Mapping the Energy Cascade in the North Atlantic Ocean: The Coarse-Graining Approach
Mapping the Energy Cascade in the North Atlantic Ocean: The Coarse-Graining Approach
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DOI：
10.1175/jpo-d-17-0100.1
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发表时间：
2018-02-01
影响因子：
3.5
通讯作者：
Vallis, Geoffrey K.
中科院分区：
文献类型：
--
作者：
Aluie, Hussein;Hecht, Matthew;Vallis, Geoffrey K.
采用粗粒度框架分析了非线性过程，测量了能量传输速率，并从模拟的全球海洋数据中绘制了能量路径。从湍流理论中测量能量级联的传统工具，如谱通量或谱转移，依赖于统计均匀性的假设，或者至少在运动尺度和统计非均匀性尺度之间存在很大的分离。粗粒度框架允许在尺度和空间上同时探测完全非线性动力学，并且不受这些假设的限制。本文描述了该框架如何应用于海洋流动。由于量规自由，量尺之间的能量传递不是唯一的。本文认为，伽利略不变子滤波尺度（SFS）通量是测量海洋能量尺度转移的合适量。结果表明，SFS定义可以得到与将空间输运与能量尺度转移混为一谈的传统测量有质的不同的答案。本文提出了能量尺度转移的地理地图，这些地图既在空间上是局部的，又允许准光谱或逐比例尺的动态诊断。利用北大西洋强涡旋流模拟，发现并不是所有地方都存在高等级的能量转移。事实上，墨西哥湾流附近和赤道逆流中的某些地区有明显的小尺度转移。然而，平均而言，在0(10(3))km大小的区域内，高档转移是对温带能量尺度转移的合理平均描述。
A coarse-graining framework is implemented to analyze nonlinear processes, measure energy transfer rates, and map out the energy pathways from simulated global ocean data. Traditional tools to measure the energy cascade from turbulence theory, such as spectral flux or spectral transfer, rely on the assumption of statistical homogeneity or at least a large separation between the scales of motion and the scales of statistical inhomogeneity. The coarse-graining framework allows for probing the fully nonlinear dynamics simultaneously in scale and in space and is not restricted by those assumptions. This paper describes how the framework can be applied to ocean flows. Energy transfer between scales is not unique because of a gauge freedom. Here, it is argued that a Galilean-invariant subfilter-scale (SFS) flux is a suitable quantity to properly measure energy scale transfer in the ocean. It is shown that the SFS definition can yield answers that are qualitatively different from traditional measures that conflate spatial transport with the scale transfer of energy. The paper presents geographic maps of the energy scale transfer that are both local in space and allow quasi-spectral, or scale-by-scale, dynamics to be diagnosed. Utilizing a strongly eddying simulation of flow in the North Atlantic Ocean, it is found that an upscale energy transfer does not hold everywhere. Indeed certain regions near the Gulf Stream and in the Equatorial Countercurrent have a marked downscale transfer. Nevertheless, on average an upscale transfer is a reasonable mean description of the extratropical energy scale transfer over regions of O(10(3)) km in size.