Spectrum analysis on the dry/wet variations in warm half year in the Middle to Lower Reaches of the Yangtze River during 1470 to 2000

Using the Dryness/wetness grads covering the period of 1470~2000 and instrumental measured precipitation from May to September during 1951 to 2000 from eight stations,with the methods of EOF,moving-ttest and wavelet analysis,we analyzed spatial and temporal characters of the dryness/wetness variations in the lower-middle reaches of the Yangtze River.The results show(1) the dryness/wetness grades could accurately reveal the spatial and temporal characters of dryness/wetness variations in the study area;(2) the first three modes which are featured by accordant all over the region,contrary in west-east and in north-south,indicate the main spatial patterns of the dryness/wetness variation,and the first mode accounts for 42.3% of whole variance;(3) at the 30-year scale,the abrupt changes of dryness/wetness occurred before 1850s,conversions from dry condition to wet condition occurred in 1550s,1740s,1820s and 1837s and conversions from wet condition to dry condition occurred in 1620s,1520s and 1850s;the greatest variability occurred in the past 50 years;(4) the dryness/wetness variations had the significant cycles of 117-year,40-year,30-year,18-year,5-year and 2~3-year;excluding the cycles of 18-year and 2~3-year,other's amplitude changed evidently through the 18th century.It may be implicated that the 19th century was not the copy of the 17th,though both of centuries were featured by cold condition.