UC San Diego

Abstract: Submesoscale currents and internal gravity waves achieve an intense turbulent cascade near the ocean surface (0 m – O(100) m depth), which is thought to give rise to significant energy sources and sinks for mesoscale eddies. Here, we characterise the contributions of Non-Wave Currents (NWCs; including eddies and fronts) and Internal Gravity Waves (IGWs; including near-inertial motions, lee waves and the internal wave continuum) to near-surface submesoscale turbulence in the Drake Passage. Using a numerical simulation, we combine Lagrangian filtering and a Helmholtz decomposition to identify NWCs and IGWs and to characterise their dynamics (rotational vs. divergent). We show that NWCs and IGWs contribute in different proportions to the inverse and forward turbulent kinetic energy cascades, based on their dynamics and spatiotemporal scales. Purely rotational NWCs cause most of the inverse cascade, while coupled rotational– divergent components of NWCs and coupled NWC–IGWs cause the forward cascade. The cascade changes direction at a spatial scale at which motions become increasingly divergent. However, the forward cascade is ultimately limited by the motions’ spatiotemporal scales. The bulk of the forward cascade (80 – 95%) is caused by NWCs and IGWs of small spatiotemporal scales (L <10 km; T <6 hours), which are primarily rotational: submesoscale eddies, fronts, and the internal wave continuum. These motions also cause a significant part of the inverse cascade (30%). Our results highlight the requirement for high spatiotemporal resolutions to diagnose the properties and large-scale impacts of near-surface submesoscale turbulence accurately, with significant implications for ocean energy cycle study strategies.