Lunch Talk Series by Earth, Ocean and Atmospheric Sciences (EOAS) Thrust, HKUST (GZ)  - Observed Cross-Scale Kinetic Energy Transfers in the Ocean

Mesoscale eddies contain the bulk of the ocean’s kinetic energy (KE), but fundamental questions remain on the cross-scale KE transfers linking eddy generation and dissipation. The role of submesoscale flows represents the key point of discussion, with contrasting views of submesoscales as either a source or a sink of mesoscale KE. Here, the observational assessment of the annual cycle of the KE transfer between mesoscale and submesoscale motions is performed in the upper layers of a typical open-ocean region. The cross-scale KE transfer exhibits two distinct stages, whereby submesoscales energize mesoscales in winter and drain mesoscales in spring. An exploratory appraisal of the dynamics governing mesoscale–submesoscale KE exchanges suggests that the upscale KE transfer in winter is underpinned by mixed layer baroclinic instabilities, and that the downscale KE transfer in spring is associated with frontogenesis. The frontogenetic rate is found to correlate more strongly with horizontal convergence as the scale decreases, suggesting that convergent flows are the main driver of submesoscale frontogenesis. We further quantify the rate of energy exchange between low-frequency background flows (mesoscale and submesoscale sub-inertial motions) and the internal wave field. In general, energy transfer rates are bidirectional, with energy transfers occurring both from low- to high-frequency, and vice versa. The directionality of those transfers is linked to both the energetics of the low-frequency flows and the internal waves interacting with that flow as well as their relative phasing.