The MMD-A1B models project an increase in precipitation in East Asia in all seasons. The median change at the end of the 21st century is +9% in the annual mean with little seasonal difference, and a large model spread in DJF (Table 11.1). In winter, this increase contrasts with a decrease in precipitation over the ocean to the southeast, where reduced precipitation corresponds well with increased mean sea level pressure. While the projections have good qualitative agreement, there remain large quantitative differences among the models, which is consistent with previous studies (e.g., Giorgi et al., 2001a; Hu et al., 2003; Min et al., 2004).
Based on the MMD models, Kimoto (2005) projects increased Meiyu-Changma-Baiu activity associated with the strengthening of anticyclonic cells to its south and north, and Kwon et al. (2005) show increased East Asia summer precipitation due to an enhanced monsoon circulation in the decaying phase of El Niño. A 20-km mesh AGCM simulation shows that Meiyu-Changma-Baiu rainfall increases over the Yangtze River valley, the East China Sea and western Japan, while rainfall decreases to the north of these areas mostly due to the lengthening of the Meiyu-Changma-Baiu (Kusunoki et al., 2006). Simulations by RCMs support the results from AOGCMs. For example, Kurihara et al. (2005) show an increase in precipitation over western Japan in summer.
Kitoh and Uchiyama (2006) investigated the onset and withdrawal times of the Asian summer rainfall season in 15 MMD simulations (Figure 11.10). They find a delay in early summer rain withdrawal over the region extending from Taiwan to the Ryukyu Islands to the south of Japan, but an earlier withdrawal over the Yangtze Basin, although the latter is not significant due to large inter-model variation. Changes in onset dates are smaller.
Figure 11.10. (a) The ensemble mean change in withdrawal date of the summer rainy season between the MMD-A1B projections in 2081 to 2100 as compared with the 1981 to 2000 period in the 20C3M simulations. A positive value indicates a later withdrawal date in the A1B scenario. Units are 5 days. (b) Fraction of the models projecting a positive difference in withdrawal date. (Kitoh and Uchiyama, 2006).
Yasunaga et al. (2006) used a 5-km mesh cloud-resolving RCM to investigate summer rainfall in Japan. They find no changes in June rainfall but increased July rainfall in a warmer climate. The increase in July can be attributed to the more frequent large-precipitation systems.
Intense precipitation events are very likely to increase in East Asia, consistent with the historical trend in this region (Fujibé et al., 2005; Zhai et al., 2005). Kanada et al. (2005) show, using a 5-km resolution RCM, that the confluence of disturbances from the Chinese continent and from the East China Sea would often cause extremely heavy precipitation over Japan’s Kyushu Island in July in a warmer climate. An increase in the frequency and intensity of heavy precipitation events also occurs in Korea in the long RCM simulation of Boo et al. (2006). Similarly based on RCM simulations, Y.L. Xu et al. (2005) report more extreme precipitation events over China. Gao et al. (2002) find a simulated increase in the number of rainy days in northwest China, and a decrease in rain days but an increase in days with heavy rain over South China. Kitoh et al. (2005) report similar results in South China from an AOGCM simulation.
Kimoto et al. (2005) suggest that the frequencies of non-precipitating and heavy (≥30 mm day–1) rainfall days would increase significantly at the expense of relatively weak (1–20 mm day–1) rainfall days in Japan. Mizuta et al. (2005) find significantly more days with heavy precipitation and stronger average precipitation intensity in western Japan and Hokkaido Island. Hasegawa and Emori (2005) show that daily precipitation associated with tropical cyclones over the western North Pacific would increase.
The previously noted weakening of the East Asian winter monsoon (e.g., Hu et al., 2000) is further confirmed by recent studies (e.g., Kimoto, 2005; Hori and Ueda, 2005).