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, typically occurring during the boreal fall, influencing weather and climate in surrounding regions and even China. The IOD is affected by both the El Niño-Southern Oscillation (ENSO) and internal variability within the Indian Ocean. However, the quantitative contributions of two types of ENSO, namely the eastern Pacific (EP) and the central Pacific (CP), and internal variability to the IOD remain unclear. Here, a binary combined linear regression method is used to separate and estimate the contributions of these three factors. The results show that internal variability is the primary source of IOD sea surface temperature (SST) changes, accounting for more than 60% of the variance. The contribution of ENSO is about one-third, predominantly driven by the CP ENSO, whereas the EP type tends to exert a stronger influence on the IOD during extreme events. Their influencing mechanisms are different: ENSO affects the Indian Ocean wind field primarily via the Walker circulation, with the efficiency depending on the location of the warming cores (EP vs. CP). In comparison, internal variability tends to induce SST anomalies through oceanic processes within the Indian Ocean, facilitating IOD development. Due to the longer lifetime of El Niño events, a co-occurring positive IOD has a higher chance of transforming into an Indian Ocean basin-wide warming event in the following spring, for which ENSO contributes more than 70% of the transition. Although internal variability does not show a significant statistical relationship with this transition, a strong positive IOD still has the potential to induce subsequent basin-wide warming. These findings improve our understanding of climate modes and inter-basin interactions.