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

概述：南大洋在全球环流和气候中起着举足轻重的作用。德雷克海峡纬度带没有陆地屏障，这使得西风有可能驱动强大的东南极环极流（ACC），它在南大洋环绕地球，连接各个海洋盆地。由于南大洋的密度面是陡峭倾斜的，并且有从深海到海面的上升流，因此南大洋的翻转环流与同样剧烈的北大西洋/北欧海的翻转环流截然不同。在海面附近，上升流的深水分裂成更轻的水（浮力更强），并最终循环进入亚热带温跃层（上层），以及密度更大的水，并为全球底层水（下层）提供水源。在全球热量、淡水、碳和营养物质的再分配中，很大一部分是由南大洋的翻转环流造成的。在过去的50年里，南大洋的变暖比北半球要弱，这可能是因为德雷克海峡限制了向南的海洋热通量，因此保持了寒冷、被冰覆盖的水域。然而，由于其体积非常大，南大洋吸收了气候系统的很大一部分。地球的多余热量和大约60%的海洋人为二氧化碳储存在南半球的海洋中。来自南大洋深水上涌的营养物质通过上层细胞进入温跃层，在那里它们支持南纬30°以北75%的初级生产力。理论价值：纬向平均的南大洋翻转环流通常被假设为有一个上层环流和一个下层环流，由流入的印度、太平洋和大西洋深水提供，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。这些细胞的分离程度（或不分离）将作为这项工作的一部分进行检查。观察表明，上层细胞最有可能是由源自印度洋和太平洋而非大西洋的营养丰富的深水滋养的，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掩盖了对环流至关重要的纬向不对称性。利用剩余环流框架和中性密度坐标下提出的新的等环流和准环流体积输送在线诊断，研究人员将定量研究南大洋状态估计（SOSE）中区域对剩余环流的贡献，包括ACC的作用、地形特征、副热带和南极环流系统。他们还将量化三维剩余环流路径中涡流驱动和稳定流动的相对贡献。上层池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，淡水和热量几乎同等重要，但底旋混合，特别是在等旋露头区域，也可能很重要的假设将得到检验。利用提出的新的在线sse诊断方法，将在每个模式时间步长量化热量和盐度强迫对转化的相对、局部贡献。在区域范围内，通过SOSE和这些新的诊断方法，研究人员将检查导致连贯净加热区域的过程的平衡，找到最重要的上升流/海气交换点，并量化海冰过程在上层单元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响应，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。现有的公共宣传工作（学校、教师团体、图书馆、大学俱乐部）将通过该项目得到加强，包括可视化。建议开发的3D残余（即等环流平均）环流在线诊断将使MITgcm用户和SOSE用户受益，因为它包含了所有绝热过程对慢环流速度的贡献。实现在线中性密度计算的代码将提供给所有MITgcm用户。各州估算日益成为综合数据的首选工具。该项目支持快速增长的sse用户群，包括许多学生。随着海冰下的原位观测和生物地球化学参数的激增，sse将在未来几年成为了解南大洋动力学、热力学和生物地球化学的广泛工具。将对其水团结构和海气通量进行关键的核查。研究生将接受指导。

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.