Collaborative Research: The upper branch of the Southern Ocean overturning in the Southern Ocean State Estimate: water mass transformation and the 3-D residual circulation

合作研究：南大洋上支翻转 南大洋状态估计：水团转化和 3-D 剩余环流

• 批准号: 1357072
• 负责人: Lynne Talley
• 金额: $ 81.33万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-02-01 至 2018-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1357072&HistoricalAwards=false
• 关键词: Collaborative; Research; upper; branch; Southern

Overview: The Southern Ocean plays a pivotal role in the global circulation and climate. The absence of land barriers in the Drake Passage latitude band makes it possible for westerly winds to drive the strong eastward Antarctic Circumpolar Current (ACC) that encircles the globe in the Southern Ocean, connecting the individual ocean basins. Due to its steeply sloping density surfaces with upwelling from great depths to the sea surface, the Southern Ocean's overturning circulation is extremely different from the equally vigorous North Atlantic/Nordic Seas overturn. Near the sea surface, the upwelled deep waters split into waters that become lighter (more buoyant) and eventually circulate into the subtropical thermocline (upper cell), and waters that become denser and feed the global bottom waters (lower cell). The Southern Ocean's overturning circulation is responsible for a large portion of the global redistribution of heat, freshwater, carbon and nutrients. Warming in the Southern Ocean over the past 50 years is weaker than in the Northern Hemisphere, possibly because Drake Passage limits southward oceanic flux of heat, hence maintaining cold, ice-covered waters. However, because of its very large volume, the Southern Ocean is absorbing a significant fraction of the climate system?s excess heat and about 60% of the total oceanic anthropogenic carbon dioxide inventory is stored in the Southern Hemisphere oceans. Nutrients from the upwelling deep waters in the Southern Ocean enter the thermocline via the upper cell, where they support 75% of primary productivity north of 30°S. Intellectual Merit : The zonally averaged Southern Ocean overturning circulation is commonly hypothesized to have an upper cell and a lower cell, fed by inflowing Indian, Pacific and Atlantic Deep Waters, that upwell to the ocean surface where surface buoyancy fluxes convert them to lighter and denser waters, respectively. The degree of separation (or not) of these cells will be examined as part of this work. Observations indicate that the upper cell is most likely fed by nutrient-rich deep waters that originate in the Indian and Pacific rather than from the Atlantic, a hypothesis that will be examined in detail.A second hypothesis is that the two-dimensional, zonally averaged meridional overturning circulation hides significant zonal asymmetries that are essential to the circulation. Using a residual circulation framework with proposed new online diagnostics of isopycnal and diapycnal volume transport in neutral density coordinates, the investigators will quantitatively examine regional contributions to the residual circulation in the Southern Ocean State Estimate (SOSE), including the role of the ACC, topographic features, and subtropical and Antarctic gyre systems. They will also quantify the relative contributions of eddy-driven and steady flow in the three-dimensional residual circulation pathways.Water mass transformation and formation processes in the upper cell are also three-dimensional. The hypothesis that air-sea fluxes dominate with nearly equal importance of freshwater and heat, but that diapycnal mixing, particularly in isopycnal outcrop regions, can also be important will be tested. Using the proposed new online SOSE diagnostics, the relative, localized contributions of heat and salinity forcing to transformation will be quantified at every model time step. Regionally, with SOSE and these new diagnostics, the investigators will examine the balance of processes that lead to coherent net heating regions, find the most important upwelling/air-sea exchange sites, and quantify the role of sea ice processes in the essential freshwater inputs to the upper cell.Broader Impacts: The project will inform understanding of Southern Ocean response to climate change, including changes in surface temperature, upper ocean heat content and sea ice cover. The results will be published in scientific journals and presented at major meetings. Existing public outreach efforts (schools, teacher groups, libraries, university clubs) will be enhanced by the project, including visualizations. The proposed development of online diagnosis of the 3D residual (i.e. isopycnally-averaged) circulation will benefit MITgcm users as well as SOSE users through the inclusion of the contributions to the diapycnal velocity from all diabatic processes. The code implementing online neutral density calculation will be made available to all MITgcm users. State estimates are increasingly a tool of choice for synthesizing data. This project supports the rapidly growing user base for SOSE, including many students. SOSE will be a broadly used tool for understanding Southern Ocean dynamics, thermodynamics, and biogeochemistry for years to come, as the numbers of in situ observations under sea ice and of biogeochemical parameters soar. Crucial verification of its water mass structure and air-sea fluxes will be undertaken. A graduate student will be mentored.

概述：南大洋在全球环流和气候中发挥着举足轻重的作用。德雷克海峡纬度带没有陆地屏障，使得西风有可能驱动强大的南极绕极流（ACC）东移，在南大洋环绕地球，连接各个洋盆。由于其陡峭倾斜的密度表面以及从深处到海面的上升流，南大洋的翻转环流与同样剧烈的北大西洋/北欧海翻转极为不同。在海面附近，上升的深水分裂成变得更轻（更有浮力）并最终循环到副热带温跃层（上单元）的水，以及变得更稠密并供给全球底层水域（下单元）的水。南大洋的翻转环流造成了全球热量、淡水、碳和营养物重新分配的很大一部分。过去 50 年南大洋的变暖程度弱于北半球，可能是因为德雷克海峡限制了向南的海洋热量流动，从而维持了寒冷、冰雪覆盖的水域。然而，由于其体积非常大，南大洋正在吸收气候系统的大部分多余热量，并且约 60% 的海洋人为二氧化碳库存总量储存在南半球海洋中。来自南大洋深水上升流的营养物质通过上层细胞进入温跃层，支持南纬 30°以北 75% 的初级生产力。 智力优点：纬向平均的南大洋翻转环流通常被假设有一个上部单元和一个下部单元，由流入的印度、太平洋和大西洋深水供给，上升到海洋表面，在那里表面浮力通量将它们分别转化为较轻和较稠密的水域。作为这项工作的一部分，将检查这些细胞的分离程度（或不分离程度）。观察表明，上层细胞很可能是由源自印度洋和太平洋而不是大西洋的营养丰富的深水供给的，这一假设将被详细检验。第二个假设是，二维纬向平均经向翻转环流隐藏了对环流至关重要的显着纬向不对称性。利用残余环流框架和提出的中性密度坐标中等密度和二密度体积输送的新在线诊断，研究人员将定量研究南大洋状态估计（SOSE）中残余环流的区域贡献，包括ACC的作用、地形特征以及亚热带和南极环流系统。他们还将量化三维残余循环路径中涡流驱动和稳定流的相对贡献。上部单元中的水团转化和形成过程也是三维的。空气-海洋通量占主导地位的假设与淡水和热量几乎同等重要，但双重混合，特别是在等重露头区域，也很重要，将得到检验。使用拟议的新在线 SOSE 诊断，将在每个模型时间步长量化热量和盐度强迫对转化的相对局部贡献。在区域上，利用 SOSE 和这些新的诊断方法，研究人员将检查导致相干净加热区域的过程的平衡，找到最重要的上升流/海气交换地点，并量化海冰过程在上层细胞的基本淡水输入中的作用。更广泛的影响：该项目将有助于了解南大洋对气候变化的反应，包括表面温度、上层海洋热含量和海冰覆盖的变化。研究结果将发表在科学期刊上并在重要会议上发表。该项目将加强现有的公共宣传工作（学校、教师团体、图书馆、大学俱乐部），包括可视化。所提议的 3D 残余（即等密度平均）环流在线诊断开发将通过包含所有非绝热过程对径流速度的贡献而使 MITgcm 用户以及 SOSE 用户受益。实现在线中性密度计算的代码将提供给所有 MITgcm 用户。状态估计越来越成为综合数据的首选工具。该项目为 SOSE 快速增长的用户群提供支持，其中包括许多学生。随着海冰下的现场观测和生物地球化学参数的数量激增，SOSE 将成为未来几年了解南大洋动力学、热力学和生物地球化学的广泛使用的工具。将对其水团结构和海气通量进行关键验证。将指导一名研究生。

项目成果

Episodic Southern Ocean Heat Loss and Its Mixed Layer Impacts Revealed by the Farthest South Multiyear Surface Flux Mooring

• DOI: 10.1029/2017gl076909
• 发表时间: 2018-05-28
• 期刊: GEOPHYSICAL RESEARCH LETTERS
• 影响因子: 5.2
• 作者: Ogle, S. E.;Tamsitt, V.;Weller, R. A.
• 通讯作者: Weller, R. A.