An unusually intense sea surface temperature drop (ΔSST) of about 10.8°C induced by the Typhoon Kai-Tak is observed in the northern South China Sea (SCS) in July 2000. Observational and high-resolution SCS model analyses were carried out to study the favorable conditions and relevant physical processes that cause the intense surface cooling by Kai-Tak. Upwelling and entrainment induced by Kai-Tak account for 62% and 31% of the ΔSST, respectively, so that upwelling dominates vertical entrainment in producing the surface cooling for a subcritical storm such as Kai-Tak. However, wind intensity and propagation speed alone cannot account for the large ΔSST. Prior to Kai-Tak, the sea surface was anomalously warm and the main thermocline was anomalously shallow. The cause was a delayed transition of winter to summer monsoon in the northern SCS in May 2000. This produced an anomalously strong wind stress curl and a cold eddy capped by a thin layer of very warm surface water west of Luzon. Kai-Tak was the ocean’s perfect storm in passing over the eddy at the “right time,” producing the record SST drop and high chlorophyll-a concentration.

The northward-flowing Kuroshio often intrudes westward and modulates the water masses of the South and East China Seas. These intrusions transcend multiple scales in time and space, which we demonstrate here using various independent data sets. There are two hot spots of intrusion, one in the Luzon Strait and the other off northeast Taiwan, which occur synchronously when the upstream Kuroshio weakens during winter. Beyond seasonal time scales, the two intrusions were not synchronous during 1993–2013. While intrusions into the South China Sea echoed the Pacific Decadal Oscillation, the intrusion northeast of Taiwan decreased markedly before 2002 but regularly reached the shelf thereafter. This change was due to the influence of westward impingements of cyclonic eddies from the open ocean on the Kuroshio main stream in place of anticyclonic eddies. During 1993–2001, decreasing cyclonic eddy impingements moved the Kuroshio away from northeast Taiwan, weakening the Kuroshio intrusion onto the East China Sea shelf. Thereafter, enhanced cyclonic eddy impingement during 2002–2013 weakened the Kuroshio transport, moving it closer to the shelf and enhancing its intrusion into the East China Sea.