云南2009—2012年4年连旱的气候成因研究

Climatic causes of continuous drought over Yunnan Province from 2009 to 2012

利用云南124个气象站逐月降水资料、NCEP/NCAR大气环流再分析资料、全球海表面温度(SST)资料以及向外长波辐射(OLR)资料,分析了云南2009—2012年连续4年干旱的气候特征,并从异常大气环流、海温、局地对流等方面分析了4年连续夏季干旱发生的成因.结果表明:1云南2009—2012年干旱是一次持续时间长,跨越春、夏、秋、冬四季、影响范围广的全省性严重干旱,最严重的区域主要是滇中及滇东南.并且是发生在云南降水减少的气候背景下.22009—2012年夏季500 h Pa欧亚地区高、中、低纬度的高度场分布不是很相似,但4年的环流配置形势表明东亚冷空气路径偏东,影响中国西南地区的冷空气偏弱,不利于冷空气南下影响云南.另外2009年和2010年副高位置偏西偏强,云南在其控制之下,不利于降水;而2011年和2012年副高位置偏北偏东,其外围的水汽不易输送到云南.32009—2012年云南大部地区及其南侧的孟加拉湾地区高度场持续偏高,孟加拉湾附近的印缅槽不活跃或偏弱,也不利于南方水汽向云南界内输送.4El Nino事件发展期、La Nina事件衰弱期以及印度洋海温偏暖时都有利于云南干旱的发生和发展.52009—2012年夏季云南局地及其以南大部地区基本维持低层辐散、高层辐合的垂直散度场配置,不利于上升运动的生成和发展.孟加拉湾及南海一带基本为西北或偏东气流控制,向北输送的水汽较常年偏弱,是造成云南连续4年夏季干旱少雨异常气候的直接原因之一.62009—2012年连续4年夏季云南局地对流相对常年偏弱,这也是云南降水偏少的重要因素之一.

By using the observed monthly precipitation data（ 1961—2012） of 124 stations of Yunnan Province,the monthly mean atmospheric circulation data and global SST data from NCEP / NCAR reanalysis and the outgoing longwave radiation（ OLR） data,the climatic features of continuous drought over Yunnan from 2009 to2012 and the causes of summer drought over Yunnan from 2009 to 2012 with the atmospheric circulation anomaly,SST anomaly and local convection have been studied. The results indicate that firstly,the continuous drought over Yunnan from 2009 to 2012 was a serious drought across whole Province,which lasted for a longer time and took place from spring,summer,autumn to winter. The most serious areas were mainly located in the middle and the southeast of Yunnan. Secondly,the continuous drought over Yunnan from 2009 to 2012 took place against the big climatic background of precipitation reduction. Thirdly,the summer 500 h Pa height field distributions of high,medium and low latitudes over Eurasia were not very similar from these in 2009 to 2012,but the summer circulation configurations in the four years indicated that the cold air over East Asia shifted more east than the normal and the impacts of cold air in southwest China were weaker,so it was unfavourable to cold air southward affecting on Yunnan. In addition,subtropical high extended to the west of its normal position and its strength was stronger in 2009 and 2010 so that it was unfavourable to rainfall over Yunnan Province under the control of subtropical high. In 2011 and 2012,subtropical high extended to the north and east of its normal position so that it was unfavourable to transporting its peripheral vapor to Yunnan Province. The height of most of Yunnan Province and Bay of Bengal was above normal continuously from 2009 to 2012. The India-Burma trough near the bay of Bengal was not active or weak so that it is unfavourable to transporting the south vapor to Yunnan. In addition,during the continuous 4 years summer drought over Yunnan,the SST of tropical Pacific Ocean shows different abnormal stages respectively. Developing stages of El Nino events,decaying stages of La Nina event and the warmer Indian Ocean SST were favorable to the occurrence and development of summer drought over Yunnan. Tropical SST anomaly was one of the important external force factors of summer drought over Yunnan from 2009 to 2012. Finally,the divergence at lower level companied with the convergence at the higher level over most of Yunnan and its south from 2009 to 2012 was unfavourable to the occurrence and development of upward movement. The bay of Bengal and south China sea area was mainly controlled by northwest wind or easterly wind and the northward vapor transport was below normal,which was one of direct causes of continuous 4-year summer drought over Yunnan from2009 to 2012. The local summer convection over Yunnan was weaker than normal,which was one of important factors of summer rainfall being less than normal over Yunnan from 2009 to 2012.