Quantifying impacts of ENSO and internal variability on the Indian Ocean Dipole

The Indian Ocean Dipole (IOD) is an intrinsic climate mode in the Indian Ocean that typically peaks during boreal fall and influences weather and climate across surrounding regions. It is influenced by both the El Niño–Southern Oscillation (ENSO) and internal variability within the Indian Ocean. However, the relative contributions of the two ENSO types—namely, Eastern Pacific (EP) and Central Pacific (CP) ENSO—and internal variability to the IOD remain poorly quantified. Here, we employ a binary combined linear regression approach to isolate and quantify the contributions of these three factors. The results show that internal variability is the dominant driver of IOD-related sea surface temperature (SST) anomalies, explaining over 60% of the variance. ENSO accounts for approximately one-third of the variance, primarily through the CP type, whereas the EP type tends to influence the IOD mainly during extreme events. Their underlying mechanisms differ. ENSO primarily modulates the Indian Ocean wind field through the Walker circulation, whose effectiveness depends on the longitudinal position of the equatorial Pacific warming—eastern for EP events and central for CP events. In contrast, internal variability generates SST anomalies through local ocean–atmosphere feedbacks that sustain the IOD. Because El Niño tends to persist longer, co-occurring positive IOD events are more likely to evolve into basin-wide Indian Ocean warming in the following spring, a transition to which El Niño contributes more than 70%. Although internal variability shows no significant statistical association with this transition, a strong positive IOD alone still has the potential to trigger the basin-wide warming in the subsequent spring. These findings enhance our understanding of climate modes and inter-basin interactions.