ENSO Modulation of Interannual Variability in the First Occurrence of MJO and BSISO Events

This study investigates the interannual variability in the first occurrence dates of the first Madden Julian Oscillation (FMJO) and the first Boreal Summer Intraseasonal Oscillation (FBSISO) events over the Indian Ocean during 1940–2022. The FMJO typically occurs in November, whereas the FBSISO mainly occurs in early May. The FMJO onset exhibits substantially larger interannual variability. The timing of both phenomena is strongly influenced by the El Niño Southern Oscillation (ENSO), which modulates large scale atmospheric conditions, particularly through column moisture anomalies. The leading Empirical Orthogonal Function (EOF) mode of April column moisture anomalies displays a meridional dipolar pattern over the Indian Ocean. Its associated principal component (PC) time series shows a statistically significant correlation with the FBSISO onset date, indicating that a strong meridional moisture contrast with positive anomalies over the northern Indian Ocean favors an earlier FBSISO onset. The leading EOF mode of November column moisture anomalies exhibits a zonal dipolar pattern extending from the western equatorial Indian Ocean to the Maritime Continent, with a pronounced meridional contrast over the Maritime Continent. The corresponding PC is significantly correlated with the FMJO onset date, suggesting that a zonal moisture contrast with higher moisture over the western Pacific favors an earlier FMJO onset. These moisture anomalies are closely linked to ENSO conditions. Consequently, ENSO also modulates the onset timing of these events, although the onset dates show weaker correlations with ENSO than with monthly moisture anomalies. The physical mechanisms underlying the distinct responses of the FMJO and FBSISO to ENSO remain to be elucidated.