Extremes in a Tree-ring Drought Reconstruction for Australia's Monsoonal North are Modulated by Central Pacific Ocean Sea Surface Temperatures

Understanding the drivers of tropical hydroclimate variability and how hydroclimate is likely to change into the future are acutely hampered by short instrumental records. Here we present a 246-year tree-ring reconstruction of drought during the wet-dry transition season (March - May) for northern Australia. This reconstruction extends the instrumental drought record back by 150 years. Around one third of total annual rainfall typically falls during the wet-dry transition season (March - May), making it a crucial component of the monsoonal cycle. This wet-dry transition is also the season most impacted by the differential decay process of Central Pacific (as opposed to Western Pacific) El Niño events that are linked with dry conditions over northern and northwestern Australia. In the reconstruction, we found that hydroclimate extremes in the region had a strong, but asymmetric relationship with central Pacific sea surface temperatures (SSTs) and El Niño-Southern Oscillation indices. Years with extreme wet transition seasons were associated with cooler SSTs, above average rainfall across much of Australia, and often coincided with La Niña events. Although the spatial relationship between dry extremes and Pacific SSTs is generally consistent with the SST signature of central Pacific El Niño events, it is less clear than for wet extremes. The relationship between drought in the far north and dry conditions across Australia is also less extensive and weaker than for wet events. Our results suggested that more extreme wet events in the Australian far north likely reflect cool central Pacific SSTs and later termination of the Australian monsoon. Persistent extreme drought became more frequent over the latter part of the 20^th Century while the probability of a sudden transition from a dry extreme to a wet extreme peaked in the mid 20^th Century and has since declined.