Periodicity in rainfall at Madras

Abstract

Monthly rainfall data from Madras for the period 1813–1976 are spectrally analysed for detection of significant oscillations using the new data adaptive technique of maximum entropy developed by Burg. Spectra for annual, NE monsoon and SW monsoon rainfall are separately presented and compared with the spectra derived from Fourier transform using FFT. The long series of data are divided into several subsets and analysed to determine the stability of the periodicities. The effect of changing the length of PEF on the resultant spectra is also discussed. The analysis reveals that longer period variations (>20 years) in the rainfall is either absent or non-stationary. Solar-cycle periodicity of 11 years is also missing in the rainfall. A quasibiennial oscillation in the annual and monsoon rainfall is observed in all the subsets and in the full series. Stacked spectrum derived from the spectra of monthly rainfall series for 30-year spans is dominated by the annual line and its harmonics. The biennial pulse is absent in the monthly rainfall series.