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近20年华北地区干旱期大气环流异常特征
CHARACTERISTICS OF ATMOSPHERIC CIRCULATION ANOMALIES DURING PERSISTENT DROUGHTS IN NORTH CHINA FOR LAST TWO DECADES
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摘要: 利用NCAR/NCEP再分析资料和中国气象局整编的中国160站气温、降水月平均资料, 计算并分析了1951年1月-2000年10月中国华北地区Palmer drought severity index (PDSI)。研究表明:近50年来华北干旱有显著的年际和年代际变化。从20世纪80年代以来, 华北出现了持续性严重干旱现象; 另外, 华北地区干旱持续时间一般都在两年以上。华北夏季典型干旱年的前期 (冬季和春季) 及同期环流特征是:北半球中高纬度500hPa高度距平场出现EU型遥相关分布, 华北地区长期处于大陆暖高压控制下。Abstract: The persistent droughts during the last two decades in North China are studied.The main results are as follows :(1) The Palmer Drought Severity Index (PDSI) shows that there is inter-annual variation in droughts in North China.The inter-decadal variation is even more remarkable.Before the mid-1970s, there was wetsummer climate, while since 1970, there were more drought years in North China.Moreover, the most severe droughts happened after the 1980s.(2) Summer droughts in North China are the consequence of persistent anomalous circulation over the Eurasia.The main features of the anomalous circulation include that there is height anomaly in the region of 55°to 65°N, 60°to 135°E and that there was EU teleconnection and it is the main type of atmospheric circulation anomalies that causes the persistent summer droughts in North China.(3) During the summer drought years, there is continuity in the distribution of the 500 hPa height anomaly field from winter to summer.(4) The anomalies of the general atmospheric circulation during droughts in North China are not local phenomena, but the responses to the Northern Hemisphere general atmospheric circulation anomaly.
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图 4 华北地区夏季典型干旱年 (a、b) 和湿润年 (c、d) 的前期500 hPa高度距平合成图
(a) 干旱年前期1月, (b) 干旱年前期4月, (c) 湿润年前期1月, (d) 湿润年前期4月 (其它说明同图 3)
图 5 (a) PDSI指数第一模态空间分布, (b) PDSI时间系数求出的500 hPa高度场第一回归模态, (c) 第一时间系数
(实线:高度场序列, 虚线:PDSI指数序列)
表 1 Palmer指数干湿等级
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