Spectrally resolving the ocean’s Lorenz energy cycle

中尺度涡旋是海洋环流的一个重要特征，其空间尺度约为5~200公里。它们的产生机制及其对输送和混合的影响是众所周知的，从小尺度到大尺度的逆能量串级通量涡动动能(EKE)似乎是海洋动力学的一个强有力的观测特征。它遵循从较小尺度的涡旋产生到较大尺度的消散的反向涡流能量级联的传统范例。这一模式的问题在于，没有多少过程可以解释大尺度的耗散(例如，海洋模型中的摩擦倾向于小尺度)。然而，这一传统模式最近受到了挑战，因为它在高分辨率海洋模式中对涡旋能量产生项进行了光谱解析：我们和其他作者在中高纬度发现了EKE产生项的尺度依赖迹象。在那里，eke在大范围内被转移回势能，这几乎平衡了反向能量级联。因此，由于在较高纬度可以预期馈送EKE的能量库中的较小尺度的正向级联，因此建议对所谓的洛伦兹能量循环的所有其他项进行频谱解析，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消散途径的缺失理解。这项工作需要解决两个问题：恒定深度平均而不是等周期平均，以及可用势能的定义。由于这两个问题在现实的海洋模型中都很难解决，我们建议在返回到现实的海洋模型之前，在理想化的海洋模型中这样做，在那里问题可以很容易地解决。我们的基本假设是，这两个问题都不会从本质上改变结果，这将为我们在现实模型中对光谱分辨洛伦兹能量循环的新解释提供信心。我们的工作结果将是对海洋不同纬度的中尺度涡旋耗散和涡旋能量循环的新看法。

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.