Flash droughts are a recently defined type of extreme climate phenomenon characterized by the sudden onset and rapid intensification of drought conditions, which can have serious consequences on agriculture and natural vegetation. Australia is the driest continent globally, with more than 80% of the country having an average annual rainfall of less than 600 mm. Drought is one of Australia’s major agricultural problems, affecting food production, farmers’ living conditions and costing the government billions of dollars in aid. 

Dr. Hanh Nguyen, Dr. Matthew Wheeler, Dr. Harry Hendon, Dr. Eun-Pa Lim at the Bureau of Meteorology, Melbourne, Australia, in collaboration with Dr. Jason Otkin from the University of Wisconsin-Madison, conducted a study on the evolution of flash drought conditions in subtropical eastern Australia during 2019 and investigated aspects of its predictability using linear regression with global climate drivers. The paper was published in the Weather and Climate Extremes journal in June this year.

Dr. Nguyen and colleagues focused on the Australian 2017-2019 drought period with embedded flash drought occurrences. “This multi-year drought across many parts of Australia was the driest and hottest 3-year period since 1911 due to a massive precipitation shortfall. During 2017–2019, the drought impacts increased to become most severe in 2019 after several episodes of rapid intensification that we have objectively identified as flash drought,” added Dr. Nguyen. To monitor the point-by-point drought advancement and define occurrences of flash drought, the team uses the Evaporative Stress Index (ESI), and multiple linear regression with the ESI is used to make conclusions on the extent to which flash drought may be predicted. 

The authors investigated the progression of flash drought by the ESI, reflecting the combined effects of many factors that can lead to vegetation stress, such as lack of moisture in the soil and elevated evaporative demand. “Monitoring of drought conditions, using ESI, allows us to identify and fully characterize flash drought events in terms of timing, span, and severity, and based on the ESI and its relation to the climate drivers, we’ve learnt that 2019 is not an entirely unexpected drought,” said Dr. Nguyen. The drought intensity expanded to be extreme in 2019 after a few weeks of rapid drought intensification fostered by the consolidated impacts of near-record strong positive Indian Ocean Dipole Mode, solid Central Pacific El Niño Modoki near-record strong negative Southern Annular Mode.  

Then the research team suggested that the simultaneous occurrence of strong positive Indian Ocean Dipole mode, El Niño Modoki and strong negative Southern Annular Mode provided a certain degree of predictability in flash drought occurrence and worsening. “However, while climatic regimes are mostly well predicted at several weeks lead time by both statistical and dynamic methods, predicting the exact timing and location of a flash drought will require more local and current information than such major climatic factors, as can be provided by initial atmospheric conditions for a dynamic model,” said Dr. Nguyen. Dr. Nguyen and colleagues’ research also examines the predictability of flash droughts using the Australian Community Climate and Earth-System Simulator-Seasonal v1.0 (ACCESS-S1) model. The findings of this extensive research will guide future studies on flash drought predictability.

Journal Reference and main image credit:

Nguyen, H., Wheeler, M.C., Hendon, H.H., Lim, E.P. and Otkin, J.A., 2021. The 2019 flash droughts in subtropical eastern Australia and their association with large-scale climate drivers. Weather and Climate Extremes32. DOI: https://doi.org/10.1016/j.wace.2021.100321

About the Author

Dr. Hanh Nguyen, Ph.D.

Dr. Hanh Nguyen is a senior climate scientist at the Bureau of Meteorology, Australia. She completed her Ph.D. on Equatorial Africa synoptic rainfall variability and its link to equatorial convectively coupled Kelvin waves at University Pierre and Marie Curie in Paris, France. Her research interests include tropical climate variability and climate change, the science of droughts and seasonal climate prediction. Dr. Hanh Nguyen currently works on flash droughts as part of the Northern Australian Climate Program (NACP). Her team was the first to publish on flash droughts over Australia in 2019. Her publication record can be accessed via Researchgate or Google Scholar.