Eddy Dynamics and Energy Pathways from 4-Dimensional Glider Observations and Numerical Simulations.

Mesoscale features and the corresponding submesoscale structures can vertically transport heat, freshwater, and biogeochemical tracers (i.e., phytoplankton, oxygen, and carbon) from the surface to the interior. These structures may grow, decay, and transfer energy through various processes. This study examines the small (~20 km) mesoscale eddy evolution and the associated energy transfers from a four-dimensional, three-month-long glider fleet survey in the Western Mediterranean Sea. The combined glider fleet covered nearly 15978 km over the ground, performing 704 glider days while doing over 4837 dives to as deep as 700 m, measuring physical and biochemical parameters. The sources of eddy kinetic energy are examined and compared with numerical eddy-resolving simulations (2-km grid-scale). The comparison allows us to identify whether the energy exchange is local or has a broader interaction between the mean and eddy flow. We study the redistribution of energy during the eddy merging and splitting processes, and how these processes relate to changes in the flow divergence and vertical velocities. During the eddy merging, the vertical velocity reaches up to 20 m/day. However, we observed a reduction in the areas where significant vertical motion occurs, which was associated with a decrease in frontogenesis in the periphery of the eddy and a redistribution of kinetic energy across the merging eddy. During the eddy splitting, the vertical velocity was significantly reduced (less than 10 m/day) by a frontolytic event in the northern eddy. Eddy splitting caused a significant reduction of the positive and negative divergence, and the energy of the two newly formed cyclonic eddies (CEs) decayed (vertical velocities decreased from ~20 m/day to ~10 m/day). The eddy merging event can be considered as a large-scale energy pump in the regions where an inverse energy cascade occurs. The observed imbalance in the transfer of EKE during eddy splitting suggests that the northern CE decays quicker and maintains less kinetic energy than the southern one. We examine the energy transfer terms of the baroclinic and barotropic components, taking into account both the horizontal and vertical energy transfer (baroclinic horizontal term, the baroclinic vertical term, and the barotropic term), which provides a better understanding of the instability processes responsible for the eddy formation.