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