Interannual Variability of the Indonesian Rainfall and Air–Sea Interaction over the Indo–Pacific Associated with Interdecadal Pacific Oscillation Phases in the Dry Season

The interannual and interdecadal variabilities of Indonesian rainfall in dry seasons (June–November) are investigated by using rainfall data from the Climate Research Unit (CRU) from 1939 to 2016 and from the Global Precipitation Climatology Project (GPCP) from 1979 to 2016. The first principal component (PC1) of both the CRU and GPCP data shows that the canonical El Niño–Southern Oscillation (ENSO), ENSO Modoki, and Indian Ocean Dipole (IOD) are major climate modes influencing the interannual variability of rainfall in Indonesia, and the Interdecadal Pacific Oscillation (IPO) is the major decadal phenomenon affecting the decadal variability of the rainfall. Furthermore, the IPO modulates the influence of IOD on Indonesian rainfall, with a weaker influence during the positive IPO phase during 1979 – 1997 and a stronger influence during the negative IPO phases during 1939 – 1978 and 1998 – 2016. The dependency of Indonesian rainfall response to the canonical ENSO and ENSO Modoki on IPO phases is not significant, although the response to the ENSO Modoki (canonical ENSO) becomes significant (less significant) in the negative IPO phase during 1998 – 2016 when compared with earlier periods.