Understanding the essential characteristics of surface ocean boundary layer turbulence in frontal zones

本研究的动机是观察到海洋混合层锋是整个世界海洋的共同特征，目前在边界层表示中被忽略。与锋面不稳定有关的环流能够将物质从表面输送到海洋内部，可能是海洋环流的关键因素。这项研究将产生这些输送过程的定量估计，可用于改进气候研究中使用的海洋模式的参数化。计划在项目期间由研究生参与，研究结果将发表在公认的同行评审期刊上。这项调查的内容将被纳入俄勒冈州立大学的研究生课程，并通过一个涉及高中生的暑期实习项目纳入公共学习努力。本研究的重点是紊流海洋边界层中相干结构的性质和动力学，以及支持其发展的条件。这项工作将扩展和补充以前的研究，这些研究已经检查或援引朗缪尔或对称不稳定过程来解释海洋边界层观测。据推测，在初步的大涡模拟数值实验中观察到的相干大涡结构可能最好地被理解为一种基本状态的非对称线性不稳定性，该基本状态包含地转平衡水平密度梯度的表面应力和浮力通量修正。这项工作的第一个目标将是专门解决这一假设。第二个更广泛的目标是，以第一部分的结果为背景，利用大涡模拟来探索和表征海洋边界层湍流过程，特别是与相干结构相关的垂直通量和混合，对平均水平梯度的强度和特征的依赖。将进行一系列大涡模拟，以评估锋面梯度和地表强迫对相干结构发展和特征的相对影响。研究的具体强迫参数将包括表面应力和浮力（热）通量、表面波（朗缪尔环流）和锋面（水平密度或温度梯度）强度，包括适当的推导出的Ekman浮力通量作为标度参数。数值模拟将以广义基本状态线性稳定性分析形式的理论研究作为补充，这将为海洋边界层不稳定性的表征提供一个概念和数量框架。具体目标包括：1)发展广义线性不稳定性理论，用于与大涡模拟和初始假设的测试进行比较；2)利用大涡模拟研究湍流结构和通量标度机制对地表通量和平均水平梯度的依赖性；3)用广义线性不稳定性理论的结果对大涡模拟机制进行物理解释和合理化。

This study is motivated on the observation that ocean mixed layer fronts are common features throughout the world ocean and are currently ignored in boundary layer representations. Circulations associated with frontal instabilities are capable of transporting material from the surface into the ocean interior and are likely a key element in the ocean general circulation. This research will yield quantitative estimates of these transport processes that can be used to improve parameterizations for ocean models used in climate studies. Participation by a graduate student for the duration of the project is planned and results will be published in recognized peer-reviewed journals. Elements of this investigation will be incorporated into Oregon State University graduate curriculum as well as in public learning efforts through a summer internship program involving high school students.This research focuses on the properties and dynamics of coherent structures in the turbulent ocean boundary layer and on the conditions that support their development. This work will extend and complement previous studies that have examined or invoked Langmuir or symmetric instability processes to explain ocean boundary layer observations. It is hypothesized that the coherent, large-eddy structures observed in preliminary large-eddy simulation numerical experiments may be best understood as non-symmetric linear instabilities of a basic state that incorporates surface stress and buoyancy flux modification of a geostrophically balanced, horizontal density gradient. The first goal of this work will be specifically to address this hypothesis. The second, broader goal is, with the results of the first part as context, to use the large-eddy simulation to explore and characterize the dependence of ocean boundary layer turbulent processes, including especially the vertical fluxes and mixing associated with coherent structures, on the strength and character of mean horizontal gradients. A range of large-eddy simulations will be conducted to assess the relative influence of frontal gradients and surface forcing on coherent structure development and characteristics. Specific forcing parameters examined will include surface stress and buoyancy (heat) flux, surface waves (Langmuir circulation), and frontal (horizontal density or temperature gradient) strength, including where appropriate the derived Ekman buoyancy flux as a scaling parameter. The numerical simulations will be complemented by theoretical studies in the form of linear stability analysis for a generalized basic state that will provide a conceptual and quantitative framework for characterization of instabilities in the ocean boundary layer. Specific objectives include: 1) Development of a generalized linear instability theory for comparison with large-eddy simulations and testing of the initial hypothesis; 2) Characterization of the dependence of turbulent structures and flux scaling regimes on the imposed surface fluxes and mean horizontal gradient, using large-eddy simulations; 3) Physical interpretation and rationalization of the large-eddy simulation regimes in terms of the results of the generalized linear instability theory.