Analysis Of Extreme Drought Events And Its Mechanism Over The Middle And Lower Reaches Of The Yangtze River Basin

Drought disaster is one of the major natural disasters in China, the extreme drought event frequency which affected by global warming is increasing. It is necessary to analyze the evolution process and the formation mechanism of the extreme drought events in the middle and lower reaches of the Yangtze River because its abundant resources, economic development and the loss caused by drought. Based on the daily temperature and precipitation data of 90 stations from 1981 to 2013 in the middle and lower reaches of Yangtze River (MLRYR) in China,the NCEP/NCAR daily reanalysis data and monthly NOAA Extended Reconstructed Sea Surface Temperature v4 (ERSSTv4) data, the general characteristics and effect mechanism of the drought events in 2011 winter-spring and summer 2013 over the middle and lower reaches of the Yangtze river region (MLRYR) has been investigated. The results were shown as follows.(1) The precipitation of both two extreme drought events over the middle and lower reaches of the Yangtze River reached the lowest value in the same period of history, and everywhere in MLRYR subjected to different degrees of drought. The rainfall was less 51.51% than the historical average in 2011 winter-spring, partial area precipitation less than 70%, the most severe drought was happened in April, and some local areas of MLRYR were reached middle and even high drought. The rainfall was less 49.43% than the historical average in 2013 summer, partial area precipitation less than 80% and the most severe drought was happened in the middle and last dekad of July, some local areas of MLRYR were reached special drought with the daily mean temperature on the high side 5 ?.(2) One of the most important reason about the precipitation anomalies in the middle and lower reaches of the Yangtze River is the strong East Asian Monsoon,because the cold and warm air could not converge in this place. In winter-spring of 2011, the East Asian winter monsoon and the East Asian trough were unusually strong so that dry cold air and warm moist air was insufficient in MLRYR, and affected by the sinking airflow, the water vapor was scattered and it is difficult to form precipitation. In the summer of 2013, the east Asian summer monsoon was strong, the western Pacific subtropical high (WPSH) was larger, stronger and the position was westward than normal, controlled MLRYR for a long time, the warm-moist and cold-dry air did not match in MLRYR, combined with the descending motion effects, which was not conducive to the formation of precipitation.(3) The anomaly meridional and zonal circulation which caused by the Pacific and Indian Ocean SST in the low latitudes are closed to the precipitation in MLRYR.The equatorial Indian Ocean - west Pacific - middle East Pacific Ocean SST of both two extreme drought events were present"-+-" SST anomaly distribution,strengthening the Walker circulation at the same time caused local Hadley circulation anomalies, so the MLRYR controlled by abnormal sinking airflow for a long time, he conditions for producing precipitation were insufficient.(4) The Rossby wave energy which from the North Atlantic can be dispersed eastward to the East Asia-Pacific region through the teleconnection wave train over Asia-Europe continent, deepening the members of the East Asian monsoon system and strengthening the East Asian monsoon to affect the climate in East Asia, which in turn affects the precipitation in MLRYR. Influenced by NAULEA teleconnection pattern, Rossby wave energy propagated eastward to the east of Eurasian and Pacific region, the east Asia trough were maintained in the vicinity of 120 0 E for a long time which strengthen the East Asian winter monsoon and further affected precipitation of MLRYR in Winter-Spring of 2011; With the mutual influence of the negative phases by teleconnections of silk road and EAP, the Rossby wave energy were accumulated in the eastern Eurasian continent and the Pacific, resulted in the WPSH westward extension abnormally which strengthen the East Asian summer monsoon and further affected precipitation of MLRYR in summer of 2013.(5) The summer SST anomalies in the North Atlantic are significantly related to the location of the East Asian subtropical westerly jet, and can change the location and configuration of the East Asian summer monsoon system,which is one of the influencing factors of precipitation anomalies in the middle and lower reaches of the Yangtze River.