Low frequency modes and their link with summer monsoon activity described by wavelet analysis throughout India

The purpose of this paper is to propose a method and its application to attempt investigating temporal-frequency characteristics of low frequency modes throughout India. This diagnostic method combines wavelet analysis (WT) interlaced by a factor D and empirical orthogonal function (EOF). It is shown that the stability related to WT is ensured by the adequate choice of factor D and the sampling frequency. Thus, the reconstruction of a time series from its wavelet coefficients becomes possible (i.e., “wavelet synthesis,” which can constitute a narrow bandpass). This method has been applied to daily pressure during summer monsoon activity for the year 1977. The results of analysis indicate a link between summer monsoon activity and both a 10-20-day and 25-50-day band. This link is expressed by the simultaneous occurrence of both modes throughout India with a different amplitude from one region to another expressed by the following factors. During summer monsoon onset, it is the 10-20-day mode that is predominant, while during the break, both modes have nearly the same amplitude. These modes have a maximum activity situated nearly at 23°N, 83°E