This study deals with standing wave or seiche events using cruise observations, satellite altimeter data, and theoretical analysis. Cruise missions in summer 2007 and 2009 detected internal oscillation signals in the South China Sea (SCS) deep basin. The signals have average wavelengths of 320 and 390 km and the maximum amplitudes of 50–100 m at layers 500–700 m and 1500–1700 m. Satellite altimeter sea level anomaly (SLA) images and the second intrinsic mode function (IMF2) images derived from the empirical mode decomposition (EMD) analysis show that the observed internal oscillations are a portion of 2‐D seiche modes, which lasted for at least 2 weeks. We recognize that the observed internal oscillation signals represent seiche modes H_5,3 and H_5,1 derived from a rectangular model ocean basin with a uniform depth, a west‐east length of 1000 km and a north‐south width of 800 km. Statistical analysis of standing wave modes H_4,0, H_5,1, and H_5,3 with the average wavelength of 500, 390, and 320 km indicates that from 1993 to 2012 (1045 weeks), total 94 events with total temporal coverage of 218 weeks are affirmed. The total occurrence frequency is 20.9%. Histograms of annual distributions of seiche events and timespans show an interannual variability of about 9 years, with peak years 1993, 1994, 1998, 2001, and 2011. While monthly distributions show an intraseasonal variability double‐peaked in May and October, transit periods of East Asia monsoon in the SCS.

The ocean hosts a variety of fluid motions characterized by contrasting dynamical regimes and spatiotemporal scales. A practical decomposition of multiscale motions in realistic oceanic settings is crucial to advancing dynamical interpretation and prediction of oceanic processes, but remains a major challenge. To this end, methodology is developed in this study for decomposing multiscale oceanic motions based on their respective dynamical characteristics. Specifically, large‐scale currents and barotropic tides have the largest horizontal scales but contrasting frequencies; low‐mode internal gravity waves (IGWs) are well constrained by linear dispersion relations, whereas mesoscale flows are of relatively low frequency and with horizontal scales above the first baroclinic deformation radius; the intrinsic frequency of high‐mode IGWs (submesoscale flows) is above (well below) the inertial frequency. The validity and usefulness of the proposed methodology are demonstrated with a proof‐of‐concept application to simulated flows in the central basin of the South China Sea.