Origin of the springtime South China Sea Warm Current in the southwestern Taiwan Strait: Evidence from seawater oxygen isotope

Oxygen isotope (δ18O) of seawater is an excellent proxy for tracing the origins of water masses and their mixing processes. Combining with hydrographic observation, hybrid coordinate ocean model (HYCOM) analysis data, and seawater oxygen isotope, we investigated the source of the South China Sea Warm Current (SCSWC) in the southwestern Taiwan Strait and its underlying mechanism. Results show that the Kuroshio subsurface water (KSSW) can intrude the continental slope in the southwestern Taiwan Strait, and thereby climb up the continental slope coupled with upwelling. The δ18O-salinity relationship further indicates that in spring, the SCSWC in the southwestern Taiwan Strait originates from the upslope deflection of the slope current formed by the KSSW intrusion into the South China Sea, rather than from the west segment of the SCSWC formed to the east of Hainan Island. In addition, the southward flowing Zhe-Min Coastal Current (ZMCC) can reach as far as the Taiwan Bank (TB) and deflects offshore over the western TB at approximately 23.5°N, to some extent affecting the SCSWC. Moreover, this study reveals that seawater δ18O is exquisitely sensitive to the determination of the origin and transport of water masses as compared with traditional potential temperature-salinity plot (θ-S) and HYCOM analysis data. In addition, their coupling can more reliably interpret the mixing processes of shelf water masses.