Typhoon Kai-Tak: An Ocean's Perfect Storm

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.