摘要
With the methods of correlation and composite analyses, Ju et al. discussed the relationship between low-frequency oscillation in the summer monsoon region in East Asia and droughts/floods in the middle and lower reaches of the Yangtze River and found that strong monsoon years usually cause more rain in the region. Studying the interdecadal variation of precipitation in three rain zones over south China, the valley of Yangtze River and north China, Tan et al. showed that mid- and higher- latitude circulation and SST have important effect on the precipitation associated with summer monsoon in east China. With data of north Pacific SST and characteristic subtropical high, Cai et al.discovered that the anomalies of precipitation distribution in rainy seasons were directly affected by the activity of the subtropical high. As shown in an observational study, anti-correlation was found between the rainfall in the middle and lower reaches of the Yangtze River during the Mei-yu season and the frequency of TCs in northwest Pacific, including the South China Sea, in the prime of summer . The less active maritime TCs are, the more southward the westerly zone is over the Asian mid- latitudes. It is favorable for the Southwest Monsoon to transport moisture towards the region the Yangtze and Huaihe Rivers and for frontal rains to stagnate over the middle and lower reaches of the Yangtze, increasing the rainfall during Mei-yu. Otherwise, the mid-latitude westerly is located more northward, frontal rains develop north of the Yangtze River, the Mei-yu season shortens and rainfall decreases. Statistically studying the effect of South China Sea TCs and directionturning TCs on the Mei-yu season from 1980 to 1991, Kang et al.showed that 85% of the TCs had significant effect on Mei-yu and 35% of them would cause it to end. In addition, the Chinese meteorologists have long noted the cut-off role of TCs in the transportation of moisture by Southwest Monsoon to the Mei-yu fronts in the Yangtze and Huaihe Rivers. They found that TCs active offshore China would usually take moisture from the Bay of Bengal and cut off the transportation of moisture and transformation of energy for the frontal areas of Mei-yu, weakening, or even break and end the precipitation （like TC 9106, or Zeke）, but there were few TCs over offshore waters or even the west Pacific when Mei-yu was active. As shown in a numerical study, the disturbance of Zeke triggered a horizontal flow field at lower troposphere that weakened the Southwest Monsoon and further decreased its moisture transportation to the region of Yangtze and Huaihe Rivers and the convergence of moisture flux there. At the same time, the typhoon had indirect impact on the Mei-yu through its effect on the subtropical high.
基金
Natural Science Foundation of China (40505011)
Project of Shanghai Science andTechnology Committee (05DZ05815)
