Interdecadal modulation of the effect of ENSO on rainfall in the southwestern Pacific

The El Niño-Southern Oscillation (ENSO) is the dominant driver of interannual variability on rainfall in many Pacific Islands and in countries bordering the tropical Pacific Ocean. From 1916 through to 1975, the correlation coefficient between the Southern Oscillation Index (SOI) and interannual variability in rainfall in eastern Australia was strong in negative phases of the Interdecadal Pacific Oscillation (IPO) but weak in positive phases. By examining records of rainfall over the past hundred years in central Vanuatu and on the 'dry side' of Fiji, which both lie near the southern edge of the South Pacific Convergence Zone (SPCZ), we find that such modulation by IPO has been much weaker there than in eastern Australia. This paper examines possible reasons for this difference. We also find that the correlation between rainfall and the SOI remained strong throughout each of the past three phases of the IPO, in all these places, including eastern Australia. However, at Rarotonga in the southern Cook Islands, whose position is also near the southern edge of the SPCZ, but at the southeastern end, the displacement of the SPCZ by ENSO events is greater there than further west. Consequently, the correlation between rainfall and SOI is so strong at Rarotonga in El Niño years with SOI <-5 that SOI alone becomes a good predictor of wet-season rainfall there. The difference in modulation of rainfall in eastern Australia between the two positive phases of IPO (1926-1941 and 1978-1998) may be due to the influence on Australia of other climatic oscillations, such as the Indian Ocean Dipole, though other factors may also have played a role.