Propagation of Topographic Rossby Waves in the Deep Basin of the South China Sea Based on Abyssal Current Observations.

Topographic Rossby waves (TRWs) are oscillations generated on sloping topography when water columns travel across isobaths under potential vorticity conservation. From our large-scale observations from 2016 to 2019, near-65-day TRWs were first observed in the deep basin of the South China Sea (SCS). The TRWs propagated westward with a larger wavelength (235 km) and phase speed (3.6 km day−1) in the north of the array and a smaller wavelength (80 km) and phase speed (1.2 km day−1) toward the southwest of the array. The ray-tracing model was used to identify the energy source and propagation features of the TRWs. The paths of the near-65-day TRWs mainly followed the isobaths with a slightly downslope propagation. The possible energy source of the TRWs was the variance of surface eddies southwest of Taiwan. The near-65-day energy propagated from the southwest of Taiwan to the northeast and southwest of the array over ~100–120 and ~105 days, respectively, corresponding to a group velocity of 4.2–5.0 and 10.5 km day−1, respectively. This suggests that TRWs play an important role in deep-ocean dynamics and deep current variation, and upper-ocean variance may adjust the intraseasonal variability in the deep SCS. Significance Statement: Topographic Rossby waves (TRWs) play an important role in deep-ocean dynamics and deep current variation, and they have potential impacts on sediment transport and abyssal biogeochemical processes. From large-scale observations of abyssal currents in the deep basin of the South China Sea (SCS), energetic near-65-day TRWs with a westward phase speed of 1.2–3.6 km day−1 were first observed. The ray-tracing model was used to identify the energy source and propagation features of the TRWs. The possible energy source was the variation of mesoscale eddies southwest of Taiwan. The study has implications for adjustments to the energy and intraseasonal variability generation in the deep basin of the SCS. [ABSTRACT FROM AUTHOR]