Relationship between sunspot number and seasonal rainfall over Kerala using wavelet analysis

Abstract

In this work, we investigate the periodicities of rainfall over Kerala, India, and sunspot number (1871–2016) and their possible association. Fourier and wavelet transform were performed on the rainfall and sunspot number time series to determine dominant periods. Both the data were considered on an annual scale, as well as on a seasonal scale by grouping into winter, pre-monsoon, monsoon, and post-monsoon seasons. Fourier analysis showed short periods between 2–12 years for all the seasonal rainfall and a prominent 10.7-year period for sunspot number. Further, the wavelet spectrum of rainfall over Kerala showed the most significant periodicities around 2–10 years but with intermittent character and 8–12 years for sunspot number. The periodicity of sunspot number and rainfall estimated by Fourier analysis was in agreement with wavelet analysis. Wavelet analysis gave common spectral powers with varying significance for winter and monsoon seasons around 2–3 years. Common features in the wavelet power of the two time series were visible at 8–12 years with varying significance, suggesting a relationship between them. The cross-wavelet transform, employed to study the temporal relationship, indicated high cross-power around 8–12 years, during all the seasons. The relationship between sunspot number and rainfall over Kerala during varying levels of solar activity was examined using wavelet coherence. Results showed higher coherence during the high solar activity period than during low solar activity, mainly during winter, monsoon, and post-monsoon seasons. Coherence studies were also performed to check whether the influence of solar activity on rainfall completely diminished after the onset of global warming. Results showed persistence of solar influence in different seasons during the whole period of study.