Sensitivity of Rainfall Extremes to Unprecedented Indian Ocean Dipole Events.

Strong positive Indian Ocean Dipole (pIOD) events like those in 1997 and 2019 caused significant flooding in East Africa. While future projections indicate an increase in pIOD events, limited historical data hinders a comprehensive understanding of these extremes, particularly for unprecedented events. To overcome this we utilize a large ensemble of seasonal reforecast simulations, which show that regional rainfall continues to increase with pIOD magnitude, with no apparent limit. In particular we find that extreme rain days are highly sensitive to the pIOD index and their seasonal frequency increases super‐linearly with higher pIOD magnitudes. It is vital that socio‐economic systems and infrastructure are able to handle not only the increasing frequency of events like 1997 and 2019 but also unprecedented seasons of extreme rainfall driven by as‐yet‐unseen pIOD events. Future studies should prioritize understanding the hydrological implications and population exposure to these unprecedented extremes in East Africa. Plain Language Summary: The Indian Ocean Dipole is a pattern of climate variability in the Indian Ocean, characterized by opposite‐signed sea surface temperature differences from normal in the west and east. During positive events (pIOD), the western Indian Ocean is warmer than usual, which disrupts the normal circulation of the atmosphere. When pIOD is strong, extreme rainfall and flooding are common in East Africa. This happened in 1997 and 2019. According to future predictions, the frequency of extreme pIOD events will increase. However, our understanding of these events is limited as there is not a large enough sample of past events in the historical data to study. To address this, we evaluate a large collection of climate model simulations called seasonal reforecasts. The results show no evidence of an upper limit to the impact of pIOD on regional rainfall. The frequency of rain days increases during pIOD, whilst extremely heavy rain days increase even more. This means that more frequent extreme pIOD seasons will likely bring more heavy rainfall impacts. It also means that if pIOD reaches a high magnitude which has not yet been recorded, the rainfall impacts are likely to be even larger than those seen in 1997 or 2019. Key Points: Unprecedented strong positive Indian Ocean Dipole (IOD) events are likely to bring extreme rainfall to East AfricaThe extreme component of the seasonal distribution of rain days is particularly sensitive to the magnitude of the IODThe impact of larger IOD events on extreme rainfall frequency increases non‐linearly with the IOD magnitude [ABSTRACT FROM AUTHOR]