Wintertime fronts in the East China Sea (ECS) are identified from 26 years (1995-2020) of satellite-derived sea surface temperature (SST) gradients using an edge-detection algorithm. We focus on a U-shaped compound front in the northern ECS that exhibits significant interannual variations in position and intensity. Our analyses demonstrate that the interannual variations are influenced by large-scale climate factors, including the El Niño–Southern Oscillation (ENSO) and the East Asian Winter Monsoon (EAWM). In addition, the frontal interannual variations correlate with the transport of the Taiwan Warm Current, the Tsushima Warm Current, and the Yellow Sea Warm Current. An intensification of the Taiwan Warm Current and the Tsushima Warm Current strengthens the front, whereas a stronger Yellow Sea Warm Current suppresses the development of the interior cold water mass, thereby weakening the frontal intensity. These currents also contribute to shifts in the frontal position through their variability in transport and pathway. Furthermore, the cold-water mass embedded within the frontal zone exerts additional control on the front, as its volume and thermal properties can modulate both the front's strength and its interannual variability. In summer, the surface manifestation of the front disappears as the shelf waters warm, yet the cold water persists in the mid and deep layers of the northern ECS. The summertime volume of the cold-water mass is not only influenced by the cold-water volume in the preceding winter but also by the heat transport of the warm currents in the spring. This residual cold-water mass affects stratification throughout the summer: a more substantial summer cold-water mass weakens the thermocline, reduces the mixed layer thickness, and increases the thermocline thickness.

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