A Skewed perspective of the Indian rainfall-ENSO Relationship
Abstract. The application of higher-order wavelet analysis to India rainfall and the El Niño/Southern Oscillation (ENSO) is presented. An auto-bicoherence analysis is used to extract the frequency modes contributing to the skewness of India rainfall and ENSO. A nonlinear wavelet coherence method is proposed for diagnosing why the time-domain correlation between two time series temporally changes when at least one time series has changing nonlinear characteristics. The results indicate the India rainfall and ENSO are highly nonlinear phenomenon. It is also demonstrated that the sea surface temperature (SST) patterns associated with different nonlinear ENSO modes depend on the frequency components participating in the nonlinear phase coupling. The SST pattern associated with coupling between ENSO modes with periods of 31 and 15.5 months is reminiscent of a central Pacific El Niño and intensifies around 1995, contrasting with the coupling between the 62- and 31-month modes that became active around the 1970s ENSO regime shift. A nonlinear coherence analysis showed that the skewness of India rainfall is weakly correlated with that of 4 ENSO time series after the 1970s, indicating that increases in ENSO skewness after 1970's at least partially contributed to the weakening India rainfall-ENSO relationship in recent decades. The implication of this result is that the intensity of skewed El Niño events is likely to overestimate India drought severity, which was the case in the 1997 monsoon season, a time point when the nonlinear wavelet coherence between All-India rainfall and ENSO reached its lowest value in the 1871–2016 period.