**2. Materials and Methods**

The model domain covers the northern South China Sea, the Luzon Strait, and part of the Western Pacific and ranges zonally from 110◦ E to 129◦ E and meridionally from 16◦ N to 23◦ N. Realistic bathymetry data are derived from the general bathymetric chart of the oceans (GEBCO\_08, Figure 1). The primary research domain, i.e., the Luzon Strait, is placed at the center of the model domain. L1 is selected as a generation site, and B2 falls along the propagation path [16]. The grid resolution in the horizontal direction is set to 1/30th degree × 1/30th degree, which is fine enough to describe baroclinic tidal signals for our model domain. A total of 115 uneven vertical layers are set, ranging from 10 m in the upper ocean and gradually increasing to 200 m in the deep ocean.

**Figure 1.** Model domain and its topography. Black solid box is the main research domain, the Luzon Strait. Selected locations L1 and B2, which refer to Ramp et al., are marked with stars [16]. The dashed line is the inner boundary of a sponge layer.

The model is driven by barotropic tidal currents composed of the first eight tidal constituents (*M*2, *S*2, *K*2, *N*2, *K*1, *O*1, *P*1, *Q*1) at four open lateral boundaries. Harmonic constants of forcing are derived from the OSU TOPEX/Poseidon Global Inverse Solution (TPXO7.2). A sponge layer is specified along the open boundaries to avoid artificial reflection (Figure 1). A relaxation term that relaxes variables toward the boundary values by applying a linearly increasing relaxation time scale is added to the momentum equation. The relaxation time scale is set to one *M*<sup>2</sup> cycle at the interior termination of the sponge layer and to one thousandth of one *M*<sup>2</sup> cycle at boundaries as the *M*<sup>2</sup> signal is most significant in our case.

Initial temperature and salinity are derived from the World Ocean Atlas 2009 and are horizontally homogeneous and vertically stratified (Figure 2a,b). According to the buoyancy frequency *N* (Figure 2c), the initial pycnocline is at a depth of approximately 100 m, showing a stable ocean stratification structure. A no-slip condition is used for lateral boundaries and at the bottom. The quadratic bottom drag coefficient is 0.002. The vertical diffusivity and viscosity are calculated by the KPP vertical mixing parameterizations proposed by Large et al., which can simulate oceanic processes like convective penetration and diurnal cycling based on physical principles [28]. The model run lasts for 50 model days with 1-min time steps and hourly output.

**Figure 2.** (**a**) The temperature, (**b**) salinity, and (**c**) buoyancy frequency profiles. Gray dashed lines are the initial field. Black and blue solid lines are the background profiles at L1 and B2, respectively.
