Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
22 August 2020

Lagrangian Eddy Kinetic Energy of Ocean Mesoscale Eddies and its Application to the Northwestern Pacific

Science

Here, we propose a comprehensive concept—Lagrangian EKE (LEKE) as an additional metric which is a combination of gridded EKE calculated in Eulerian framework and tracked coherent mesoscale eddies in the Lagrangian framework. Evidence suggests that LEKE can make up these deficiencies as an effective supplement. In this study, regional application over the Northwestern Pacific Ocean is taken as an example. It clearly demonstrates that LEKE reveals more accurate and detailed characteristics of both cyclonic and anticyclonic eddies than EKE when coherent mesoscale eddies are the specific focus, such as the variation rates of kinetic energy during the eddy propagation, spatial-temporal differences of kinetic energy between cyclonic and anticyclonic eddies. Overall, using LEKE to analyze coherent mesoscale eddies 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 mesoscale eddies in the ocean.

Impact

LEKE provides a potential linkage for the further understanding of the energy cascade at both directions. Recent results have shown an enhanced mixing due to wind-driven internal waves where the EKE is relatively large. Moreover, this mixing is more powerful within the anticyclonic eddies than that within the cyclonic eddies in the region of 30°–45°N. The spatial difference between cyclonic eddies and anticyclonic eddies shields more lights on the roles played by cyclonic eddies and anticyclonic eddies in ocean mixing. For instance, the mixing is found to be enhanced in the band of 20°–22°N, which is due to the effect of anticyclonic eddies. It may correspond with larger anticyclonic eddy-LEKE (over 40%) than that cyclonic eddy-LEKE at the longitude of 143°E. This evidence does produce practical significance on the further understanding of the inverse energy cascade.

Summary

By combining EKE calculated in the Eulerian framework with tracked eddies in the Lagrangian framework, LEKE is proposed to measure the Kinetic Energy of coherent mesoscale eddies. LEKE can reveal more accurate and detailed characteristics of coherent mesoscale eddies than EKE. This is related to the three deficiencies of EKE when focusing especially on mesoscale eddies with coherent rotating structures. Firstly, EKE averaged over a given region is a mixture of the Kinetic Energy of all mesoscale motions. Secondly, EKE fails to display the time evolution of mesoscale eddies. Last and most important, EKE is incapable of distinguishing the Kinetic Energy of cyclonic and anticyclonic eddies. By taking the Kuroshio Extension region as an example, the spatial-temporal variabilities of EKE and LEKE are compared. The roles of coherent mesoscale eddies are highlighted with the usage of LEKE. More information, such as the relative contribution of cyclonic and anticyclonic eddies in south/north of Kuroshio Extension Jet, the larger temporal variability with a significant increasing trend, and so on are all provided to a deep understanding of coherent mesoscale eddies over this domain. It is evident that LEKE as an additional metric to analyze coherent mesoscale can draw further attention to their unique roles in the ocean.

 

Contact
Aixue Hu
National Center for Atmospheric Research (NCAR)
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