Summertime sea surface temperature and salinity fronts in the southern Taiwan Strait

Based on hydrographic survey and satellite data, three fronts were observed in the southern Taiwan Strait in summer. They are the Taiwan Bank Front (TBF), the Southwest Coastal Upwelling Front (SCUF), and the Pearl River Plume Extension Front (PRPEF). In addition to the temperature aspect of TBF and SCUF as indicated in previous studies, we find that TBF and SCUF could also be identified according to salinity. The TBF is closely related to the Taiwan Bank upwelling, tidal mixing, and the Pearl River Plume Extension. Different hydrographic conditions on the southern and northern sides of this front lead to a south–north asymmetric structure of the TBF. The relatively small Simpson–Hunter number around the Taiwan Bank indicates that the TBF may be a tidal front. The SCUF separates the wind-driven cold, saline coastal upwelling water from the warm, less saline offshore water. Owing to frontal instability, SCUF exhibits both short temporal (several days) and small spatial (several kilometres) scales, indicative of intense sub-mesoscale processes. Nonetheless, the weak summertime SCUF was revealed in the log-transformed satellite frontal map. Finally, apart from the commonly observed dominant PRPEF in summer, a bifurcation from the PRPEF was identified in most hydrographic sections. Once generated, this bifurcation is carried by the topography-following current away from the PRPEF.