Trends in Marine Cold-Spells across the Taiwan Strait and Taiwan’s Coral Habitats Observed Using Multi-Satellite Data

Despite intensified global warming, the frequency of Marine Cold Spells (MCS) in the Taiwan Strait and adjacent coral habitats shows a declining trend, whereas both the intensity and duration of these events appear to be increasing. Long-term statistics reveal that between 1985 and 1994, MCS events in the Taiwan Strait occurred frequently, with the southwestern region exhibiting particularly high intensities and prolonged durations. From 1995 to 2004, the number of such events decreased markedly, accompanied by a more dispersed spatial distribution. Subsequently, from 2005 to 2014, MCS events became concentrated along the coast of China, with both higher intensities and longer durations. However, data from 2015 to 2023 indicate a further decrease in the number of MCS events off the coast, yet with increased event intensity and extended duration. These patterns suggest that global warming has not entirely suppressed the occurrence of MCS; in fact, more rapid and pronounced cooling is observed in certain shallow shelf areas. In recent years, the Penghu Channel and the Taiwan Banks have continued to experience around three to four MCS events annually. Although the total number of events is comparable to previous years, the intensity in 2023 notably surged, reaching a maximum of -4.7°C. Meanwhile, the three major tropical and subtropical coral habitats—Green Island, Nanwan Bay, and Dongsha Island—have recorded fewer MCS events in recent years, yet several have coincided with cold wakes generated by typhoons or with winter monsoon systems, leading to short-lived but severe drops in sea temperature. For example, during 2021, a strong northeastern monsoon triggered nearly three weeks of cold-water intrusion into Nanwan Bay, with the maximum intensity reaching -1.7°C. Green Island and Dongsha Island generally experience about one week of cold spells under either summer typhoon activity or winter monsoon conditions. A synthesis of historical data suggests that the intensification of the Taiwan Strait’s current system and the strengthening of winter surface winds may both contribute significantly to the onset and persistence of MCS. Furthermore, large-scale climate oscillations—including the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Arctic Oscillation (AO)—collectively modulate winter sea surface temperatures in this region. Overall, although the frequency of MCS appears to have slightly declined under warming conditions, there is a continuing increase in the intensity and duration of these cold spells. At present, the Taiwan Strait region is witnessing dual extremes, characterized by marine heatwaves in summer and marine cold-spells in winter.