East Asian marginal seas surrounding China exhibit rich ocean upwelling, mostly in response to the southwesterly summer monsoon. Upwelling in the China seas, namely, the South China Sea, the Taiwan Strait, the East China Sea, the Yellow Sea, and the Bohai Sea, has become increasingly important because the potential changes in the upwelling may have dramatic ecosystem, socioeconomic, and climate impacts. This paper reviews the progress of upwelling studies in the China seas since the year 2000, by presenting the principal characteristics and new understanding of 12 major upwelling regions in the China seas. Upwelling exhibits long‐term variability at intraseasonal to multidecadal scales as well as short‐term variability frequently caused by tropical cyclones. It is also associated with the El Niño–Southern Oscillation, local environmental variation, and biogeochemical factors. The coastal upwelling around Hainan Island and the upwelling or cold dome northeast of Taiwan Island are specifically highlighted because they have attracted great interest for decades. This paper summarizes upwelling mechanisms in terms of wind, topography, tide, stratification, and background flow, with applications mostly to the China seas. Finally, we propose some topics for future upwelling research, i.e., potential intensification of coastal upwelling under global climate change, downwelling, intrusion of upwelling into coastal embayments, and the influence of upwelling on fishery and biogeochemical processes.

Both observational and reanalysis sea surface temperature data reveal that upwelling occurs frequently off the northeastern coast of Hainan Island (downstream of the change in topography off Tongluo Cape), which cannot be attributed to the along‐shelf wind alone. To identify dynamics of the upwelling, we conduct a numerical experiment using an idealized topography that is simplified from the actual topography off the eastern and northeastern coasts of Hainan Island. The result indicates that the upwelling downstream of the change in topography is associated with onshore cross‐isobath transport. Analysis of the vertically integrated momentum balance shows that the upwelling‐linked onshore transport is primarily intensified by the along‐isobath barotropic pressure gradient force (PGT), but is weakened by the along‐isobath baroclinic pressure gradient force (PGC). The along‐isobath PGT is linked to the advection of relative vorticity, the bottom stress curl and the gradient of momentum flux in vorticity equation. On the other hand, the PGC‐related process is diagnosed by potential vorticity (PV) balance. Similar to the negative PV term from wind stress, the negative PV terms of the joint effect of baroclinicity and relief and the baroclinic bottom pressure torque weaken the upwelling‐linked onshore transport downstream of the change in topography. The onshore transport is enhanced by the positive PV from bottom stress. In addition, the cross‐isobath forces play an important role in upwelling intensification in the shallow nearshore region.

Given their ephemeral nature, eddies have proven difficult to study, with contrary results from field observations that typically sample at the center during a specific stage of an eddy lifespan. Using the natural occurring radionuclide ²³⁴Th, we examined particle fluxes within two cyclonic eddies (CEs) at different evolutionary stages (mature stage eddy, C2, and a decay stage eddy, C1) in the oligotrophic western South China Sea (SCS). Using a 1‐D steady state model, ²³⁴Th derived particulate organic carbon (POC) and opal fluxes at 100 m were determined. Both POC and opal fluxes were significantly higher at the eddy edges relative to the eddy cores, with integrated eddy POC and opal fluxes of 6.2 (2.1) mmol C m⁻² day⁻¹ and 1.5 (0.28) mmol Si m⁻² day⁻¹ in C2 (C1). When compared to non‐eddy regimes, both POC (by 2.6‐fold) and opal fluxes (by 7.5‐fold) in C2 were enhanced; they were reduced and decoupled (0.9‐fold and 1.4‐fold) in C1. The difference in enhancement of particle fluxes and the coupling/decoupling between POC and opal flux likely reflects changes in phytoplankton community structures resulting from eddy evolution. Scaling these results to the entire SCS basin suggests that CEs contribute 15% of the opal flux. Therefore, CEs may regulate the biogeochemical cycling of silica to a much greater extent than carbon in the ocean.