Collaborative Research: DIMES, Diapycnal and Isopycnal Mixing in the Southern Ocean

海洋颠覆的环流在地球气候中起着主导作用，因为海洋具有容纳热量和二氧化碳的巨大能力。环绕南极洲的南大洋在这种颠覆的环流中扮演着不成比例的角色，因为这是深水上升到表面与大气交换热量和二氧化碳的主要区域之一。虽然南极绕极流(ACC)系统给海面带来了深水，但动力限制阻碍了经向交换。据信，海洋涡旋在将水向南输送穿过ACC深脊上方的过程中发挥了主导作用，为被强风驱使向北的水提供了食物。这种等轴线环流通过密度层的混合而被“短路”的程度对气候模型很重要，但尚不清楚。智慧的优点：全球经圈翻转的概念模型和对未来气候的数值预测对用于表示沿和跨南极绕极流(ACC)的混合的方法非常敏感，在那里等轴线急剧倾斜。在南大洋，既没有系统地测量到日周期混合，也没有系统地测量到等周期混合。南大洋双周期和等周期混合实验的目标是测量沿次表层海洋密度面的涡旋混合(等周期混合)和跨越这些密度层的涡旋混合(日周期混合)，并确定这些过程如何依赖于更大尺度的海洋动态，以便能够在海洋环流和气候的数值模式中适当地表示。为了揭示这些过程在ACC中的作用，一种化学示踪剂和75个沿着等径面跟随水的浮标将于2008年初在ACC中1300米深、60 S和110W附近释放。测量细结构T、S和示踪云内部及上方速度的浮标将同时释放。浮标和示踪剂将由ACC携带到东南太平洋相对平坦的海底上空，在漂流过程中沿洋流扩散。一年后，示踪剂的前沿将开始越过Drake Passage的山脊进入斯科舍海。此时，示踪剂补丁中心附近将释放另外75个等密度浮标。用一组声源声学测量的漂浮物的轨迹将用于研究和测量等周期弥散。示踪剂的扩散将给出等膜和透膜弥散的综合测量。将利用卫星高度计测量的海面高度，以及从考察船和使用示踪剂漂移的自主仪器获取的水文剖面，研究涡旋场及其垂直结构。用船载自由落体廓线仪测量湍流耗散，以研究混合的空间和时间尺度，并检查可疑的混合热点。剪切驱动这种混合将用自由落体剖面仪和随示踪剂漂移的特殊漂浮物测量，以及在表层和示踪层之间的漂浮物测量。广泛影响：DIMES将部署各种仪器，包括显微结构和精细结构剖面仪以及等径线跟踪的自主漂浮物，其中一些是首次在南大洋使用。混合结果将用于帮助改进气候模型中混合的表示。此外，对DIMES浮标进行剖面分析将扩大南大洋的ARGO数据库。该项目将涉及一名博士后研究员、佛罗里达州立大学和斯克里普斯海洋研究所的研究生，并将每年为一至两名本科生提供研究机会。该项目是对美国气候可变性和可预测性(CLIVAR)计划的贡献。

The overturning circulation of the ocean plays a governing role in the earth's climate because of the enormous capacity of the ocean to hold heat and carbon dioxide. The Southern Ocean, which surrounds Antarctica, plays a disproportionate role in this overturning circulation because this is one of the main areas where deep waters rise to the surface to exchange heat and carbon dioxide with the atmosphere. Although the Antarctic Circumpolar Current (ACC) system brings deep water to the surface, dynamical constraints inhibit meridional exchanges. Ocean eddies are believed to play a dominant role in transporting water south across the ACC above deep ridges, feeding water driven northward by the intense winds. The extent to which this Isopycnal circulation is "short-circuited" by mixing across density layers is important to climate models but is unknown.Intellectual Merit: Conceptual models of global meridional overturning and numerical predictions for future climate are strongly sensitive to the methods used to represent mixingalong and across the Antarctic Circumpolar Current (ACC), where isopycnals are steeply tilted. Neither diapycnal nor isopycnal mixing has been measured in the Southern Ocean in a systematic way. The goals of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) are to measure eddy mixing along density surfaces in the subsurface ocean (isopycnal mixing), and across those density layers (diapycnal mixing), and to determine how those processes depend on the larger scale dynamics of the ocean, so that they can be properly represented in numerical models of ocean circulation and of climate. To reveal these processes at work in the ACC, a chemical tracer and 75 floats that follow the water along isopycnal surfaces will be released in the ACC near 1300 m depth, 60 S, and 110 W, early in 2008. Floats that measure fine-structure T, S, and velocity within and above the tracer cloud will be released at the same time. The floats and tracer will be carried by the ACC over the relatively smooth bottom of the SE Pacific, spreading both across and along the current as they travel. After a year, the leading edge of the tracer will just start to pass over the ridges of Drake Passage into the Scotia Sea. Another 75 isopycnal floats will be released near the center of the tracer patch at this time. Trajectories of the floats, measured acoustically with an array of sound sources, will be used to study and to measure isopycnal dispersion. Spreading of the tracer will give integrated measures of both isopycnal and diapycnal dispersion. The eddy field, and its vertical structure, will be studied with sea surface height measured by satellite altimeters, and with hydrographic profiles taken from research vessels and from autonomous instrumentsdrifting with the tracer. Turbulent dissipation, from which diapycnal mixing can be estimated, will be measured with ship-based free-falling profilers to study the spatial and temporal scales of the mixing and to examine suspected hot spots of mixing. Shear driving this mixing will be measured with the free-falling profilers and with special floats drifting with the tracer and floats that profile between the surface and the tracer layer.Broader Impact: DIMES will deploy a variety of instruments including microstructure and finestructure profilers and and isopycnal-following autonomous floats, some for the first time in Southern Ocean. The mixing results will be made available to aid in improving representations of mixing in climate models. In addition, profiling DIMES floats will augment the Argo database for the Southern Ocean. The project will involve a postdoctoral investigator, graduate students at Florida State University and Scripps Institution of Oceanography and will offer research opportunities to one to two undergraduates per year. This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program.