Observed Variability of Bottom‐Trapped Topographic Rossby Waves Along the Slope of the Northern South China Sea. Issue 12 (26th November 2021) - Wang, Jinghong
Abstract: Strong deep‐current variabilities with periods of 8–25, 16–40, and 30–80 days observed in the northern South China Sea (NSCS) are interpreted as topographic Rossby waves (TRWs) based on the moored observations from July 25, 2017 to August 13, 2018. The TRWs present significantly spatial distribution in the frequency and intensity, which are largely controlled by the mesoscale perturbations in the upper layer. Longer‐period (30–80‐day) TRWs observed to the west of the Dongsha Islands were excited by locally enhanced perturbations with the periods of 30–80 days in the upper ocean. The bottom currents in the periods of 8–25 days observed on the slope of the northeastern South China Sea (NESCS) were influenced by upper‐layer mesoscale perturbations on their northeastern side through TRW propagation in the deep ocean in addition to local processes. In the Luzon Strait, deep currents with the periods of 16–40 days were associated with locally strong perturbations in the upper layer. The distribution of the bottom current variability was closely related to the spatial variability of the upper‐layer fluctuations. The 8‐25‐day bandpass‐filtered surface eddy kinetic energy (EKE) was overall maximal in the Luzon Strait and weakened westward to the east side of the Dongsha Islands, which explained why TRWs with the periods of 8–25 days were more energetic in the Luzon Strait. The surface fluctuations with the periods of 30–80 days were dominant in the west of the Dongsha Islands, and triggered the strong TRWs with the frequency band. Plain Language Summary: Topographic Rossby waves (TRWs) play an essential role in deep ocean circulation variability and potentially contribute to the abyssal mixing. Strong variations in the bottom currents with periods of 8–25, 16–40, and 30–80 days are interpreted as TRWs, explaining over 50% of the velocity variance in the northern South China Sea (NSCS). The TRWs present significantly spatial distribution in the frequency and intensity on the slope of the NSCS. The dominant periods of bottom currents varied geographically, with longer‐period (30–80 days) TRWs observed to the west of the Dongsha Islands and shorter‐period (8–25 days) TRWs active near the Dongsha Islands and in the Luzon Strait. Local perturbations from upper layers were thought to be the main energy source for the generation of TRWs. However, strong TRWs in the periods of 8–25 days were sometimes observed on the slope of the northeastern South China Sea when upper‐ocean fluctuations were weak. Based on a ray‐tracing model, distant surface fluctuations were identified as energy sources, impacting bottom currents through TRW propagation. These spatial patterns of TRWs are explained by the differences in surface fluctuations. Key Points: Observations reveal strong abyssal current variabilities in the northern South China Sea, which are interpreted as topographic Rossby waves High frequency topographic Rossby waves weaken westward from Luzon Strait; those with low frequency are strong to west of Dongsha Islands The spatial difference of upper‐layer fluctuation dominates the distribution of topographic Rossby waves