Lagrangian Eddy Kinetic Energy: A Proper Metric Measures the Kinetic Energy of Coherent Mesoscale eddies
Coherent mesoscale eddies (MEs) are the "weather" of the ocean, with typical spatial scales on the order of tens to hundreds kilometers and temporal scales from weeks to months. MEs have great impacts on ocean transports and global climate and possess kinetic energy that is roughly one order of magnitude larger than the mean ocean current. The surface gridded eddy KE (EKE), derived from the sea surface height anomalies field based on the geostrophic relationship, is therefore commonly used to analyze the MEs and their variabilities.
"However, there are three deficiencies of EKE when focusing on the analysis of MEs. Firstly, EKE averaged over a given region is a mixture of the KE of all mesoscale motions. Secondly, EKE fails to display time evolution of MEs. Last and most important, EKE is incapable of distinguishing the KE of cyclonic eddies and anticyclonic eddies." Said Dr. DING Mengrong, from the Institute of Atmospheric Physics (IAP) at the Chinese Academy of Sciences.
She and her collaborators—LIN Pengfei, LIU Hailong from IAP, HU Aixue from CGD/NCAR and LIU Chuanyu from KLOCAW/IOCAS—propose a comprehensive concept "Lagrangian EKE (LEKE)" as an additional metric. The additional metric LEKE is a combination of gridded EKE calculated in Eulerian framework and tracked coherent MEs in Lagrangian framework. Evidences suggest that LEKE can make up these deficiencies as an effective supplement. This new finding has been published in Scientific Reports.
By taking the Kuroshio Extension region as an example, the team compared spatial–temporal variabilities of EKE and LEKE. "Apparently using LEKE as an addition metric to analyze coherent MEs can draw further attention to their unique roles in the ocean," says LIN Pengfei, one of the corresponding authors of this paper.
Overall, according to the study, using LEKE to analyze coherent MEs gives the rise to understand the spatial–temporal contrasts between eddies with different polarities, and provides a new perspective to recognize the crucial role played by coherent MEs in the ocean.
Ding M., Lin P., Liu H., et al. Lagrangian eddy kinetic energy of ocean mesoscale eddies and its application to the Northwestern Pacific[J]. Scientific Reports, 2020, 10(1).
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