3.8.3.1 Western North Pacific

In the western North Pacific, long-term trends are masked by strong inter-decadal variability for 1960 to 2004 (Chan and Liu, 2004; Chan, 2006), but results also depend on the statistics used and there are uncertainties in the data prior to the mid-1980s (Klotzbach, 2006). Further increases in activity have occurred in the last few years after Chan and Liu (2004) was completed (Figure 3.40). Tropical cyclones making landfall in China are a small fraction of the total storms, and no obvious long-term trend can be discerned (He et al., 2003; Liu and Chan, 2003; Chan and Liu, 2004). However, Emanuel (2005a) and Webster et al. (2005, 2006) indicated that the typhoons have become more intense in this region, with almost a doubling of PDI values since the 1950s and an increase of about 30% in the number of category 4 and 5 storms from 1990 to 2004 compared with 1975 to 1989. The post-1985 record analysed by Klotzbach (2006) is too short to provide reliable trends.

The main modulating influence on tropical cyclone activity in the western North Pacific appears to be the changes in atmospheric circulation associated with ENSO, rather than local SSTs (Liu and Chan, 2003; Chan and Liu, 2004). In El Niño years, tropical cyclones tend to be more intense and longer-lived than in La Niña years (Camargo and Sobel, 2004) and occur in different locations. In the summer (JJA) and autumn (SON) of strong El Niño years, tropical cyclone numbers increase markedly in the southeastern quadrant of the western North Pacific (0°N–17°N, 140°E–180°E) and decrease in the northwestern quadrant (17°N–30°N, 120°E–140°E; Wang and Chan, 2002). In SON of El Niño years from 1961 to 2000, significantly fewer tropical cyclones made landfall in the western North Pacific compared with neutral years, although in Japan and the Korean Peninsula no statistically significant change was detected. In contrast, in SON of La Niña years, significantly more landfalls were reported in China (Wu et al., 2004). Overall in 2004, the number of tropical depressions, tropical storms and typhoons was slightly above the 1971 to 2000 median but the number of typhoons (21) was well above the median (17.5) and second highest to 1997, when 23 developed. Moreover, a record number of 10 tropical cyclones or typhoons made landfall in Japan; the previous record was 6 (Levinson, 2005). The ACE index was very close to normal for the 2005 season (Figure 3.40).