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南极绕极流（ACC）向东流动，该绕极流环绕南大洋的地球仪，连接各个海洋盆地。由于其陡峭的倾斜密度表面，从很深的地方向上涌到海面，南大洋的翻转环流与同样活跃的北大西洋/北欧海洋翻转截然不同。在海面附近，涌上来的深层沃茨分裂成沃茨和沃茨，前者变得更轻（浮力更大），最终循环进入亚热带温跃层（上部单元），后者变得更稠密，供给全球底层沃茨（下部单元）。南大洋的翻转环流是全球热量、淡水、碳和营养物重新分配的主要原因。在过去的50年里，南大洋的变暖比北方半球要弱，这可能是因为德雷克海峡限制了向南的海洋热量通量，因此保持了寒冷的、被冰覆盖的沃茨。然而，由于其非常大的体积，南大洋正在吸收气候系统的很大一部分？南半球的海洋中储存着大量的热量，约60%的海洋人为二氧化碳总量储存在南半球的海洋中。来自南大洋深层沃茨上升流的营养物质通过上层单元进入温跃层，在30 ° S以北，营养物质维持75%的初级生产力。 智力优势：纬向平均的南大洋翻转环流通常假设有一个上单元和一个下单元，由流入的印度，太平洋和大西洋深水沃茨，上升到海洋表面，表面浮力通量将它们分别转换为更轻和更密集的沃茨。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。观测结果表明，上层环流很可能是由来自印度洋和太平洋而不是大西洋的富营养深层沃茨补给的，这一假设将被详细检验。第二个假设是，二维的纬向平均纬向翻转环流隐藏了重要的纬向不对称性，这对环流是必不可少的。使用剩余环流框架，建议在中性密度坐标中对等密度线和纵贯线体积运输进行新的在线诊断，调查人员将定量研究区域对南大洋状态估计（SOSE）中剩余环流的贡献，包括ACC的作用，地形特征以及亚热带和南极环流系统。他们还将量化涡动和稳定流在三维剩余环流路径中的相对贡献。上层单体中的水团转化和形成过程也是三维的。这一假设，即海-气通量占主导地位，几乎同样重要的淡水和热量，但diapycnal混合，特别是在等密度露头地区，也可以是重要的将被测试。使用建议的新的在线SOSE诊断，相对的，本地化的贡献的热量和盐度迫使转换将量化在每个模型的时间步长。在区域范围内，利用SOSE和这些新的诊断方法，研究人员将检查导致一致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单元基本淡水输入中的作用。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，并在重要会议上介绍。该项目将加强现有的公共外联工作（学校、教师团体、图书馆、大学俱乐部），包括视觉化。建议开发的3D残余（即等密度平均）循环的在线诊断将通过纳入所有非绝热过程对纵摇速度的贡献而使MITgcm用户和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。国家估计数日益成为综合数据的首选工具。该项目支持SOSE快速增长的用户群，包括许多学生。SOSE将是一个广泛使用的工具，了解南大洋动力学，热力学和海洋地球化学在未来几年，随着海冰下的原位观测和海洋地球化学参数的数量飙升。将对其水团结构和海气通量进行重要核查。一名研究生将接受指导。

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.