Extending Abyssal Mixing Observations and Parameterizations

扩展深渊混合观测和参数化

• 批准号: 0961262
• 负责人: Douglas Luther
• 金额: $ 20.66万
• 依托单位: University of Hawaii
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-04-15 至 2013-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0961262&HistoricalAwards=false
• 关键词: Extending; Abyssal; Mixing; Observations; Parameterizations

Intellectual Merit: Observational studies suggest the existence of only weak diapycnal mixing in the ocean interior, with orders of magnitude more mixing near the bottom in regions of rough topography. In contrast, numerical circulation and climate models generally employ spatially uniform parameterizations of abyssal diapycnal mixing. However, several recent numerical studies demonstrate the sensitivity of the large scale abyssal ocean circulation to the spatial distribution of abyssal diapycnal mixing, leading modelers to recognize that mixing parameterizations mimicking the observed spatial variability are required to improve simulations of the ocean circulation.As is typical in oceanography, a major problem hampering both the development and validation of improved abyssal mixing parameterizations is the scarcity of observations. Microstructure observations, the most direct and accurate tool with which to infer diapycnal mixing, have only been accomplished in a few select regions of the global ocean and will unfortunately not become a routine measurement in the near future due to their cost and the necessity for very high levels of quality control. Consequently, much work has focused on ways to infer diapycnal mixing indirectly from standard hydrographic data, potentially yielding a much more densely sampled global map of diapycnal mixing. A now common approach is the application of fine-scale parameterizations, relating turbulent dissipation to shear and/or strain variance on scales of order 10 meters, based on nonlinear internal wave-wave interaction theory. However, it is becoming increasingly clear that fine-scale parameterizations are reliable predictors only for a very limited range of conditions. The method is known to break down in special environments (e.g., the coastal slope, canyons) and to be a poor predictor of diapycnal mixing in weakly stratified water. The stratification limitation is severe since both the global abyssal ocean (depth 1000 m) and the Southern Ocean fall in that category. The abyssal and Southern Ocean are important branches of the Meridional Overturning Circulation and thus the credibility of climate modeling studies depends on accurate representation of diapycnal mixing in these ocean volumes. The World Ocean Circulation Experiment (WOCE) hydrographic data will be re-analyze using Thorpe scale analysis. Thorpe scale analysis has been shown to be a more robust predictor of dissipation and diapycnal mixing than fine-scale parameterizations, in particular in weak stratification and regions where the characteristics of the internal wave field deviate from the canonical Garrett-Munk model, yet has not been applied extensively to the WOCE hydrographic data set. The expected results from this work are an improved observational knowledge of the spatial distribution of mixing in the abyssal ocean. We also propose a comprehensive comparison and validation of existing abyssal mixing parameterizations intended for use in general circulation and climate models. Broader Impacts: The funds requested in this proposal are intended for the support of a postdoctoral investigator. The results from this project will be disseminated in the refereed literature, made available online and presented at appropriate scientific meetings. The spatial variability of mixing significantly affects the abyssal circulation and stratification, the strength and depth of the Antarctic Circumpolar Current as well as various aspects of the meridional overturning circulation (MOC). In turn, these relate to the ability of the ocean to store and transport heat and greenhouse gasses, and thus the response of the climate system to anthropogenic and natural forcing. This project is expected to result in a better observational knowledge of the spatial distribution of mixing than achieved to date, and to contribute to the improvement of diapycnal mixing parameterizations intended for ocean general circulation models and climate models.

智力优势：观测研究表明，海洋内部只存在微弱的昼夜混合，而在地形崎岖的地区，海底附近存在更多数量级的混合。相比之下，数值环流和气候模式通常采用空间上统一的深海昼夜混合参数。然而，最近的一些数值研究表明，大尺度深海海洋环流对深海昼夜混合的空间分布非常敏感，这使得模型者认识到，需要模拟观测到的空间变异性的混合参数来改进海洋环流的模拟。微结构观测是推断昼夜混合的最直接和最准确的工具，仅在全球海洋的少数选定区域完成，不幸的是，由于其成本和对非常高水平的质量控制的必要性，在不久的将来不会成为常规测量。因此，许多工作都集中在从标准水文数据间接推断昼夜混合的方法上，潜在地产生了采样更密集的昼夜混合的全球地图。目前一种常用的方法是应用精细尺度的参数化法，基于非线性内波-波相互作用理论，将湍流耗散与10米量级的剪切和/或应变变化联系起来。然而，越来越清楚的是，细尺度参数化仅对非常有限的条件范围是可靠的预测。众所周知，该方法在特殊环境(如海岸斜坡、峡谷)中会分解，并且不能很好地预测弱分层水中的昼夜混合。分层限制是严重的，因为全球深海(深度1000米)和南大洋都属于这一类别。深海和南大洋是经向翻转环流的重要分支，因此气候模拟研究的可信度取决于这些洋量中昼夜混合的准确表示。世界海洋环流实验(WOCE)的水文数据将使用索普尺度分析进行重新分析。Thorpe尺度分析已被证明是一个比细尺度参数化更可靠的耗散和日混合预报因子，特别是在弱层结和内部波场特征偏离正则Garrett-Munk模式的地区，但尚未广泛应用于WOCE水文数据集。这项工作的预期结果是改进了对深海混合的空间分布的观测知识。我们还建议对现有的用于大气环流和气候模式的深海混合参数进行全面的比较和验证。更广泛的影响：本提案要求的资金旨在支持一名博士后研究人员。该项目的成果将在参考文献中传播，在网上提供，并在适当的科学会议上介绍。混合的空间变异性对深海环流和层结、南极绕极流的强度和深度以及经向翻转环流(MOC)的各个方面都有显著的影响。反过来，这些与海洋储存和运输热量和温室气体的能力有关，从而与气候系统对人为和自然强迫的反应有关。这一项目预计将使人们对混合的空间分布有更好的观测了解，并有助于改进海洋环流模式和气候模式的昼夜混合参数。