Air–sea interactions in tropical Indian Ocean: The Indian Ocean Dipole

The El Nino/Southern Oscillation (ENSO) has been recognized as the dominant mode of climate variations with widespread global impacts. However, ENSO variability and its teleconnections are influenced by other modes of climate variations. The Indian Ocean Dipole (IOD) is one such climate mode. For example, the IOD has changed the way ENSO influences the Indian summer monsoon rainfall (ISMR). It is found that the adverse effect of El Ninos on the ISMR has diminished a lot during recent decades when the positive IODs (pIODs) evolved more frequently. However, the frequent occurrences of pIODs have been very unfavorable for the eastern sphere of the IOD. Many parts of Australia and Indonesia report frequent droughts and forest fires during these pIOD years. Therefore the predictability of the IOD at long-lead times is sought from the operational centers to develop mitigation measures. While most models, especially the Scale Interaction Experiment-Frontier Research Center for Global Change prediction system, have shown skills to predict the IOD, it remains a challenge to predict them at sufficiently long-lead times. Nevertheless, there are some exceptions like the 2019 pIOD event. This event was unique in the sense that it was a record-breaking pIOD and was observed in absence of El Nino. This chapter discusses this unique event at length and reviews past studies in the context of this exceptional phenomenon.