Long-range propagation and associated variability of internal tides in the South China Sea

Abstract

The variability of internal tides during their generation and long‐range propagation in the South China Sea (SCS) is investigated by driving a high‐resolution numerical model. The present study clarifies the notably different processes of generation, propagation, and dissipation between diurnal and semidiurnal internal tides. Internal tides in the SCS originate from multiple source sites, among which the Luzon Strait is dominant, and contributes approximately 90% and 74% of the baroclinic energy for M2 and K1, respectively. To the west of the Luzon Strait, local generation of K1 internal tides inside the SCS is more energetic than the M2 tides. Diurnal and semidiurnal internal tides from the Luzon Strait radiate into the SCS in a north‐south asymmetry but with different patterns because of the complex two‐ridge system. The tidal beams can travel across the deep basin and finally arrive at the Vietnam coast and Nansha Island more than 1000–1500 km away. During propagation, M2 internal tides maintain a southwestward direction, whereas K1 exhibit complicated wave fields because of the superposition of waves from local sources and island scattering effects. After significant dissipation within the Luzon Strait, the remaining energy travels into the SCS and reduces by more than 90% over a distance of ∼1000 km. Inside the SCS, the K1 internal tides with long crests and flat beam angles are more influenced by seafloor topographical features and thus undergo apparent dissipation along the entire path, whereas the prominent dissipation of M2 internal tides only occurs after their arrival at Zhongsha Island.

Generation and Propagation of M2 Internal Tides Modulated by the Kuroshio Northeast of Taiwan

“…The water depth changes abruptly from the open ocean to shallow areas to the northeast of Taiwan, resulting in significantly different properties of the internal tides. On the western Pacific side, the first mode wavelength is ~134 km and the first mode phase speed is ~3.1 m/s, which is consistent with previous estimations by Zhao () and Xu et al (). In contrast, over the slope in the ECS where the water depth is less than 200 m, the wavelength is ~17.3 km and the phase speed is ~0.4 m/s.…”

“…To summarize, the comparable multisource distributions for the M 2 internal tides to the northeast of Taiwan are different from those in the SCS, wherein the Luzon Strait is the dominant internal tide source (Alford et al, ; Buijsman et al, ; Xu et al, ). The area to the northeast of Taiwan is characterized by several source sites with comparable energy contributions, leading to complex baroclinic flux fields, which exhibit variable propagation directions and three‐dimensional inhomogeneous features (see sections and ).…”

“…The energetic generation to the northeast of Taiwan is mainly attributed to the presence of corrugated, steep bathymetric features and strong cross‐isobath barotropic currents. For example, the continental slope of the South China Sea (SCS) extends over 1,500 km from western Taiwan to the Vietnamese coast, but the M 2 conversion rate therein amounts to only ~0.76 GW (Xu et al, ), which is much smaller than that of the slope area to the northeast of Taiwan; this slope area is only ~200 km in length.…”

Long‐Range Radiation and Interference Pattern of Multisource M2 Internal Tides in the Philippine Sea

“…ROMS is a nonlinear, three‐dimensional, free‐surface, hydrostatic, primitive equation model with a terrain‐following coordinate (Shchepetkin & McWilliams, ). Because of the much larger scale of the horizontal wavelength than the vertical fluctuation of internal tides, models with hydrostatic assumptions are widely used to simulate baroclinic tidal processes (Carter et al, ; Jan et al, , ; Kerry et al, ; Osborne et al, ; Xu et al, ).…”

“…The surface elevation and cophase lines agree well with those of TPXO7.2 (Egbert & Erofeeva, ). The small‐scale differences near varying topographies are mainly caused by the signals associated with baroclinic tides (Niwa & Hibiya, ; Xu et al, ). Observed surface height data at two deepwater stations and a costal station in the Philippine Sea are available from the National Oceanic and Atmospheric Administration (NOAA) Center for Operational Oceanographic Products and Services.…”