Accelerated termination of the 2019 extreme Indian Ocean Dipole by shortwave-induced warming in a shallow mixed layer off Sumatra and Java

Abstract In 2019, a positive Indian Ocean Dipole (pIOD) event developed into one of the strongest in the past 30 years. Cold sea surface temperature (SST) anomalies in the eastern Indian Ocean peaked in September–October, and then rapidly decayed during November and December to terminate the 2019 pIOD. This study investigates the mechanisms underlying this rapid termination, focusing on the role of mixed layer depth (MLD) and surface heat flux variations. Argo floats in the tropical Indian Ocean were abundant in 2019, enabling observation-based spatiotemporal analysis of upper ocean variations. Using the Argo data, we identified a shallow mixed layer distribution extending from the coastal regions of Sumatra and Java to about 500 km offshore during October to December 2019. Meanwhile, the eastern Indian Ocean experienced positive net surface heat flux driven by increased shortwave radiation and reduced latent heat loss. The analysis shows that the anomalously shallow MLD significantly increased the sensitivity of mixed layer temperature variations to surface heating. As a result, SST in the eastern Indian Ocean warmed rapidly, effectively contributing to the abrupt termination of the 2019 pIOD. These findings emphasize the critical role of MLD variability in modulating SST variations during the IOD decay phase and will offer new insights into the potential evolution of the expected future strong pIOD events.