A Unique Feature of the 2019 Extreme Positive Indian Ocean Dipole Event

During austral spring of 2019, an extreme positive Indian Ocean Dipole (pIOD) event occurred, with cold sea surface temperature (SST) anomalies over the eastern equatorial Indian Ocean (EEIO) and warming in the west. Although the growth of the EEIO cold anomalies involves forcing by equatorial nonlinear advection, unique to the 2019 pIOD is an air-sea heat flux that was a forcing to the EEIO cold anomalies, rather than a damping as in previous extreme events. This unique thermodynamic forcing is due to a large latent cooling, which is supported by an unusually strong wind speed contributed by a large southerly anomaly as part of a long-term trend. The wind trend is underpinned by a mean state SST change featuring slower warming off Sumatra-Java. Given that a similar SST trend pattern is projected under greenhouse warming, the likelihood of such thermodynamical forcing operating more frequently in the future needs to be considered.