Climatic trends and periodicities of annual rainfall over India

Abstract

Trends and periodicities of annual rainfall for 29 sub-divisions of India and all India (area-weighted average of the sub-divisions) are studied by using the rainfall series for a period of 124 years (1871–1994). The trends are evaluated by using a linear regression technique. To identify the climatic changes, the rainfall series are subjected to 11-year moving averages. These averages show some epochs. From this, it is found that in some sub-divisions the trend in one direction reverses its direction after some years. The years pertaining to these changes are identified and the trends are calculated for the segments of the series. The significance of the trend values are tested. Power spectra are obtained using the maximum-entropy method. The periods in the range 2.0–2.5 years are present in the rainfall series of the sub-divisions to the north of 20 °N, on the west coast, and in Tamilnadu and for all India. This periodicity may be attributed to the quasi-biennial oscillation. Rainfall series of the most of the sub-divisions and all India indicate a triennial cycle. Significant periods in the range from 3.0 to 8.0 years and 8.0 to 12.0 years are also identified.