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

由于海洋储存热量和二氧化碳的巨大能力，海洋的翻转环流在地球气候中起着控制作用。围绕着南极洲的南大洋在这种翻转环流中扮演着不成比例的角色，因为这是深水上升到水面与大气交换热量和二氧化碳的主要区域之一。虽然南极环极流（ACC）系统将深水带到表层，但动力约束抑制了经向交换。海洋涡旋被认为在将水向南输送穿过深脊上方的太平洋环流中起着主导作用，并为被强风吹向北方的水提供补给。这种等平环流在多大程度上因密度层之间的混合而“短路”，这对气候模式很重要，但目前尚不清楚。知识价值：全球经向翻转的概念模式和未来气候的数值预测对用来表示沿南极环极流及其上空的混合的方法非常敏感，在南极环极流中等平线是急剧倾斜的。在南大洋都没有系统地测量过深旋和等旋混合。南大洋底流和等流混合实验（DIMES）的目标是测量沿海底密度面（等流混合）和跨密度层（底流混合）的涡旋混合，并确定这些过程如何依赖于更大尺度的海洋动力学，以便它们能够在海洋环流和气候的数值模式中得到适当的表示。为了揭示在ACC中工作的这些过程，一种化学示踪剂和75个漂浮物将在2008年初在ACC附近1300米深，60 S和110 W的地方释放。同时将释放用于测量示踪云内部和上方精细结构T、S和速度的浮子。漂浮物和示踪剂将由ACC携带在东南太平洋相对平坦的底部，随着漂浮物和示踪剂的移动，漂浮物和示踪剂将在洋流中扩散。一年后，示踪器的前沿将开始越过德雷克海峡的山脊进入斯科舍海。另外75个等平浮标将于此时在示踪补丁的中心附近释放。用声源阵列声学测量浮体的轨迹，将用于研究和测量等轴色散。示踪剂的扩散将给出等旋散和异旋散的综合测量。涡旋场及其垂直结构将通过卫星高度计测量的海面高度，以及从研究船和随示踪剂漂移的自主仪器获取的水文剖面图进行研究。利用舰载自由落体剖面仪测量湍流耗散，以研究混合的空间和时间尺度，并检查疑似的混合热点。将使用自由落体剖面仪和随示踪剂漂移的特殊浮子以及在表面和示踪剂层之间的浮子来测量驱动这种混合的剪切力。更广泛的影响：DIMES将部署各种仪器，包括微结构和精细结构分析器，以及等斜面跟踪自主浮筒，其中一些是首次在南大洋进行。混合结果将用于帮助改进气候模式中混合的表示。此外，对DIMES浮标的分析将增加Argo对南大洋的数据库。该项目将包括一名博士后研究员、佛罗里达州立大学和斯克里普斯海洋学研究所的研究生，每年将为一到两名本科生提供研究机会。该项目是对美国CLIVAR（气候变率和可预测性）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.