Causal effects of Indian Ocean Dipole on El Niño–Southern Oscillation during 1950–2014 based on high-resolution models and reanalysis data

Abstract

Uncertainty exists regarding the interaction between the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) where ENSO is normally expected to be the leading mode. Moreover, the effect of global warming on the relationship between these two modes remains unexplored. Therefore, we investigated the ENSO–IOD linkage for the years 1950–2014 using reanalysis data and high-resolution climate model simulations. The 1950–2014 period is of particular interest as rapid Indian Ocean warming since the 1950s has had a huge impact worldwide. Our results showed that the IOD had robust causal effects on ENSO, whereas the impact of ENSO on IOD exhibited lower confidence. All models demonstrated that the IOD was unlikely to have no causal effects on ENSO, whereas eight out of 15 studied models and the reanalysis data showed significant causal effects at the 10% significance level. The analyses provide new evidence that ENSO interannual variability might be forced by changes in Indo-Pacific Walker circulation induced by the IOD. Weak control of ENSO on the IOD is likely due to nonsignificant effects of ENSO on the western tropical Indian Ocean, implying that the rapid warming environment in the Indian Ocean may fundamentally modulate the relationship between the IOD and ENSO. We find high agreement between the models and reanalysis data in simulating the ENSO–IOD connection. These results indicate that the effects of the IOD on ENSO might be more significant than previously thought.