It is reported that turbulent mixing is enhanced in the South China Sea (SCS), and it is highly variable in both space and time. Generation and breaking of internal tides has been identified as the main process to drive turbulent mixing in the SCS, while the contributions from other processes are not clear enough. Here we investigate the potential contribution from mesoscale eddies to turbulent mixing in the SCS using a high resolution numerical simulation. Our results show that mesoscale eddies in the SCS effectively dissipate over complex rough topography and indicate that the generation of submesoscale motions and lee waves are two pathways for the transfer of mesoscale eddy energy down to small dissipation scales. The energy loss from mesoscale eddies near the Xisha Islands is estimated to be sufficient to sustain turbulent kinetic energy dissipation rate of O (10−8) W/kg. This study suggests an alternative and potentially efficient mechanism to internal tides for the local maintenance of turbulent mixing in the SCS.
The intensity of deep‐ocean mixing critically shapes the global overturning circulation, but the energy cascade participating in the elevated mixing above rough topography remains poorly understood. Using 350‐day moored observations, the energy cascade triggered by eddy‐topography interactions is for the first time revealed in the deep South China Sea. The deep‐ocean observations show bottom‐intensified near‐inertial wave (NIW) pulses during periods of eddy occurrences. Eddy‐induced NIWs appear to catalyze nonlinear wave‐wave interactions, which further cascade energies from NIWs and internal tides to higher‐frequency internal waves (HFIWs) above fk1 frequency. Corresponding to the HFIW enhancement, the kinetic energy spectrum at 3–9 cycles per day increases by 2–9 times, and the spectral slope becomes ~ −1 rather than ~ −2, as predicted by the Garrett‐Munk oceanic internal wave spectrum. A cascade of internal waves above rough topography potentially promotes extracting energy from eddies and tides and dissipating this energy to mix deep waters.
In the ocean, various dynamic systems coexist, which span a vast range of spatiotemporal scale, including largescale circulations, mesoscale eddies, submesoscale processes, small-scale internal waves, and so on. They interact with each other, which ultimately leads to microscale dissipation both at ocean boundaries and in the interior of the deep ocean, and plays a primary role in the thermodynamic balance of the ocean. Among them, internal lee waves act not only as an energy sink of geostrophic flow but also as a significant energy source of deepocean mixing (e.g., Nikurashin & Ferrari, 2011). Although the magnitude of global energy input into lee waves is smaller than that into internal tides, lee wave-driven mixing significantly affects the ocean state. It can reduce ocean stratification associated with warming of the abyssal ocean, accelerate global meridional overturning circulation because of its promotion of deep-water upwelling, and hence facilitating the renewal of deep water (e.g.,
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