Characteristics And Mechanism Analysis Of Dry/wet Rhythms In Winter And Summer Precipitation Over The Mid-lower Reaches Of The Yangtze River Basin

As one of the most important bases for the climate prediction in China, the rhythm phenomenon has attracted considerable attention from scholars home and abroad about its formation process, forming reason and physical mechanism. Based on the monthly precipitation data in 160 China stations during 1951-2013 provided by NCC and the NCEP/NCAR reanalysis data, this paper reveals a significant rhythm phenomenon in winter and summer precipitation in the mid-lower reaches of the Yangtze River Basin, then analyzes the atmospheric circulation anomalies between dry and wet rhythm years, and gives a preliminary study and discussion to its possible physical mechanism. The main conclusions are as follows:(1) There is a significant rhythm phenomenon in winter and summer precipitation in the mid-lower reaches of the Yangtze River Basin, that is, if in winter there is less(more)precipitation than the normal over this region, precipitation in the following summer tends to be less(more) as well. The dry/wet rhythms phenomenon has provided an important precursory signal for the precipitation prediction during flood season in the mid-lower reaches of the Yangtze River Basin. In winter(summer) during the wet rhythm years,distribution of the atmospheric circulation anomalies in East Asia is similar to that in weak East Asian winter(summer) monsoon years, they both have a positive effect on the increasing precipitation in the mid-lower reaches of the Yangtze River Basin; on the contrary, during the dry rhythm years, distribution of the atmospheric circulation anomalies in East Asia is similar to that in strong East Asian winter(summer) monsoon years, they both have a negative effect on the increasing precipitation in the mid-lower reaches of the Yangtze River Basin.(2) The winter-summer dry/wet rhythms in the mid-lower reaches of the Yangtze River Basin are intimately associated with intensity of the winter and summer monsoon activities in East Asia; i.e., the wet rhythms usually correspond to both weaker winter and summer monsoons, while the dry rhythms to both stronger winter and summer monsoons. By analyzing the relationship between East Asian winter monsoon and surface temperature in the same and later time, it is showed that if there is strong East Asian winter monsoon, thetemperature in east Asian continent and eastern and southern ocean would be lower. Because of ocean’s characteristics of high specific heat and large capacity, its temperature remains,and when summer comes, there is a distinct land-sea thermal contrast, which strengthens East Asian summer monsoon. Contrarily, weak East Asian winter monsoon causes weak East Asian summer monsoon, and the high correspondence between East Asian winter and summer monsoons’ strength closely related to the dry/wet rhythms in winter and summer precipitation in the mid-lower reaches of the Yangtze River Basin.(3) The research shows that there is a good relationship between the winter plateau snow cover and the dry/wet rhythms in winter and summer precipitation in the mid-lower reaches of the Yangtze River Basin. By combining the existing research, it is showed that if there is more winter plateau snow cover, the temperature of the plain would be lower, and the heat source is weaker, leading to weak East Asian trough and weak East Asian winter monsoon, in that case that the mid-lower reaches of the Yangtze River Basin is charged by southerly,resulting more precipitation in that area. And the effect of the winter plateau snow cover could continue until the following spring and summer, leading to weak land-sea thermal contrast, which causes weak East Asian summer monsoon, so the mid-lower reaches of the Yangtze River Basin is still charged by southerly, resulting more precipitation in that area as well. Contrarily, the situation is opposite in the less winter plateau snow years, there are strong East Asian winter and summer monsoons, leading to less precipitation in the mid-lower reaches of the Yangtze River Basin and the dry rhythm.