Impact of vertical coordinate systems on simulations of barotropic and baroclinic tides in the Yellow Sea

Abstract. An accurate representation of tidal dynamics is critical for simulating physical and biogeochemical processes in marginal seas, such as the Yellow Sea, where energetic tides and strong seasonal stratification coexist. This study assessed the impact of vertical coordinate systems on the simulation of barotropic and baroclinic tides using Modular Ocean Model version 6, evaluating two configurations: the z* coordinate system (ZSTAR) and hybrid z*-isopycnal coordinate system (HYBRID). The model outputs were validated against satellite-derived sea surface temperatures, in situ temperature profiles, and TPXO tidal harmonics, with a focus on contrasting winter and summer conditions. HYBRID more accurately reproduced sea surface temperatures and vertical thermal structures, particularly during strongly stratified summers, and maintained a sharper and deeper thermocline, as confirmed by long-term temperature diagnostics and age tracer experiments, indicating reduced vertical mixing and improved stratification. For barotropic tides, HYBRID showed better agreement with TPXO for the dominant M2 constituent (RMSE: 24.19 cm; correlation: 0.76) than ZSTAR (RMSE: 32.55 cm; correlation: 0.67). A similar improvement is found for the K1 constituent, further confirming the superior barotropic tidal performance of HYBRID across multiple tidal frequencies. HYBRID also produced stronger barotropic tidal energy fluxes across key regions, yielding 13.5–27.6 % larger fluxes in winter and 17.2–51.1 % larger fluxes in summer relative to ZSTAR. Baroclinic tidal dynamics exhibited more contrasting model behavior. In winter, ZSTAR produced 19.5–36.8 % greater baroclinic kinetic energy (KE). However, in summer, HYBRID simulated consistently stronger baroclinic KE, with 9.2–33.6 % larger magnitudes across all regions, reflecting more realistic baroclinic tide generation under strong stratification. Analysis of the baroclinic energy budget further revealed that, although ZSTAR often yielded greater barotropic-to-baroclinic conversion, a substantial portion of this converted energy was locally dissipated rather than radiated, resulting in a much larger residual dissipation term. This indicates that spurious diapycnal mixing in ZSTAR rapidly removes internal-tide energy, degrading propagation and vertical energy transfer. In contrast, HYBRID preserved baroclinic tidal energy more effectively, enabling more coherent energy radiation away from generation hotspots. These results highlight that vertical coordinate design critically influences tidal energetics and stratification-dependent processes in high-resolution regional models. The improved stratification maintenance enabled by HYBRID offers substantial advantages for accurately representing internal-tide dynamics and associated vertical energy pathways in the Yellow Sea.