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Using observational and reanalysis data sets, we investigated interannual variability in the Kuroshio in the East China Sea (ECS‐Kuroshio). We exhibited that the surface velocity and position of the ECS‐Kuroshio were synchronized on a quasi‐3‐year interannual timescale during 2005–2016. We further demonstrated that: (a) during 2005–2016, wind stress curl variability related to the El Niño–Southern Oscillation (ENSO) played a leading role in the interannual ECS‐Kuroshio variability by exciting baroclinic Rossby waves along the subtropical countercurrent (STCC) zone east of Taiwan; (b) mesoscale eddy activities in the STCC zone, especially long‐lived (≥150‐day lifetime) cyclonic eddies, probably played a secondary role in the interannual ECS‐Kuroshio variability. In addition, we showed that the occurrence of the quasi‐3‐year interannual variability of the ECS‐Kuroshio since 2005 was likely linked to the following changes in the ENSO‐related atmospheric circulation: (a) The primary ENSO timescale changed from a quasi‐5‐year period in 1993–2004 to a quasi‐3‐year one in 2005–2016; (b) Over the central equatorial Pacific along with the eastern tropical Indian Ocean, the sea surface temperature in 2005–2016 varied with a more intense amplitude than in 1993–2004, which resulted in a different western North Pacific atmospheric response to the ENSO in 2005–2016 from that in 1993–2004. Plain Language Summary: The Kuroshio, which serves as the western boundary current of the North Pacific subtropical gyre, is one of the most important elements that control the oceanic and atmospheric conditions around the East China Sea (ECS). In this study, we found that the surface velocity and position of the ECS‐Kuroshio were synchronized on a quasi‐3‐year interannual timescale during 2005–2016. We further determined that, during 2005–2016, baroclinic Rossby waves along the subtropical countercurrent (STCC) zone east of Taiwan, which were excited by wind forcings related to the El Niño–Southern Oscillation (ENSO), played a leading role in the interannual ECS‐Kuroshio variability, and mesoscale eddy activities in the STCC zone probably played a secondary role. Moreover, we suggested that, since the mid‐2000s, the change of the primary ENSO timescale and the amplification of the sea surface temperature variability in the tropical ocean have led to the occurrence of the quasi‐3‐year interannual variability of the ECS‐Kuroshio through the western North Pacific atmospheric response to the ENSO. Key Points: Kuroshio in the East China Sea varied on a quasi‐3‐year interannual timescale during 2005–2016El Niño–Southern Oscillation‐related atmospheric circulation played a leading role in this interannual variability by exciting baroclinic Rossby wavesMesoscale eddy activities, especially long‐lived (≥150‐day) cyclonic eddies, probably contributed secondarily to this interannual variability [ABSTRACT FROM AUTHOR]

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]