What Caused the Extreme Indian Ocean Dipole Event in 2019?

An extreme positive Indian Ocean Dipole (IOD) event occurred in 2019 boreal autumn, which has induced severe climate impacts around the Indian Ocean basin. In this study, the cause for 2019 IOD event and the related mechanism are explored. We find that the remarkable strengthening of Australian high and weakening of sea level pressure over South China Sea/Philippine Sea have been evidently visible since May 2019. Such a record-breaking interhemispheric pressure gradient (IHPG) induced northward cross-equatorial flow over the western Maritime Continent, which triggered strong wind-evaporation-SST and thermocline feedbacks in 2019. In addition to 2019 case, historical IOD events are highly correlated with IHPG from boreal late spring to summer. We show that skillful IHPG prediction can be made in March by European Centre for Medium-Range Weather Forecasts seasonal forecast system, which makes it quite possible for the early warning of extreme IOD events by two to three seasons ahead.