Abstract: 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. It is affected by both the El Niño-Southern Oscillation (ENSO) and internal variability within the Indian Ocean. However, the quantification of two types of ENSO, namely the Eastern Pacific (EP) and the Central Pacific (CP), and internal variability remains unclear. Here, a binary combined linear regression method can be 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 influence the IOD during extreme events. Their influencing mechanisms are different: ENSO primarily influences the Indian Ocean wind field via the Walker circulation, of which efficiency depends on the location of the warm core with EP and CP types accordingly. In comparison, internal variability tends to cause SST anomalies through oceanic processes that facilitate IOD. Due to the longer lifetime of El Niño events, the co-occurring positive IOD has a higher chance of transforming into the Indian Ocean basin-wide warming in the following spring, in which ENSO contributes more than 70%. Although internal variability does not show a significant statistical relationship with this transition, a strong positive IOD still has the potential to induce the following basin-wide warming. These findings would improve understanding of the climate modes and inter-basin interactions.

Key words: Tropical Indian Ocean, IOD, ENSO, IOB