Meiyu of the Huaihe Basin in Recent 52 Years

Climate data show that the Meiyu period exists not only in the mid lower Yangtze Basin (MLYB), but also prevails in central southern part of the Huaihe Basin. Then using the daily rainfall data of 5 stations evenly distributed in the latter region with the consideration of seasonally moving process of the pentad ridge's line of west Pacific high, Meiyu periods of the Huaihe Basin (HB) in recent 52 years (1953—2004) are delimited. They are very significant as the same exist in the MLYB, their average onset and ending dates are June 25 and July 21, later than the average Meiyu periods of MLYB by 5 days and 7 days respectively. However the inter annual changes of the HB Meiyu are large, the range of onset days reaches 41 days and the amplitude of ending dates approaches 2.5 months in the recent 52 years. In 8 years (occupying 15.4%), the HB Meiyu are absent, but plenty numbers of the HB Meiyu(exceeding the average value of more than 20%)also occur in 18 years (occupying 34.6%). As early Meiyu period (EMP) has appeared in MLYB, it also occurs in Huaihe Basin during 5 years, they arethe certain kind of Meiyu periods appearing abnormally early in May, however after they end, the summer monsoon rain belts of east China (SMRBEC) retreat to south un til the beginning of the main Meiyu period in June/July. The inter annual change of the HB Meiyu plays a key role which causes the summer trend of whether drought or flood in these regions. Most anomalies of the Meiyu amount both in the Huaihe Basin and in the MLYB have the same sign; however the opposite signs to each other have appeared in 1/4 of the cases. The inter annual changes of the HB Meiyu have enhanced since late 1970s, such abrupt change of the HB Meiyu means the weak condition with earlier ending dates in former stage abruptly transforms into a new stage around 1979, having the following characteristics:higher amount of HB Meiyu with later ending dates accompanied by strong short waves of 2.2—2.3 years. The average HB Meiyu amount of the latter 26 years since 1979 is higher than the former 26 years by nearly 30%, such contrast is especially significant between the following 2 stages:the anomalous weak HB Meiyu in 1958—1966 and the anomalous plenty HB Meiyu in 1979—1987. From the point of atmospheric circulation, it is found that the July coastal trough (at 500 hPa level) in the mid latitudes of East Asia (JCTEA) tends to move southward more frequently since 1979, then the west Pacific high and its northern SMRBEC retreat accompanied by the delay of HB Meiyu and the higher amounts. Such JCTEA has bigger influence on the HB Meiyu and its ending dates than those of either Okhotsk high or Ural high. First analysis shows that the abrupt change since 1979 may be closely associated with many effects of anthropogenic activities in different scales, such as the enhanced global warming, the increasing smoke and dusts over central eastern China due to the acceleration of industrialization etc.