Patterns and mechanism of wintertime penetrating fronts in the East China Sea

Penetrating fronts play an important role in the cross-shelf transport of terrestrial materials in the East China Sea(ECS). Using long-term satellite remote sensing data and numerical simulation data, the most likely period of occurrence and region of the penetrating fronts in the western ECS are analyzed in this study, and the evolutionary process and mechanism are also investigated. The statistical results of satellite-derived chlorophyll data from 1998 to 2022(25 years) reveal that penetrating fronts occur most frequently near 27°N in winter, with the frequencies of occurrence in January, February and March being 47%,65% and 64%, respectively. Backward Lagrangian tracer experiments demonstrate that the penetrating water near 27°N originate from three different regions. The core penetrating low-salinity water originates from the Zhejiang-Fujian coast, while the northern marginal water originates from the northern part of Taiwan Island, and the southern marginal water originates from the central Taiwan Strait. Mechanism analysis reveals that the core penetrating low-salinity water evolves in three successive stages.First, under the influence of the Zhejiang-Fujian Coastal Current driven by strong northeasterly winds, the low-salinity water along the Zhejiang-Fujian coast moves southwestward parallel to the coastline and accumulates in the western Taiwan Strait.Second, during the relaxation stage of the northeasterly wind, the northeastward Taiwan Strait Current strengthens, and the lowsalinity water accumulated in the western Taiwan Strait moves to the northeast. Third, when the northeastward-moving lowsalinity water runs into the Western Kuroshio Branch off the northeastern Taiwan Island, the low-salinity water is rapidly stretched eastward, and a significant penetrating front eventually appears. Since the synoptic northeasterly wind and the Western Kuroshio Branch are dominant dynamic factors in this region during wintertime, we believe that the synoptic wintertime penetrating fronts near 27°N are common and important phenomena that play important roles in the cross-shelf transport of terrestrial materials in the ECS.