Evolution and dynamics of a summertime penetrating front off the Zhejiang-Fujian coast, China

Penetrating fronts are frequently observed off the Zhejiang-Fujian coast, but their life cycles are poorly understood because of the lack of time series data. In this study, Geostationary Ocean Color Imager data are used to describe the complete evolutionary process of a penetrating front, and the impacts of wind and circulation on evolution are explored. Based on the horizontal coverage and the front of the penetrating water, the evolutionary process is divided into three stages of development, maturation and decay. During the development stage, the area of the penetrating water increases rapidly with eastward extension, and the penetrating front is well connected to the Zhejiang-Fujian coastal front in the southwestern area. During the maturation stage, the penetrating water continues to extend eastward at a low speed and finally arrives at the Kuroshio area on the continental slope. Moreover, the southwestern part of the penetrating front is gradually separated from the coastal front. During the decay stage, the coverage of the penetrating water decreases rapidly, and the penetrating front disappears first in the south. In this stage, the penetrating front is rarely moves east but completely separated from the coastal front south of 30°N. Dynamic analysis suggests that the typhoon-induced southward extension of the Changjiang Diluted Water off the Zhejiang-Fujian coast is an important precondition for the generation of the penetrating front. The eastward extension of the penetrating front during the development stage is mainly driven by a southwesterly wind, while in the maturation stage, it is affected by both the wind and the offshore branch of the Taiwan Warm Current. The detachment of penetrating water from coastal water is caused by the inshore branch of the Taiwan Warm Current. Numerical tracer experiments confirm the wind and Taiwan Warm Current impacts on the eastward extension and detachment of penetrating water. In addition, the Kuroshio frontal eddy may play an important role in the dissipation of the penetrating front during the decay stage.