Cases for the sole effect of the Indian Ocean Dipole in the rapid phase transition of the El Nino-Southern Oscillation

Abstract

The role of the Indian Ocean sea surface temperature (SST) in the rapid phase transition of the El Nino-Southern Oscillation (ENSO) has received much attention over the last few decades. However, the distinctive role of the Indian Ocean Dipole (IOD) and its underlying dynamics in controlling the rapid transition remains debatable, since it is difficult to isolate the sole effect of the IOD using observation. By conducting model experiments, this study demonstrated that the positive IOD cases could contribute to the rapid phase transition from El Nino to La Nina without the assistance of the Indian Ocean Basin-wide Mode. The westerly wind anomalies over the tropical western Pacific induced not only the El Nino phase via descending Kelvin waves in the equatorial eastern Pacific but also the SST cooling in the off-equatorial western Pacific via ascending Rossby waves. The positive feedback between the upwelling Rossby waves and cool SST maintained the Philippine Sea anticyclone anomalies. The accompanying easterly wind anomalies over the tropical western Pacific triggered upwelling oceanic Kelvin waves, leading to the occurrence of La Nina. This study shows the possible role of the IOD in the rapid phase transition of ENSO.