On the Variability and Prediction of Rainfall in the Post-monsoon Season over India

Considerable rainfall occurs in India during the post-monsoon period from October to December, particularly over north-eastern, eastern and southern regions, and this is of great significance in agricultural and allied sectors. For the first time, we have attempted to provide detailed information on the variability and predictability of the post-monsoon rainfall (PMR) of the country. Details on the summer monsoon rainfall from June to September are extensively documented. This study comprises four sections: (i) examination of large-scale rainfall features; (ii) examination of small-scale (or regional) rainfall features; (iii) diagnostic study in order to identify possible regional forcings and global teleconnections; and (iv) modelling long-period rainfall series and extrapolation of future trends for the next 10 years. To understand large-scale features from year-to-year, expansion/contraction of the wet area (October–December rainfall ]200 mm) was examined, involving data from 306 well-separated locations spanning 114 years (1871–1984). The post-monsoon wet area (PMWA) series (1871–1984) is homogeneous, but its interannual variations are quite large. In the low-frequency mode fluctuations however, some distinct wet/dry epochs can be identified that are broadly consistent with the wet/dry epochs of summer monsoon fluctuations, implying that the same forcing factors affect the large-scale rainfall activities of both periods. Variations in the regional rainfall were studied in order to provide some useful information for practical purposes. The longest-possible instrumental period from the October to December rainfall series for each of the six zones of the country was reconstructed by applying the objective optimization technique to the limited observations available. Rainfall series were produced for the following regions of India, the reference period of reconstruction being 1871–1980: northwest India, 1844–1996; North Central India, 1842–1996; northeast India, 1829–1996; West Peninsular India, 1841–1996; East Peninsular India, 1848–1996; and South Peninsular India, 1813–1996. The characteristics of different zonal series are documented; they do not possess a significant long-term trend, and are weakly correlated. The longest-possible all-India October–December rainfall series (1813–1996) has also been reconstructed, since it can be easily updated using the same optimum set of 116 India Meteorological Department (IMD) raingauge stations that are used for the six-zonal series; the PMWA series (1871–1984) would require the data from all 306 raingauges. The correlation between the PMWA and the all-India rainfall series is 0.92. To identify possible causes of rainfall variation, correlation of zonal, as well as all-India series, with 20 selected regional/global circulation parameters was examined. Correlation, albeit weak, of particularly large-scale features with the Southern Oscillation, El Niño, Quasi-Biennial Oscillation and surface air temperature was found to be of the same sign as that found with the large-scale features of the summer monsoon. This provided support for the previously reported theory that the sources of variation in the occurrence of rainfall during the two periods are the same. In the fourth section, different zonal and all-India series are subjected to time series analysis. A flexible approach, singular spectrum analysis (SSA), was applied first to smooth out the predictable portion of the low-frequency mode variation in the individual series. Variable harmonics of the smooth series were then estimated at a wavelength resolution of 0.1 year. A subset of a few variable harmonics was objectively identified whose combination was extrapolated to enable prediction of future trends of PMR patterns across India over a 10-year period. There are differences in the future rainfall trend over different parts of the country. Copyright © 1999 Royal Meteorological Society.