Summary

The 2019 flash droughts in subtropical eastern Australia and their association with large-scale climate drivers.

Scientific journal article

The drought in subtropical eastern Australia from 2017 to 2019 was the driest and hottest 3-year period since 1911. The most severe period of the drought occurred in 2019 with several manifestations of rapid intensification of drought conditions called flash droughts. In this study, these events were measured using the Evaporative Stress Index (ESI). The ESI is a metric of vegetation moisture stress- based on several factors including rainfall, temperature, humidity, and wind speed. ESIs of -1 are considered drought conditions. Changes in ESI over a 2-week period that are among the 20% largest decreases in ESI and result in an ESI of -1 are considered flash droughts. Under this definition, a flash drought commenced in the Central Slopes of the Upper Darling Basin in June 2019 and persisted for 6 months. In east coast regions the flash drought developed in November and December.

Researchers investigated the extent to which major climate drivers contributed to these flash droughts. Three major climate drivers influence drought in these regions of Australia. The first is El Niño which relates to the eastward shift of warmer water in the Pacific Ocean. The second is the Indian Ocean Dipole (IOD), which relates to changes in the sea surface temperatures across the tropical western and eastern Indian Ocean. The third is the Southern Annular Mode (SAM) that relates to the north-south latitudinal movement of the westerly winds circling the southern hemisphere between 40° and 60° latitude. The flash drought in the Central Slopes developed at the same time as a positive IOD and the Central Pacific El Niño. However, these two factors and the long-term ESI trend explained only half the magnitude of the drought. Similarly, about half the magnitude of the flash droughts in the east coast regions were explained by a strong positive IOD, a moderately strong Central Pacific El Niño, a strong negative SAM, and the long-term ESI trend.

Although major climate drivers did play a role, their influence was not strong enough to assist with predicting the precise timing and magnitude of the droughts in the various regions. Successful forecasts of flash droughts will require local and current information as well as information on the major climate drivers.