Using daily rainfall data for the 30-year period of 1960-1989 and following an objective criterion, the pre-active, active and post-active phases of the summer monsoon (June-September) rainfall are clearly delineated for all-India and homogeneous regions of India. It is seen that on average, the monsoon is active for 103 days over northeast (NE-India) India, for 75-78 days over central-northeast (CNE-India) India and so on for all the regions. Average daily normal rainfall (ADNRF) is at a maximum of 14.7 mm over NE-India and at a minimum of 4.6 mm over NW-India. To investigate possible periodic oscillations in daily rainfall, the daily normal rainfall (DNRF) of all the regions is subjected to power spectrum analysis. This analysis reveals that DNRF exhibits periodicities of 5, 8-12 and 20 days over NE-India, about 5 and 40 days over WC-India, 8-12 and 40 days over NW-India and 8-10 and 40 days over PEN-India. Inter-annual and decadal scale variability of the summer monsoon rainfall over homogeneous regions is studied using the summer monsoon rainfall for the period of 1871-1990. It is discovered that summer monsoon rainfall of WC-India and NW-India is dominated by a quasi-biennial oscillation (QBO), whereas summer monsoon rainfall of NE-India and PEN-India is dominated by ENSO-type periodicities. Epochs of increasing and decreasing rainfall are also observed in the summer monsoon rainfall over homogeneous regions of India
