Spectrum analysis relevant to Indian monsoon

Recent investigations on Indian monsoon by the method of spectrum analysis are reviewed from the viewpoint of various aspects of its application. The assessment is also made on their results and discussions. Through these processes, it emerged that there exist at least two kinds of quasi-periodic variations in the summer monsoonal fluctuation over India and adjacent regions. One appears as a spectral peak around the 5-day period and another is around the 15-day period.The former one is conspicuous along the monsoon trough region in northern India. The results of cross-spectrum analysis show that it reflects the passage of westward-moving, synoptic-scale disturbances called monsoon lows. The latter seems to be related to the large-scale variation of monsoon activity itself, though it is also likely to have some associations with the cyclogenesis of monsoon depressions. The recent application of spectral method to this large-scale interrelation is also discussed along with the results of some non-spectral studies and the remaining problems are pointed out.