Periodicities in Indian monsoon rainfall over spectrally homogeneous regions

Abstract

This work presents results of a sharper search for significant periodicities in Indian monsoon rainfall, based on the recognition of the area's meteorological heterogeneity. Towards this end, a quantitative definition of spectral homogeneity is proposed, and the concept is used to classify India into distinct spectrally homogeneous regions (SHR) by two independent methods. The analysis is then carried out for each of the 10 SHRs, which may cut across or be subsets of homogeneous-rainfall zones defined earlier by various workers based on different criteria. A particularly interesting region is SHR7, the largest spectrally homogeneous cluster identified by both methods, which includes sub-divisions from west central and peninsular India. The spectrum here shows a significant dip in the frequency band 0.2-0.31 per year, flanked on either side by a rich structure characterised by nearly coincident spectral peaks in all the seven sub-divisions constituting the region. The significant peaks (confidence level = 99%) in SHR7 are 3.0, 5.7, 10.9, 13.3, 24.0, 30.3 and 60.6 years. The spectral dip is conjectured to be associated with the ENSO (EI Niño-Southern Oscillation) phenomenon, which occurs on the period scales of 3-5 years and is known to be anti-correlated with monsoon rainfall.