Characteristics of Surface Humidity and Extreme Drought/Wet Events in Yangtze River Delta
2017, 32 (9):
Based on the Penman-Monteith Model and the daily meteorological data of precipitation, temperature, wind speed, sunshine duration and relative air humidity at 43 stations in Yangtze River Delta from 1957 to 2014, reference crop evapotranspiration, surface humidity index and extreme drought/wet events were detected and counted. The spatial and temporal variation of humidity index and extreme drought/wet events were analyzed using linear regression, TFPW-MK trend test and Morlet wavelet. The results indicated: 1) Annual and seasonal mean humidity indexes showed similar distribution patterns in recent 58 year in Yangtze River Delta except in summer that higher values distributed in the south and lower values distributed in the north, which were mainly influenced by latitude, distance from the sea, topographic conditions, and general atmospheric circulation. In summer, the low values of relative humidity index emerged along the axis of “North-central of Anhui-South-central of Jiangsu-North of Zhejiang-Shanghai”. 2) The climate showed the tendency of turning arid that the annual humidity index decreased at the rate of -0.021/10 a. 1980s and 1990s are humid period. The sudden change of annual relative humidity occurred in 2003 according to TFPW-MK abrupt change test, and there were several oscillation periods of annual relative humidity according to wavelet analysis, such as 12-18-year period from 1957 to 1990, 8-10-year period since 1977. At seasonal scale, the climate in spring and autumn turned dry and the climate in winter and summer turned humid. The mutational years of spring, autumn and winter were 1999, 1988 and 1985 respectively. Moreover, there were also several oscillation periods at seasonal scale. The change characteristics were mainly influenced by the variation of precipitation, evapotranspiration, Southern Oscillation and subtropical high over the western Pacific. Spatially, the aridification mainly happened in southeast, east and part of the west regions, while the humidification mainly emerged in the north-central and southwest regions. The variation coefficient of annual relative humidity had the spatial pattern that high values occurred in the northwest and low values occurred in the southeast. 3) The frequency of annual extreme drought events showed upward trend with the rate of 0.121/10 a, and the frequency of annual extreme wet events showed downward trend with the rate of -0.052/10 a. Spatially, the frequency of annual extreme drought events increased in the west and southeast of Anhui, south central of Jiangsu and most parts of Zhejiang, while it decreased in north and east of Anhui, northwest of Jiangsu, and northeast of Zhejiang. The frequency of annual extreme wet events presented the opposite spatial patterns with the frequency of annual extreme drought events, i.e. the frequency of annual extreme wet events increased in northwest region and decreased in southeast region.
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