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淮河夏季降水与赤道东太平洋海温对应关系的年代际变化
Decadal Variation of the Relationship Between Summer Precipitation along the Huaihe River Valley and SST over the Equatorial Eastern Pacific
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摘要: 基于我国台站降水观测资料、NCEP/NCAR再分析资料和NOAA的ERSST资料讨论了淮河夏季降水与赤道东太平洋海表温度对应关系的年代际变化。结果表明:淮河夏季降水在20世纪70年代中期发生了一次明显的突变, 在突变之前, 其降水呈线性减少趋势, 而在其后降水呈线性增加趋势; 同时, 淮河夏季降水与前期ENSO的对应关系在突变前后有明显的差异, 在突变前, 降水与赤道东太平洋的海温为显著的负相关, 而在突变后, 相关值转为弱的正相关。滑动相关结果显示:近20年是整个研究时段中二者对应关系最弱的时期。进一步研究发现:淮河降水与西太平洋副热带高压位置和强度的对应关系也在20世纪70年代中期前后经历了一个年代际变化, 在突变前, 淮河夏季降水与副高强度呈正相关, 突变后则为负相关。其中淮河8月降水在突变前主要受副高脊线位置和副高北界影响, 突变后则主要受副高面积和强度影响。即ENSO对淮河夏季降水预测的参考意义正在减弱, 同时副高对降水的作用方式正在转变。Abstract: The precipitation along the Huaihe River valley plays an important role for the position and the intensity of the rainbelt in eastern China in summer. Based on monthly rainfall data from 160 primary stations in China during 1951—2003 and the NOAA extended reconstruction sea surface temperatures (ERSSTs) data in the same period, the decadal variation of the relationship between summer precipitation along the Huaihe River valley and the SSTs over the equatorial eastern Pacific is studied. Results show that the precipitation is characterized by a remarkable interannual variability, especially by a quasi-biennial oscillation (QBO) during the 1950s to mid-1960s and after the 1980s. Besides the interannual variation, it also contains a distinct abrupt change in the mid-1970s. The precipitation decreases significantly before the abrupt change time while increases after that. The relationship between the summer precipitation and the SSTs in previous winters over the Niño3 regions also differs remarkably with significant negative correlation coefficients before 1974 but with weak and positive values after 1980. Similar patterns can also be found in the composed global SST anomaly (SSTa) fields according to five wettest and driest years in each period along this valley. A 24-year moving window correlation results reveal that the relation in the last two decades is the weakest among the whole research periods. This weakening relationship can also be found in the SSTa fields in previous autumns. Besides this, the relationship between the precipitation and the intensity of the western Pacific subtropical high (WPSH) also possesses a decadal variation with positive coefficients before 1974 but negative after 1980, especially for that of August. During August, the correlation coefficients between the precipitation and the longitudes of the ridge location of the high are -0.56 and 0.03 respectively in the first and second period, while -0.18 and 0.53 respectively for the intensity of the high. That is to say, the rainfall in August is mainly affected by the position of the WPSH in the first period while by its intensity in the second. Above results indicate that the significance of ENSO as a predictor of the summer precipitation along the Huaihe River valley is weakening and the influencing approach of the WPSH is changing. This weakening significance has increased the difficulties of the prediction of China summer rainfall.
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图 1 1951—2003年淮河9站夏季降水量及其线性趋势分布
(细实线: 降水量, 粗实线: 1971—2000年平均降水量, 单位: mm; 虚线分别为1951—1974年和1980—2003年两个时段降水线性趋势线, 单位: mm·a-1)
表 1 不同时段Niño3区海温与淮河夏季降水的相关
表 2 6—8月副高特征参数与淮河降水的同期相关
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