Temporal Evolution of Dominant Periodicity in All India and Kerala Monsoon Rainfall (1901-2024): A Comparative Spectral Analysis

Abstract

Understanding the temporal organization of monsoon rainfall variability is essential for characterizing long-term climatic behavior over the Indian subcontinent. This study investigates the dominant periodicities in All-India and Kerala annual monsoon rainfall using long-term observational records spanning 1901–2024. Fast Fourier Transform (FFT) analysis is applied to the complete datasets to identify primary variability scales. To assess potential non-stationarity, the 124-year record is divided into three equal epochs and examined through epoch-wise spectral analysis. The results reveal that All-India rainfall exhibits pronounced temporal shifts in dominant periodicities, transitioning from interannual (~2–3 years) variability in the early twentieth century to decadal (~8–14 years) dominance in subsequent epochs. In contrast, Kerala rainfall demonstrates persistent decadal-scale variability throughout the record, with a gradual increase in dominant periodicity from approximately 10 years to around 14 years across successive epochs. The complete Kerala dataset further indicates a prominent quasi-decadal to multi-decadal peak (~17–18 years), underscoring the importance of longer-term oscillatory components at the regional scale. The comparative spectral analysis demonstrates clear regional contrasts in the temporal organization of monsoon rainfall variability. While the aggregated All-India rainfall series reflects evolving and non-stationary dominant periodicities, the Kerala record maintains relatively stable decadal-scale dominance. This contrast suggests that national-scale rainfall integrates multiple time-varying variability modes, whereas regional rainfall may preserve more persistent oscillatory characteristics. Overall, the findings provide statistical evidence of structural evolution in dominant variability scales and contribute to improved understanding of the evolving multi-scale behavior of Indian monsoon rainfall.