Intraseasonal Periodicities in Indian Rainfall

Abstract

Spectral analysis of a 70-year (1901?70) record of daily precipitation from 3700 stations in the country of India is carried out to search for periodicities on subseasonal time scales during the summer monsoon. Two statistically significant spectral peaks are found. A 40?50 day spectral peak corresponding to the Madden-Julian Oscillation is found over most of the portion of India south of 23°N. The phase of the oscillation is such that the precipitation maximum appears first over the relatively dry southeastern portion of the peninsula. Ten to 12 days later the precipitation peaks simultaneously all along the coast west of the Western Ghats and along a line running across India between 20° and 25°N. The precipitation maximum then spreads slowly northward and loses significance. Cross-spectral analysis shows strong coherence between the precipitation patterns and wind oscillations. The zonal wind oscillations at 850 and 200 mb am about 180 degrees out-of-phase equatorward of about 20°N, but in-phase poleward of 20°N. Compositing of the 40?50 day variance shows the structure of the wind variations associated with the precipitation cycle over India. At the southern tip and along the west coast of India, precipitation variations on the 40?50 day time scale seem to be, at least in part, orographically controlled, with upslope winds simultaneous with the precipitation maxima. In central India the precipitation maxima are more closely related to large-scale divergence and convergence patterns, with the largest precipitation clearly associated with cyclonic circulations at low levels which have about the same horizontal structure as the precipitation maximum. In addition to the 40?50 day oscillation, significant spectral peaks and coherent structures in precipitation are found whose characteristics correspond to those of monsoon lows. These are associated with a peak in the precipitation spectrum between about 5 and 7 days, which is strongest on the Bay of Bengal coast near 21°N. The 5?7 day oscillation in precipitation extends west-northwest across the breadth of India along the known track of monsoon lows. The phase of the oscillation indicates that the precipitation anomalies propagate westward across India with a phase speed of about 5 m s?. This oscillation explains as much as 20% of the variance of precipitation in some locations, Evidence for a spectral peak near 15 days is sparse, being limited to a few regions in the north of India where the 40?50 day oscillation does not dominate the low-frequency variance.