Abstract
The Indian Ocean Dipole (IOD) is a prominent mode of ocean-atmosphere interannual variability with great climate and socioeconomic impacts. Early positive IOD (pIOD), a newly discovered type of pIOD, induces pronounced rainfall anomalies in boreal summer more than canonical pIOD. It also contributes to more frequent consecutive pIODs, causing devastating droughts and floods. How early pIOD responds to global warming remains unknown. Here, we show that early pIOD has increased substantially in the past decades, reaching the same frequency as canonical pIOD. The increase is caused by intensified Bjerknes feedback and an early summer monsoon onset, which is the major trigger for early pIOD. Model simulations suggest that the increased frequency of early pIOD is likely to continue under greenhouse warming by the same mechanisms as in the observations, increasing boreal summer climate variability and leading to more climate extremes in affected regions.