Submesocale eddies in the abyssal Southern Ocean

The ocean circulation is powered primarily by winds at ocean basin scales and dissipates its energy by viscous friction at much smaller, centimetre scales. To connect the ocean power input to its dissipation, internal wave and turbulent motions are generated in the ocean interior and boundary layers, resulting in energy cascades to small dissipation scales and leading to mixing of tracers such as heat, carbon and nutrients. In the surface ocean, submesoscale motions generated from the instability of strong currents and mesoscale eddies, have been shown to dominate the energy transfer to smaller scales and contribute to the uptake and mixing of tracers. In the deep ocean, where currents tend to be weak, the energy transfer to smaller scales and mixing are believed to be dominated by the generation and subsequent breaking of internal waves enhanced at rough bottom topography. Here, we use realistic, submesoscale-resolving and internal wave-permitting simulations of the sector of the Southern Ocean south of Tasmania and New Zealand and show that there are abundant submesoscale eddies in the deep Southern Ocean. Deep submesoscale eddies are dominated by topographic wakes in regions of strong deep-reaching fronts of the Antarctic Circumpolar Current. The kinetic energy in the submesoscale wavenumber range in the deep ocean is comparable to that in the upper ocean even in the simulation in which small-scale rough topography generating internal waves is removed. Our results suggest the presence of energetic submesoscale motions in the deep ocean and hence imply a new route to the energy dissipation and mixing in the deep Southern Ocean.