Enhanced energy dissipation around intrathermocline eddy: direct observations in the South China Sea
Abstract
High-resolution numerical simulations and field efforts have revealed ubiquitous lateral buoyancy gradients around surface-intensified mesoscale eddies and emphasized the role of submesoscale turbulence in the energy cascade of geostrophic flows and vertical communication of tracers. However, active fronts and submesoscale turbulence, along with associated geostrophic energy dissipation, remain poorly understood in subsurface eddy fields, partly due to high-resolution observational limitations in the ocean interior. In this study, a field campaign observed a mesoscale intrathermocline eddy (ITE) with lens-shaped isopycnals and enhanced energy dissipation in the thermocline. The turbulent kinetic energy (TKE) dissipation rates measured by vertical microstructure profiler are significantly elevated to 10 −7 –10 −8 W kg −1 in the ITE periphery, which are one to two orders of magnitude higher than surrounding subsurface areas (∼10 −9 W kg −1 ), and even comparable to mixed-layer values. Diagnostic results from the Gregg-Henyey-Polzin finescale parameterization do not support internal wave activity as the primary source of observed high dissipation rates, although contributions from high-frequency nonlinear internal waves cannot be resolved here. Meanwhile, 600-m-resolution Triaxus observations across the ITE reveal submesoscale-enhanced lateral buoyancy gradients, increased geostrophic shear, and decreased potential vorticity (PV) in regions of elevated TKE dissipation, indicating conditions favorable for frontal submesoscale instability. The consistency of density fronts, negative PVs, and symmetric instability implies that elevated dissipation in the ITE periphery are highly associated with submesoscale instabilities. These direct observations emphasize active fronts and submesoscale turbulence around the ITE and highlight their key role in the forward energy cascade and dissipation of subsurface geostrophic eddies.