Analysis Of The Low-frequency Oscillation Characteristics Of Drought Over Mid-Lower Reaches Of The Yangtze River In Spring And Summer Based On Improved Comprehensive Meteorological Drought Index

Using the daily NCEP/NCAR reanalysis dataset and the observation data during 1963-2013 and basa on improved comprehensive meteorological drought index, the low-frequency oscillation(LFO) characteristics over Mid-Lower Reaches of the Yangtze River in spring and summer are analyzed, the circumfluent causes of LFO of CI arestudied. The result show that:(1) The major LFO period of the spring drought is 30-60d and above 60d. The major LFO period of the summer drought is 30-60d. The intensity of LFO of summer is stronger than that in spring.(2) The zonal direction of propagation in spring is westward and the meridional direction of propagation is southward and northward from Yangtze River. The zonal direction of propagation in summer is westward and the meridional direction of propagation is northward.(3) The spring drought have an important connection to westward movement of vorticity system over western Pacific and eastward of vorticity system over Qinghai-Tibet Plateau or west Asia. Southeastward movement of positive anomaly of 500hPa low-frequency height field over Balkhash Lake, eastward movement of positive anomaly of 500hPa low-frequency height field over the Arabian Peninsula, westward and northward movement of 850hPa low-frequency cyclone over western Pacific, westward movement of 850hPa low-frequency anticyclone over south of Japan, eastward and northward movement of 850hPa low-frequency anticyclone over Alab Sea are conducive to spring drought. The summer drought have an important connection to westward movement of vorticity system over western Pacific in Augest, northward movement of vorticity system over South China Sea in June and southward movement of vorticity system over North China. Eastward movement of positive anomaly of 500hPa low-frequency height field over Ural Mountains region, westward and southward movement of positive anomaly of 500hPa low-frequency height field over western Pacific, eastward movement of 850hPa low-frequency anticyclone over east of Qinghai-Tibet Plateau and eastward movement of 850hPa low-frequency cyclone over western Pacific are conducive to summer drought.(4) Westward movement of positive anomaly of 500hPa low-frequency height field over western Pacific, eastward and northward movement of positive anomaly of 500hPa low-frequency height field over upper reaches of southern branch frontal zone and southward movement of lower level low-frequency anticyclone over eastern of northwest China may be the important reasons why low-frequency CI varied in 2011. Southern China-ward movement of positive anomaly of 500hPa low-frequency height field over west Asia and western Pacific and westward and northward movement of lower level low-frequency anticyclone over the Bay of Bengal and western Pacific have an important connection to low-frequency CI variation in 2013.