Tropical Indo‐Pacific Compounding Thermal Conditions Drive the 2019 Australian Extreme Drought.

Australia suffered a long‐lasting extensive drought in 2019 with catastrophic wildfires creating about $4.4 billion damages, the worst record in the recent four decades. Concurrent with this extreme drought, the tropical Indo‐Pacific oceans exhibited an extraordinary combination of sea surface temperature (SST) anomalies, characterized by a Central‐Pacific (CP) El Niño event with westernmost location and the third strongest positive Indian Ocean Dipole (IOD) event. We here show evidence that this unique combination was responsible for the pan‐Australian drought as a westward located CP El Niño event and a strong positive IOD event can exacerbate precipitation reduction in northeastern and southern Australia, respectively. These continent‐wide extreme droughts could become more frequent over Australia in a warming world, considering projected increases in both CP El Niño and extreme positive IOD events superimposed on secular warming and drying trends. Plain Language Summary: Australia is the driest inhabited continent in the world, 70% of which is either arid or semiarid land. During 2019, Australia was ravaged by the worst drought in decades with devastating wildfires destroying around 3,000 homes and affecting about one billion animals. The physical mechanisms driving this extreme drought remains unclear. Concurrent with this long‐lasting continent‐wide drought, the Indo‐Pacific oceans showed pronounced sea surface temperature anomalies associated with a record westernmost‐located Central Pacific (CP) El Niño event and the third strongest Indian Ocean Dipole (IOD) event co‐occurring. Our observational diagnostics and numerical model experiments show that these two compounding events caused atmospheric circulation anomalies that were responsible for the extreme precipitation deficiencies over Australia. With global warming, both CP El Niño and extreme positive IOD events are projected to become more frequent, which hints to increased risk of the severe Australian droughts in the future. Key Points: Australia suffered a long‐lasting extensive drought in 2019, the worst record in the recent four decadesThis extreme drought can be well explained by combined effects of the westernmost‐located CP El Niño and third strongest positive Indian Ocean Dipole (IOD) eventsSevere Australian droughts could occur more frequently in a warming world with projected increases in both CP El Niño and extreme IOD events [ABSTRACT FROM AUTHOR]