Decadal shifts in the modulation of ENSO on the Indian ocean basin mode

Abstract This study investigates the decadal variation in the modulation of the El Niño–Southern Oscillation (ENSO) on the summer Indian Ocean Basin Mode (IOBM), revealing a weak–strong–weak pattern since the 1950s. Since the 1950s, the ENSO–summer IOBM relationship has experienced two decadal shifts, with the 1980s and 2000s as transition points. Notably, even during the two weak-correlation subperiods, the spatiotemporal evolution of Indian Ocean SST shows distinct differences, and ENSO’s modulation processes vary significantly. A strong ENSO–summer IOBM relationship occurs only during 1982–2003, maintained by slower ENSO decay, prolonged atmospheric bridge effects, effective propagation of oceanic Rossby waves in the southern Indian Ocean, and a well-established wind–evaporation–SST (WES) feedback sustaining the IOBM through summer. Weak relationships are observed before 1981 and after 2004, as the IOBM decays in early spring (1958–1981) or late spring (2004–2022). The rapid decay of the IOBM in the first subperiod is linked to fast ENSO decay, which terminates both atmospheric teleconnections and oceanic processes early, causing the warming to vanish by early spring. In the third subperiod, although slower ENSO decay allowed longer persistence, anomalous local winds disrupted the WES feedback, preventing basin-wide warming after late spring. These findings emphasize the importance of process-based analyses in understanding and predicting ENSO’s delayed impacts on the Indian Ocean.