Kinetic energy transfers between mesoscale and submesoscale motions

Mesoscale flows contain the bulk of the ocean’s kinetic energy, but fundamental questions persist on their lifecycle. In classical views, mesoscale motions are powered through a direct kinetic energy cascade, in which potential energy set up by basin-scale atmospheric forcing is released by baroclinic instability of the major current systems. Dissipation of mesoscale flows is thought to occur through small-scale turbulence primarily at boundaries –although characterisation of the mechanisms involved remains tentative. Here, we provide an assessment of how consideration of upper-ocean submesoscale motions –smaller, faster-evolving and less energetic than mesoscale flows– modifies this picture of ocean energetics. Submesoscale motions may dampen the mesoscale via their generation by instabilities of mesoscale fronts; yet they may also power the mesoscale, and bypass the direct kinetic energy cascade from basin scales, via an inverse cascade evolution. We quantify these multi-scale interactions using a unique dataset of two nested mooring arrays and a pair of gliders deployed in a representative mid-gyre region of the North Atlantic for one year. Mesoscale and submesoscale motions exhibit pronounced seasonal cycles that are broadly synchronous and dynamically coupled by substantial kinetic energy transfers. These transfers reverse in sign, and are underpinned by distinct processes, at different stages of the year. In winter (spring), as the mixed layer deepens (shoals), kinetic energy is transferred upscale (downscale). We use observational and theoretical diagnostics to show that upscale and downscale transfers are respectively associated with submesoscale instabilities and mesoscale frontogenesis. We discuss implications for our understanding of ocean energetics, and for how these are represented in state-of-the-art ocean models.