Winter fronts in the East China Sea (ECS) are identified using twenty-five years (1995-2020) of satellite observations of the sea surface temperature (SST). This paper focuses on a "U" shaped front in the East China Sea shelf. Compared to the literature, the "U" shaped front is better defined because of longer time scale and higher resolution data, and it is regarded as a composite front with obvious interannual variations in the position and intensity. The position of the front shows a trend towards onshore and offshore in different years. Moreover, the interannual variations of position and intensity have good correlations with the transport of the Taiwan Warm Current, Tsushima Warm Current and Yellow Sea Warm Current. Similarly, the cold water mass inside the front also has a great influence on its interannual variations. In summer, this front disappears from the surface due to warming of the shelf water. However, the cold water inside is preserved and exists in the middle and deep layers of the northern East China Sea, which can affect the stratification through the summer. The volume of the cold water mass in summer is determined not only by the volume of cold water mass from the previous winter, but also by the flow of warm currents in spring and the intrusion of subsurface Kuroshio water into the East China Sea shelf. The larger summer cold water mass tends to cause the rise of the thermoclin, but there is no strong correlation between its strength and thickness.

Composite front,East China Sea,cold water mass,stratification