Persistent, Energetic Bottom Trapped Topographic Rossby Waves Observed in the Southern South China Sea

Strong diapycnal mixing in the South China Sea (SCS) is usually thought as the result of breaking internal waves associated with the complex topography. However, the presence of Topographic Rossby Waves (TRWs) in the SCS has not been investigated, and their roles in the SCS deep circulation and diapycnal mixing are still unknown. Energetic fluctuations with periods of 9-14 days are found from five years’ direct measurements below 1400 m in the southern SCS, which are interpreted as TRWs as they obey the dispersion relation. Persisted from May 2009 to August 2013, the bottom current speed of TRWs with maximum value of ~10 cms^-1 was one order larger than the mean current and comparable to the tidal currents at bottom. The bottom-trapped TRWs had an approximately trapping depth of 325 m and a referred wavelength of ~82 km, which were likely excited by eddies above. The frequency peak of current at the period of ~2 months in upper layers could offer the energy sources for the persistent TRWs in the southern SCS. The energetic bottom TRWs may have a comparable role to tides in the deep circulation, contributing to the abyssal diapycnal mixing in the area with complex topography.