2026/06/17 by Kun Liu
Earth and Planetary Sciences · Environmental Science · #Oceanographic and Atmospheric Processes #Marine and coastal ecosystems #Marine and fisheries research
paper · doi:10.1175/jpo-d-25-0155.1
Abstract Internal tides (ITs) in the northern South China Sea exhibit pronounced spatiotemporal variability modulated by complex oceanic background conditions, complicating assessments of their predictability and intrinsic regularity. Using a high-resolution numerical simulation [Massachusetts Institute of Technology General Circulation Model (MITgcm) LLC4320], I evaluate IT predictability by quantifying how IT-induced baroclinic sea surface height (SSH) accounts for observed variability in along-track satellite altimetry and assess IT regularity using both conventional stationarity and a newly developed dynamical regularity (DR) index based on the Hilbert–Huang transform. Stationary IT is reasonably captured by the model, whereas nonstationary variability substantially reduces the predictability of the total IT, highlighting the need for data assimilation strategies and improved internal wave parameterization. Stationarity diagnosed from horizontal kinetic energy and baroclinic energy flux is generally high near the Luzon Strait and along main propagation beams, but results are sensitive to the diagnostic metric and sampling window length. Further analysis of baroclinic SSH suggests that nonstationary ITs retain notable dynamical regularity, rather than behaving random or unpredicatable. By explicitly incorporating variability in instantaneous frequency and energy, DR provides a time-resolved and physically consistent characterization of IT regularity. DR confirms high IT regularity near generation sites and reveals that low-mode ITs maintain pronounced regularity far downstream within the deep basin, in contrast to the downstream decay indicated by stationarity. DR also reveals reduced diurnal IT regularity in the northern Luzon Strait, underscoring the significant role of local dynamics such as the Kuroshio intrusion. Overall, the DR index captures a broader spectrum of IT dynamical behaviors than stationarity, offering a new framework capable of characterizing IT regularity in complex oceanic environments.