Indian Ocean Dipole Diversity in Both Positive and Negative Phases: Impacts on ENSO Phase Transitions

Recent studies have identified the Indian Ocean Dipole (IOD) as a critical precursor to subsequent El Niño-Southern Oscillation (ENSO) evolution. However, the dynamic mechanisms through which its spatiotemporal diversity influences ENSO phase transitions remain poorly understood. This study explores how different IOD types affect ENSO transition. Utilizing the Liang-Kleeman information flow method, we show that IOD exerts a causal influence on ENSO with a lead time of 13 months. Spectral clustering and composite analyses reveal distinct teleconnection mechanisms associated with different IOD subtypes. Positive type-I IODs, which co-occur with El Niño and feature a strong western pole, trigger rapid warming in the eastern Indian Ocean via atmospheric forcing, evolving into the Indian Ocean Basin Mode (IOBM). This persistent thermal forcing excites an anomalous anticyclonic circulation over the northwestern Pacific (WNPAC), generating equatorial easterly wind stress that drives eastward-propagating oceanic Kelvin waves. These waves shoal the eastern Pacific thermocline and induce the local cooling, accelerating the transition towards La Niña. Conversely, negative type-I IODs, with a pronounced cold western pole, similarly facilitate the shift from La Niña to El Niño through analogous air-sea interactions. Type-II IODs, lacking a strong western pole in both phases, do not produce a comparable effect.