利用贺兰山北部树轮资料重建过去270年以来6～8月平均干燥指数

采用油松树轮晚材宽度,重建了贺兰山北部地区过去270年以来夏季(6～8月)干燥指数,重建序列的方差解释量(R2)为52%。并发现干燥指数大于均值的年份有1745～1756年、1847～1864年、1925～1933年,小于均值的年份有1731～1744年、1775～1788年、1798～1808年、1865～1877年、1973～1992年;11年滑动平均后发现18世纪50年代前后、19世纪50～60年代、20世纪20年代后期以及30年代初为较干旱时期,干旱时期干燥度变率较小,而湿润时段干燥度变率较大,反映了贺兰山地区气候变化的一个特点,即干旱年会连续出现;周期分析表明,干燥度指数存在22年的准周期。

RECONSTRUCTION OF MEAN DROUGHT INDEX FROM JUNE TO AUGUST FOR THE LAST 270 YEARS USING TREE-RING DATA IN THE NORTHERN HELAN MOUNTAIN

In terms of correlation analysis between tree-ring data, such as tree-ring width, early-wood width, late-wood width, and drought index calculated from meteorological data of nearby stations, we found a good relationship between late-wood ring width and mean drought index from June to August. The relationship could be well explained by the physiology of tree growth. Because drought index is liable to these months with a little precipitation, we adjust it through “borrowing” precipitation of one to two days of adjacent months. Then, we reconstruct the summer drought index （June to August） for the past 270 years using late-wood width of Pinus tabulaeformis in the northern Helan Mountain. The explained variance of the reconstruction series is 52%. We find that these years with drought index higher than 270-a mean are: 1745～1756a, 1847～1864a, and 1925～1933a and these years lower than mean are: 1731～1744a, 1775～1788a, 1798～1808a, 1865～1877a, and 1973～1992a. After 11-a moving average, it has been found that the variability of drought index is relatively low in the drought periods, such as 1750s, 1850s～60s, and 1920s. In another aspect, the variability is relatively high in wet periods, which hints that drought years often appear continuously. Power spectral analysis shows that the reconstruction series exits a 22-a quasi-cycle.