Spectrally resolving the ocean’s Lorenz energy cycle

空间尺度约为5 ~ 200公里的中尺度涡旋是海洋环流的重要特征。它们的产生机制及其对输运和混合的影响是众所周知的，从小尺度到大尺度的逆能量级联涡流动能（EKE）似乎是海洋动力学的一个强大的观测特征。它遵循传统的逆涡能量级联模式，从较小尺度的涡产生到较大尺度的耗散。这种模式的问题在于，没有多少过程能够合理地解释大尺度上的耗散（例如，海洋模式中的摩擦倾向于小尺度）。然而，这种传统模式最近受到了高分辨率海洋模型中涡旋能量产生项的频谱解析的挑战：我们和其他作者在中高纬度地区发现了EKE产生项的尺度依赖标志。在那里，EKE在大尺度上被转换回势能，这几乎平衡了逆能量级联。由于在高纬度地区可以预期在供给EKE的能量储层中有一个更小尺度的正向级联，因此建议对所谓的洛伦兹能量循环的所有其他项进行光谱解析，以了解洛伦兹能量循环中的能量级联，并解决我们对涡旋能量耗散途径的缺失理解。这一努力需要解决两个问题：恒定深度的平均而不是等轴平均，以及可用势能的定义。由于这两个问题都难以在现实海洋模型中解决，因此我们建议在回归现实海洋模型之前，在理想海洋模型中解决这些问题。我们的基本假设是，这两个问题都不会定性地改变结果，这将为我们在现实模型中对光谱解析洛伦兹能量循环的新解释提供信心。我们的研究结果将对海洋不同纬度的中尺度涡旋耗散和涡旋能量循环提供新的认识。

Mesoscale eddies with spatial scales of ca. 5 to 200 km are an important feature of the ocean circulation. Their generation mechanism and effects on transport and mixing are well known, and an inverse energy cascade fluxing eddy kinetic energy (EKE) from small to larger scales appears to be a robust observed feature of ocean dynamics. It follows the traditional paradigm of an inverse eddy energy cascade from the smaller scales of eddy production to dissipation at larger scales. Problematic in this paradigm is that not many processes are plausible to account for dissipation at large scales (e.g. friction in ocean models favors small scales). However, this traditional paradigm has been challenged recently by spectrally resolving the eddy energy production terms in high-resolution ocean models: We and other authors found a scale-dependent sign of the EKE production term at mid- to high latitudes. There, EKE is transferred back to potential energy at large scales, which nearly balances the inverse energy cascade. Since a forward cascade to smaller scales in the energy reservoirs feeding EKE can therefore be expected at higher latitudes, it is proposed to spectrally resolve all other terms of the so-called Lorenz energy cycle to understand the energy cascades in the Lorenz energy cycle, and to resolve our missing understanding of the eddy energy route to dissipation. Two issues need to be resolved for this endeavor: averaging at constant depth instead of isopycnal averaging, and the definition of available potential energy. Since both issues are difficult to resolve in realistic ocean models, we propose to do so in idealized ocean models instead, where the issues can be readily resolved, before we return to the realistic ocean model. Our underlying hypothesis is that both issues will not qualitatively change results, which will provide confidence of our novel interpretation of the spectrally resolved Lorenz energy cycle in the realistic model. The result of our work will be a new view on mesoscale eddy dissipation and eddy energy cycle at different latitudes of the ocean.