Abstract: Against the backdrop of global warming, summer high sea temperature extremes have become more frequent and show an intensifying trend, posing serious threats to coral reefs, fisheries, and mariculture. The northeastern South China Sea continental shelf–Taiwan Strait (NESCS-TS) is a critical region for China’s marine economy and ecological security. Using multi-source satellite remote sensing data and high-resolution ocean–atmosphere reanalysis products, this study investigates the spatiotemporal distribution and interannual variability of the total number of summer marine heatwave (MHW) days in the NESCS-TS from 1993 to 2024, and reveals their association with ENSO. The results show that summer MHWs exhibit significant interannual modes, with the first three principal modes accounting for approximately 90% of the total variance. The first mode (~74% variance) features enhanced MHW activity over the offshore region of the northeastern South China Sea coast, with high-value areas mainly covering the eastern Taiwan Strait and the offshore waters to its south. This mode is closely linked to ENSO (represented by the preceding winter ONI index), and El Niño in particular may significantly influence the occurrence of summer MHWs during the decaying phase, with an 84% increase in the Taiwan Strait and a 171% increase across the entire study region. The second mode (~12% variance) is characterized by enhancement over the northeastern South China Sea continental shelf–Taiwan Strait, and the third mode (~4% variance) by enhancement over the Taiwan Strait alone. Global warming has extended the interannual signal of more frequent MHWs toward the northeastern South China Sea shelf coast and the western Taiwan Strait. Under El Niño conditions, an anomalous high-pressure system and anticyclonic circulation are induced over the northeastern South China Sea, which increase net heat flux and, together with a shallowing thermocline, promote MHW occurrence in the region. The influence of the Western North Pacific Subtropical High (WNPSH) is reflected more in its role in persistent maintenance and intensification (rather than positional migration such as westward extension) on interannual timescales. Meanwhile, mixed-layer heat budget analysis reveals that ENSO-related MHW occurrence in the Taiwan Strait—especially in its eastern part—is dominated by net heat flux and advection, whereas that over the northeastern South China Sea continental shelf is dominated by mixing/diffusion and net heat flux. In the region west of the Luzon Strait, MHWs are primarily governed by oceanic processes such as advection and mixing/diffusion, with the Kuroshio influencing MHW occurrence mainly through a looping current pattern.

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