| 01 Jun 2026
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.
The study investigates the regional differences (q) in the local dissipation efficiency of internal tides between the Luzon Strait and the Nansha Islands based on the LLC4320 simulation outputs. A physically-based empirical formulation and an XGBoost model are subsequently developed to parameterize q . This study contributes much to improving the understanding and parameterization of internal tide dissipation in the SCS. However, the manuscript still has some shortcomings, and the following points should be addressed further by the authors before publication. 1. fig 1: Since q is calculated from area-integrated conversion and dissipation, its magnitude may be sensitive to the choice of regional boundaries. Could the authors clarify the criteria used to determine the boundaries of the Luzon Strait and the Nansha Islands?
2. Lines 100–108: The manuscript only retains the first five baroclinic modes in the modal decomposition. Please state why five modes are sufficient for the analysis presented in this study? Given that high-mode internal tides are significant and important over steep and rough topography, it would be helpful to either provide the proportion of total baroclinic kinetic energy captured by modes 1–5, or discuss the potential influence of unresolved higher modes on the diagnosed local dissipation.
3. Lines 40–44 and 122–125: w is defined as the fraction of locally dissipated internal tidal energy relative to locally generated internal tidal energy, which would conventionally be expected to fall between 0 and 1. However, subsequent results presented in the manuscript report q > 1 in the Nansha Islands and negative q values for some individual modes in the Luzon Strait. Please provide the definition and sign convention of q, and explicitly explain the physical meanings of q > 1 and q < 0 in the present energy-budget framework?
4. Lines 90–92and 125–127: The LLC4320 simulation outputs used in this study only span approximately 14 months, from September 2011 to November 2012. In the analysis, January and August are used to represent winter and summer, respectively. As a result, the summer-winter differences reported in this manuscript are based on a single annual cycle rather than a multiyear climatology. Could the authors point out this limitation explicitly and avoid interpreting these differences as climatological seasonal variations?
5. Figs 2–5: Since the first and final months of the dataset are incomplete, could the authors specify how the monthly mean valuesof q for these two months were treated in the time-series plots? If these mean values were calculated from incomplete monthly records, this should be stated clearly; alternatively, the corresponding data points can be removed or marked separately.
6. Regarding Fig. 5j–m: These panels appear identical to Fig. 3j–m. Additionally, they are seeminglyinconsistent with the results described in Lines 205–213, such as the reduced magnitude of energy-budget terms and the relatively large mode-1 q Could the authors examine whether the proper data for the Nansha Islands have been plotted?