中国とモンゴルの乾燥・半乾燥地域における夏季降水量の経年変動 : その地域性とアジアの夏季モンスーンとの関係

Abstract

In this study, interannual variations of summertime precipitation over arid and semi-arid regions in China and Mongolia are investigated. To clarify the regionality of the interannual variability in summer precipitation, an analysis technique of rotated empirical orthogonal functions is applied for a recent 40-year (1951-1990) period of summer precipitation. As a result of the REOF, five regions have been determined : I) Taklimakan Desert, II) Loess Plateau, III) North China to central and the southeastern part of Mongolia, IV) the north of Tianshan Mountains, and V) the northern part of Mongolia. Summertime precipitation over Region III) shows a significant decreasing trend after 1955. Next, to examine how the variations in precipitation in these regions are influenced by the Asian (Indian)summer monsoon activity in the mid-latitudes, correlations with all-India monthly and seasonal rainfall (IMR) are investigated. Further, the change of atmospheric circulation patterns with the interannual variation of summer precipitation of Regions I and II are also examined. The results are summarized as follows : 1) The interannual variation of summer precipitation of Region I (Taklimakan Desert) is mainly related to the windward mid-latitude circulation and eastward (westward) shift of the Tibetan High in a wet (dry)year. This region shows a clear negative correlation with IMR in June and July, and the relationships are caused by a rather local circulation change with IMR variation over Central Asia. 2) In Region II (Loess Plateau, the middle reaches of Yellow River), interannual variation of summer precipitation shows a positive correlation with IMR through the summer monsoon season. It shows a clear 2-3 year periodic oscillation, and seems to be closely related to the atmosphere/ocean interaction in the equatorial Pacific.