2009—2010年云南特大干旱的气候特征及成因

Analysis on climatologic characteristics and causes of the severe drought during 2009- 2010 in Yunnan province

基于云南122个站降水、气温资料分析2009—2010年特大干旱气候的特征。2009年7月至2010年6月云南气温持续偏高,降水持续偏少,干旱从2009年9月出现,一直持续到2010年5月,许多县(市)干旱持续时间接近200 d,单日的重、特旱县(市)站数破1961年以来记录。利用NCEP/NCAR再分析和OLR资料,应用环流分析、水汽输送等方法分析此次干旱形成和持续关键时段2009年9—10月、2010年3月和5月的环流异常。结果表明,2009年9—10月乌拉尔山、青藏高原东部为脊,冷空气活动偏弱;赤道印度洋对流层低层为异常东风、高层为异常西风,导致纬向的季风环流圈偏弱;孟加拉湾、中南半岛对流偏弱;西南季风通道进入云南的水汽输送偏弱,云南上空水汽含量偏少并为异常下沉区。2010年3月欧亚中高纬位势高度距平场为北负南正,以纬向环流为主,冷空气偏北;西太平洋副高与北非副高连通并在低纬形成坝状高压带;南支槽上游地区高度场偏高、为异常下沉区;西风带无明显波动,导致南支槽不活跃,云南处于水汽辐散区。2010年5月欧亚中高纬距平分布为"-+-",低槽和冷空气偏北,西太平洋副高西伸至安达曼海,阻挡了越赤道气流进入孟加拉湾,致使孟加拉湾南部、中南半岛西南季风爆发偏晚,云南为异常西北水汽输送,为水汽辐散区。

The rainfall and temperature data from 122 meteorological stations in Yunnan province are used to analyze the climatologic characteristics of the severe drought in 2009-2010. It is noted that the higher temperature and less precipitation occurred from July 2009 to June 2010, drought emerged in Sep- tember 2009 and lasted to May 2010 with 200 drought persisting days in many counties （cities） , and the number of one-day extraordinary and severe drought county （city） broke the record after 1961. By using the NCEP/NCAR reanarysis data and OLR data, anomalous circulations at the drought-forming（ Sep.- Oct. ,2009） and drought-developing（ March--May, 2010） stages are explored. The results indicate that in the forming stage, anomalous ridges existing over Ural Mountain and Tibetan Plateau was associated with weak cold air, moreover, anomalous easterlies and westerlies at low and upper levels of troposphere covered the tropical Indian Ocean, which weakened the zonal monsoon circulation, convection over the Bay of Bengal and the Indo-China Peninsula, as well as the southwestwarding water vapor transportation to Yunnan. Hence, the water vapor content around Yunnan province was very low and the anomalous sub- sidence air flow was evident. In March, the geopotential height anomalies were negative in the north while positive in the south over the mid-high latitude in Eurasia, which related to strong zonal wind and north- warding cold air activities. The West Pacific subtropical high combined with North Africa subtropical high, and then formed a broad belt-like high in the low latitude. The geopotantial height in the upstream of the southern-branch trough was higher than the normal and was associated with anomalous subsidence air flow. In addition, there were no prominent eddies in the westerlies and the southern-branch trough was inactive, which caused water vapor divergence over Yunnan province. In May, 2010, the anomalous geopotential height exhibits ＇ -＋-＇ pattern, low level trough and cold air shited northward, and the West Pacific subtropical high extended to Andaman Sea inhibiting the cross-equatorial flow from entering the Bay of Bengal, which lead to late summer monsoon onset in the southern Bay of Bengal and Indo-China Peninsula. Therefore, Yunnan province is dominated by anomalous Northwest water vapor transportation and water vapor divergence.