The variation of the Asian summer monsoon in warmer climates

In recent three decades, North and Northeast China have suffered from severe and persistent droughts while the Yangtze River basin and South China have undergone much more significant heavy rainfall/floods events. This long-term change in the summer precipitation and associated large-scale monsoon circulation features have been examined by using about 123-yr (1880–2002) records of precipitation in East Asia. One dominating mode of the inter-decadal variability of the summer precipitation in China is the near-80-yr oscillation. Then, on this basis, a possible explanation of this long-term change in relation to significant weakening of the Asian summer monsoon, possibly due to the abrupt increase in the preceding winter and spring snow over the Tibetan Plateau and warming of the sea surface temperature in tropical central and eastern Pacific since about 1978,has been set forward. But we cannot answer whether the anthropogenic forcing has caused the changes of patterns of rainfall, floods/droughts in China, and East Asian monsoon. Natural fluctuation of climate change can also play an important role.

For the next century it seems that the major rainfall belts would move northward by about 2040, but unstable. Afterwards, the summer precipitation in North China would increase considerably and stably. Furthermore, this anthropogenically-driven precipitation shift would appear to be consistent with the occurrence of rainfall peak period caused by the natural near-80-yr cycle.

The present study has further shown that the differential increases of the upper-tropospheric temperature over the tropical Indian Ocean (TIO) and Tibetan Plateau (TP) lead to the changed relationship between the South Asian summer monsoon (SASM) intensity and tropospheric thermal contrasts over the SASM regions in a GHG-induced warmer climate. The weakening of the SASM circulation is directly related to the decrease of upper-tropospheric TP-TIO thermal contrast, which in turn is caused by the larger upper-tropospheric warming over the TIO than over the TP. The fact that the SASM weakens as the lower-tropospheric thermal contrast increases in the 21st century implies a smaller role of this thermal contrast in determining the SASM intensity than that suggested by previous studies for the 20th century. Therefore, in the future warm climate, the SASM is likely to have a different trend (weakening) from the Asian summer monsoon (increasing).