长江中下游夏季旱涝并存及其异常年海气特征分析

The Summer Drought-Flood Coexistence in the Middle and Lower Reaches of the Yangtze River and Analysis of its Air-Sea Background Feathers in Anomalous Years

利用国家气候中心提供的1957~2000年中国720站夏季(5~8月)逐日降水资料,对长江中下游地区夏季旱涝并存现象进行研究,并定义了一个季时间尺度的旱涝并存指数(IDFC),再结合NCEP/NCAR的500 hPa高度场、850 hPa风场等再分析资料以及Reynolds海温资料,对该地区旱涝并存异常年的海气背景特征进行了统计分析。结果表明:近50年长江中下游夏季旱涝并存异常的发生频率呈现上升趋势;夏季旱涝并存异常年,西太平洋副高空间活动范围较大,同期东亚夏季风偏弱;在其前期6个月中,阿拉伯海、孟加拉湾和中国南海海温显著偏高,另外赤道东太平洋海温呈现上升趋势,对应着El Ni^no的成熟阶段或发展阶段。所有这些为长江中下游夏季旱涝并存现象的预测,提供了有参考意义的前兆信号。

Based on the 1957 - 2000 summer daily precipitation of 720 Chinese stations supplied by the National Climatic Center, the drought - flood coexistence phenomenon has been investigated and a season time- scale Drought - Flood Coexistence Index （IDFC） in the middle and lower reaches of the Yangtze River has been defined. Background features of atmospheric circulation and sea surface temperature （SST） in drought - flood coexistence anomaly years have been studied statistically using 850 hPa wind field, 500 hPa height field from NCEP/NCAR reanalysis datasets with resolution 2. 5°×2. 5°and Reynolds SST data with resolution 2. 0°× 2. 0°. Results show that： the occurrence frequency of DFC （Drought - Flood Coexistence） has increased remarkably in the past 50 years, especially since 1980s; there exists a significant correlation between drought - flood coexistence phenomenon and the intensity of East Asian Summer Monsoon （EASM）, high IDFC summers associated with weak EASM; the Western Pacific Subtropical High （WPSH） is very active in high IDFC summers associated with its position a bit southward in May, a remarkable westward and northward shift in June and July and flashback southward in August, which implies WPSH plays an important role in resulting in summer DFC phenomenon in the middle and lower reaches of the Yangtze River; the SST from the Arabian Sea, across the Bay of Bengal, the South China Sea to the equatorial western Pacific is of high correlation with summer IDPC in the middle and lower reaches of the Yangtze River, which means a smaller land-sea thermal difference in high IDPC summers and the equatorial eastern Pacific also exhibits a warmer SST phase in the preceding months of high IDPC summers, which corresponds to the mature phase or the developing phase of El Nino. All these offer some predictive signals for the DFC phenomenon forecasting in summer in the middle and lower reaches of the Yangtze River. The emphasis of this paper is concentrated on the long time scale, season scale, investigation to DFC phenomenon. However, DFC is a very complicated problem, especially when the time scale gets shorter. Short - range anomaly （such as one flood and one drought and so on） and medium-range anomaly （such as floods and droughts） etc. come out, which need different studies according to different situations. This is still an open question and needs further investigation.