Study On The Characteristics Of The Drought-Flood Abrupt Alternation And Its Atmospheric Circulation During The Flood Season In South China

Based on the daily mean precipitation data for the period 1960-2015 from 412 meteorological stations in eastern China and NCEP/NCAR monthly mean and daily reanalysis,the study is performed of the time variation,spatial distribution and mutation characteristics of precipitation during the flood season(April-September)in South China.And then we investigate the annual change,abnormal distribution of precipitation and its atmospheric circulation characteristics of long-term drought-flood abrupt alternation during flood season in South China,and discuss the decadal change,abnormal distribution of precipitation and its atmospheric circulation characteristics of short-term drought-flood abrupt alternation during flood season in South China by defining a evaluation index of short-term drought-flood abrupt alternation.The main conclusions are as follows:(1)The distribution of precipitation during the flood season in South China is roughly bimodal,and drought-flood abrupt alternation are more likely to occur in the flood season.The total amount of precipitation during the flood season in South China changed significantly from decreasing to increasing in 1992.(2)During the flood season from 1960 to 2015,the frequency of the long-term(drought and flood of about 3 months)drought-flood abrupt alternation in Southern China decreases first and then increases.In the long-term drought-to-flood years,during the pre-flood season(April-June),the West Siberian Ridge and East Asian Trough weaken,so the meridional circulation weakens in the mid-high latitudes,which is not conducive to the northern cold air to the south;During the post-flood season(July-September),the Western Pacific Subtropical High(WPSH in short)jumps to the north and locates eastward to its normal position eastward,and Central Siberian Ridge and Okhotsk Trough strengthen,so the meridional circulation enhances in the mid-high latitudes,which is conducive to the northern cold air to the south,and the trough in the eastern side of the Sichuan Basin is anomalously enhanced,which causes warm and wet air flows northward.Therefore,the long-term drought-to-flood event occurs in South China during the flood season.In the long-term flood-to-drought years,during the pre-flood season,WPSH is northerly and stronger,and South China is located in the northwestern side of WPSH with stronger water vapor transport,and the West Siberian Ridge enhances,which is conducive to the northern cold air to the south;During the post-flood season,WPSH jumps northwards and stretches westward,and the South China is on the southwest side of WPSH with weaker water vapor transport,and the latitudinal circulation is relatively straight in the mid-latitudes.Therefore,the long-term flood-to-drought event occurs in South China during the flood season.(3)In South China during the flood season of the past 56 years,there are 17 short-term(drought and flood of about 15 to 40 days)drought-flood abrupt alternation events,and the frequency of the events reduces first and then increases.According the abrupt-alternation time,the events are divided into two groups:drought-to-flood events in the pre-flood season,flood-to-drought events in the post-flood season.During the drought period of drought-to-flood events in the pre-flood season,the eastern part of China mainly affected by the dry and cold West Siberian Ridge and WPSH is weaker and more southerly,which decrease the precipitation in South China accompanying by the weakening of the water vapor transport;during the flood period of drought-to-flood events in late spring and early summer,WPSH stretches toward the north,the south branch trough of the Bay of Bengal is anomaly stronger,South China is located in the west side of WPSH and southeast side of South Asia high,which increase the precipitation in South China accompanying by the strengthening of the water vapor transport;during the flood period of drought-to-flood events in midsummer,the location of WPSH is more westerly,South China is located in the northwest side of WPSH and the southeast side of South Asia high,which causes the ascending movement and water vapor transport strengthen in South China,so the precipitation increase;during the drought period of drought-to-flood events in the post-flood season,WPSH jumps significantly to the north,South Asia high extends eastward and northward to the middle and lower reaches of the Yangtze River,South China is controlled by WPSH and the upper east wind jet,the descending movement strengthen in South China,so the precipitation decrease.