Wavelet Analysis of Summer Rainfall over North China and India and SOI Using 1891-1992 Data

Localization of interannual and decadal variations of summer rainfall in North China and India and seasonal mean Southern Oscillation Index (SOI) from 1891-1992, and the favored time scales of the significant correlations among them are investigated with the wavelet transform (WT) analysis method. Strong localization and non-stationary evolution in the interannual and decadal variations of the rainfall in North China and India are demonstrated. The dominating time scales in the rainfall variations in North China and India are mainly located in two time scale bands: shorter than 10 years, and 14-28 years. The correlations between the rainfall variations in North China and India are time-scale dependent. The significant positive correlations are concentrated in two time-scale bands: shorter than 7 years and longer than 14 years. The correlations are insignificant on time scales of 7-14 years. El Nino and Southern Oscillation (ENSO) cycle is a nonstationary process. The dominant time scales for the variation of SOI are shorter than 5-7 years, and the decadal variations are obvious in 1891-1915 and 1970-1992. There is strong interaction between the Indian summer monsoon and the ENSO cycle. The significant positive correlations are mainly focused on the time scales shorter than 24-30 years, and the significant negative correlations on time scales longer than 40 years in summer and winter SOI cases. The correlation between the rainfall variations in North China and the ENSO cycle in various seasons are less significant, more scattered and complex, but there are some similarities in the correlation pattern compared with that of India, especially in the shorter time scales. The calculation shows that the summer rainfall variations in North China and India at different time scales are related with different general circulation anomalous patterns in middle and high latitudes of Eurasia and the western Pacific. The correlation patterns over Eurasia and the western Pacific between the summer rainfall variations in North China or India and the geopotential height at 500 hPa are similar to the correlation patterns between the geopotential height and the WT results of the rainfall on time scales of about 5 years, but different from those on time scales of about 11 years. Therefore, the similar behavior between the rainfall variations in North China and India may be caused by the similar associations between the rainfall variations and the ENSO cycle and the general circulation anomalies in middle and high latitudes over Eurasia and the western Pacific, and the difference may be related to the different character of these associations.

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