Non-local energy dissipation of lee waves and turbulence in the South China Sea

Breaking of internal lee waves has been shown to drive enhanced turbulence and mixing in regions where strong bottom flows interact with rough topography. However, theoretical predictions for the energy conversion from the bottom flows into lee waves differ from the corresponding turbulent energy dissipation rates observed locally at the wave generation sites. Recent idealized numerical simulations suggest that the discrepancy may be attributed to non-local wave breaking and dissipation effects: when a mean flow impinges on rough topography and generates lee waves and turbulence, a significant fraction of the generated energy gets advected downstream of the generation site and dissipates remotely. Here, we present a case study for the non-local lee wave energy dissipation in the South China Sea by using a combination of in situ mooring observations of the bottom flow interacting with two topographic features and corresponding numerical simulations. The results show that most of the near-inertial and high-frequency response observed at the mooring site is not locally generated, but rather is produced by the interaction of the subinertial flow with the topographic feature upstream. The wave and turbulent energy detected at the mooring site can be enhanced by an order of magnitude compared to the energy of the locally generated motions. The simulations confirm that up to 70% of the enhanced energy is advected into the region by the subinertial flow, while the rest is radiated into the region as waves. Implication of our results for mixing observations and ocean model parameterizations are discussed.