The Extreme Positive Indian Ocean Dipole of 2019 and Associated Indian Summer Monsoon Rainfall Response

Plain Language Summary A prominent pattern of variability in the Indian Ocean is a seesaw in sea surface temperature (SST) between the eastern and western sides of the Ocean basin, called the Indian Ocean Dipole (IOD). Its influence on the regional weather and climate is not yet fully established, but the extremely strong IOD event in 2019 provided us the opportunity to consider its impact on the Indian Summer Monsoon. By simulating the response to the anomalous SST patterns that occurred in 2019, and by observation-based analyses, we find evidence that the IOD did influence the monsoon rainfall in 2019, but that SST anomalies in the Pacific Ocean were also important. Our simulations show that the positive IOD was conducive to wetter-than-normal conditions throughout and especially at the end of the monsoon season, but that anomalous warmth in the central equatorial Pacific may have contributed to reduced rainfall in June over India. The results from this study help to understand the role of SST anomalies within and outside the Indian Ocean in affecting ISM rainfall intensity and seasonal evolution during extreme IOD events.

The positive Indian Ocean Dipole (IOD) event in 2019 was among the strongest on record, while the Indian Summer monsoon (ISM) was anomalously dry in June then very wet by September. We investigated the relationships between the IOD, Pacific sea surface temperature (SST), and ISM rainfall during 2019 with an atmospheric general circulation model forced by observed SST anomalies. The results show that the extremely positive IOD was conducive to a wetter-than-normal ISM, especially late in the season when the IOD strengthened and was associated with anomalous low-level divergence over the eastern equatorial Indian Ocean and convergence over India. However, a warm SST anomaly in the central equatorial Pacific contributed to low-level divergence and decreased rainfall over India in June. These results help to better understand the influence of the tropical SST anomalies on the seasonal evolution of ISM rainfall during extreme IOD events.