A Study On Statistical Characteristics Of Meiyu Turning From Typical To Atypical And Its Possible Causes

Abstract According to the climate zone and consistent Meiyu criterion provided by China Meteorological Administration, the precipitation characteristics of Meiyu in the Yangtze-Huaihe region and 4 climate zones are investigated. Using similarity method, the distribution of Meiyu can be divided into 8 types:the consistent wet(dry) type, southern wet(dry) northern dry(wet) type, southern and northern wet(dry) central dry(wet) type and eastern wet(dry) western dry(wet) type. In addition, the typical and atypical Meiyu have been defined by criterion of temperature, humidity and the frequency of rainy days, and its variation along climate warming are investigated The atmospheric circulation characteristics and the factors of influence in anomalous years are discussed with NCEP/NCAR Reanalysis data. The results show that:1. The precipitation characteristics are similar among south of the Yangtze, the upper Yangtze and the lower Yangtze region:Meiyu beginning date is delayed while ending date is in advance and the rainfall is reduced since 21th century. Besides the long-term evolution of rainy days and night rain tend to decrease remarkably and cloudy days tends to rise remarkably in reverse. However, the characteristics of Meiyu in Huaihe region are opposite. The contribution rate of light and moderate rain is declining while the rate of heavy rain, torrential rain and downpours are ascending in the whole of the Yangtze-Huaihe region.2. The consistent dry type is most likely to happen, its proportion over the last 53 years is 20.8%. Yet southern and northern dry central wet type and eastern wet(dry) western dry(wet) are rarely appeared. With global warming, the frequency of the consistent wet(dry) type decreases significantly, especially in recent 20 years.3. The typical degree of Meiyu has a weaken trend in south of the Yangtze, the upper Yangtze and the lower Yangtze region. Whereas the atypical degree is inclined to ascending, especially the high temperature and low humidity dry type which has the highest proportion of atypical Meiyu. On the spatial scale, the range of typical Meiyu has narrowed, by contrast, the range of atypical Meiyu has expanded. That means Meiyu is becoming less typical but more atypical in the 3 regions as the climate warms, especially in the last warmest 20 years. However, Meiyu in the Huaihe region has showed no atypical features between 1960 to 2012.4. In the consistent wet years, the west of South Asia high and is strengthened, the Ural blocking high and northeast China cold vortex are strengthened, the West Pacific subtropical high is strengthened and positioned by west, there are significant upward movement in the Jianghuai basin. In the consistent dry years, the east of South Asia high is strengthened, the polar vortex and high altitudes jam are weak, the East Asia major trough positioned by west and north, the West Pacific subtropical high is weak, there are significant downward movement in the the Yangtze-Huaihe basin. In the typical Meiyu years, the east of South Asia high is strengthened, the West Pacific subtropical high Westward become easy, the polar vortex is positioned by west, the sea of Okhotsk is blocking high, the West Pacific subtropical high is strengthened and positioned by west, the water vapor increases. In the atypical Meiyu years, the polar vortex is strengthened and larger, the sea of Okhotsk is low trough, the cold air can not be continuously transported to the Yangtze Huaihe Basin, the West Pacific subtropical high is positioned by east, the water vapor can not northward and is reduced.The Meiyu dry-wet or typical-typical condition can be affected by the anomalies of previous surface heat field in the SST near the central and eastern equatorial Pacific and Western Pacific Warm Pool through the atmospheric circulation situation.