and larger scale North Atlantic Ocean, including both time mean structure and temporal variability.