Characteristic and Variation of the High Frequency Variability (HFV) in the South China Sea (SCS)

Near-inertial oscillations (NIOs) and internal tides (ITs) have been observed in multiple locations in the SCS and are often considered to provide a crucial energy pathway for influencing the circulation in the SCS. In this study, a series of one-year current, temperature, and salinity mooring measurements at the northern, middle, and southern parts of the central SCS basin (NCB, MCB, SCB) during the South China Sea Monsoon Experiment (SCSMEX) are used to examine the spatial variation of the HFV in the upper ocean spanning the summer and winter monsoon periods (SMP, WMP). NIOs, diurnal and semi-diurnal tides are the most pronounced components in energy and vertical shear spectra at these three locations for both SMP and WMP. The tidal signals are relatively distinct in the temperature spectra and NIOs display a larger asymmetry in vertical wavenumber spectra. All these spectra agree reasonably well with the canonical internal wave spectrum, but with a larger deviation at the MCB due to local topography effects and wave-wave interaction. HFV accounts for 10-15% of the total kinetic energy (KE) in the SCS circulation, while velocity vertical shear arising from HFV is 2-3 factors larger than that from the quasi-geostrophic currents in the upper ocean. In general, NIOs are highly intermittent with stronger seasonal mean magnitude during WMP. Analyses on the coherence between wind burst and NIO episodes, accompanied with the estimate of KE budget, shows that the local wind work accounts for ~ 20-40% of the NIO generation, while energy by wave propagation and Parametric Subharmonic Instability (PSI) act as the additional sources. Seasonal mean diurnal tidal energy decreases from NCB to SCB, and larger southeastward energy fluxes occur between NCB and MCB. Both KE budget and time series of energy fluxes indicate a significant remote influence on the tidal energy through background current advection.