Spectrally resolving the ocean’s Lorenz energy cycle

• Applicant: Professor Dr. Carsten Eden
• Subject Area: Oceanography
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 520959893

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.

• DFG Programme: Research Grants