Decadal Change in Tropical Cyclone Activity in the Western Pacific

In view of the concern over of global warming and the spate of destructive tropical cyclones in recent months, there has been interest in finding a link between the two phenomena. Recently published papers (Emanuel 2005, Webster et al. 2005) showed that disturbances in the Atlantic and the Western Pacific have become significantly stronger, possibly as a result of rising sea surface temperatures resulting from global warming. However, a review by Pielke et al. (2005) emphasized that natural variability in tropical cyclones makes it difficult to establish any such connection.

Even without such a link, there is evidence that tropical cyclones are changing in other ways. Past studies have shown a rise in the annual number of tropical cyclones appearing in the Western Pacific (Chu and Clark 1999) although specific regions may exhibit a different trend (Yeung et al. 2005). Chan and Liu (2004) found that local sea surface temperatures, which have been steadily increasing with little variation over the past 40 years, were not significantly correlated to seasonal typhoon activity during the same period even if more tropical cyclones tend to appear during a positive SST anomaly. Ho et al. (2004) found a westward shift in tracks between 1980 to 2000 compared to the typical tracks that preceded this period, possibly related to the expansion of the north Pacific subtropical high.

One topic that has not yet been addressed is the changing response of tropical cyclones tracks in the Western Pacific region to the influences of the El NiƱo-Southern Oscillation (ENSO) phenomenon. The ENSO has been demonstrated to be associated with anomalies in tropical cyclone frequency and behavior in the Western Pacific (Camargo and Sobel 2004, Wu et al. 2004, Liu and Chan 2002, Wang and Chan 2002, Saunders et al. 2000). Variations in the character of these anomalies over the decades may reveal details on coming trends in tropical cyclone activity.

[Presented at the Okinawa Typhoon Center Forum, 1 October 2005, Okinawa, Japan.]