中国极端干旱——空雨频率的主模态时空特征

Extreme Drought in China——Spatial and Temporal Characteristics of Frequency of Precipitation Free Day

利用国家气象中心的站点日降水观测资料,定义一个季节内无降水日出现的频率——空雨频率作为极端干旱的表征指标,对中国487个站点1980—2014年秋、冬、春三个连续季节的空雨频率进行季节-经验正交函数分解(S-EOF),发现我国极端干旱随季节演变的时空分布主要表现为前四个主模态的特征,累积方差贡献为40.6%。第一模态为秋、冬、春三季连旱模态,空雨频率呈现显著的上升趋势,大部分地区为异常变旱,冬季华南沿海变旱最显著。第二模态为秋与冬春干旱的反位相变化模态,空雨频率出现明显的秋-冬-春季反位相的季节反差现象,且存在弱的年代际振荡。第三模态为冬-春极端干旱反位相模态,对冬春两季的代表性较好,秋冬两季空雨频率的空间分布场为以长江流域为中心的极端变旱,而春季空雨频率呈现反位相变化。空雨频率以1998年为突变点存在明显的年代际转折。第四模态为南北偶极型秋-冬-春反位相模态,呈现出长江流域以南与长江流域以北三个季节空雨频率均为相反的位相分布。不同的模态反映不同季节、不同区域的空雨频率的变化,充分表现出我国干旱分布的复杂性。

Based daily precipitation data at 487 observatory stations in China during 1980—2014 obtained from the National Meteorological Center, extreme drought variations in China in consenquent seasons-autumn （SON＋0）、winter（DJF＋0）、srping（MAM＋1） are explored. Frequency of precipitation free day （FPFD） is defined as an extreme drought index. The top four modes are leading modes of FPFD and on behalf of variation of nine-tenths observations in China. Representative sites mostly distribute in the central, southern and eastern China. Results show that the first mode appears coincident extreme drought in three seasons. Most areas in China become abnormal drought. Winter drought in the south China is of the biggest intensity. The second mode showed out-of-phase extreme drought patterns in autumn and winter-spring with weak decadal oscillation. The third mode mostly represents the distribution of FPFD in winter and spring, showing an opposite spatial pattern around the Yangtze River in winter and spring. This mode also appeared an obvious interdecadal shift in 1998. The fourth mode shows a dipole distribution in southern and northern China in the three seasons. Different modes can reflect the different variations of FPFD, which fully shows the complexity of extreme drought in China.