Local dissipation efficiency of internal tides at key topographic features in the South China Sea

Abstract. The local dissipation efficiency of internal tides, q, is a critical parameter in tidal mixing parameterizations. However, the conventionally adopted constant value (q ≈ 0.3) in large-scale ocean models neglects its significant spatiotemporal variability. Based on the MITgcm LLC4320 simulation, the internal tidal energy budgets at the Luzon Strait (a source region, LS) and the Nansha Islands (a sink region, Nansha) in the South China Sea (SCS) are analyzed. Results indicate that the barotropic-to-baroclinic energy conversion in the LS reaches approximately 45 GW, with semidiurnal constituents accounting for roughly 60 %, due to the resonance over the double-ridge topography. The value of q in the LS fluctuates between 0.3 and 0.7, primarily modulated by the high-mode local dissipation. Local internal tide generation around the Nansha Islands is less than 1.5 GW; however, this region experiences significant convergence of internal tidal energy flux, which elevates the value of q to generally greater than 1 and occasionally exceeding 2.5. Modal analysis confirms that the intensified dissipation over the Nansha Islands originates predominantly from topographic scattering and breaking of mode-1 internal tides from the far field. Parameterizations for q are developed based on both physical factors and data-driven algorithms, both of which successfully capture the macroscopic clustering of q. In the LS, q is modulated by near-field factors such as the barotropic tidal forcing and the local dissipation of high-mode internal tides. Conversely, q around the Nansha Islands is primarily contributed by mode-1 internal tidal energy coming from the far field, highlighting the jointly modulation of local extreme dissipation by far-field beam interference and nonlinear topographic scattering.