2019 Southeast Australia bushfire: examining the cause from the climate perspective

Abstract

In the southeast Australia region, 2019 austral summer was remembered as the period of devastating bushfires which was known as “Black Summer”, causing unprecedented damages. Record-breaking temperature and long-lasting drought were pointed as potential causes for this catastrophic event. It is our goal to quantitatively investigate the roles played by various weather and climate conditions on this event with Fire Weather Index (FWI), which is computed based on four climate variables. In our analysis, temperature and precipitation are key variables: a significant increasing trend of temperature in the last 40 years and a dramatic decrease of precipitation in 2019 November and December. In terms of natural climate variability, many studies suggest that positive Indian Ocean Dipole (pIOD) contributes to dry conditions in southeast Australia region. Indian Ocean Dipole index recorded the highest value in 2019 and correlation analysis showed a significantly high correlation with precipitation in the southeast Australia region. Composite of positive IOD cases also showed low precipitation in southeast Australia. Further, the trend in IOD index demonstrated skewness towards pIOD. These results project an increase in low precipitation events induced by positive IOD in the future. In summary, warm and dry conditions were key meteorological conditions to the Black Summer in southeast Australia. Both natural variability and anthropogenic climate change contributed. High positive IOD condition and associated negative precipitation anomaly indicate the former. On the other hand, steadily increased temperature highlights the latter. More importantly, number of days that satisfies dry and hot conditions is observed to reach its peak in 2019, implying influence of human-made climate change and upcoming Australian fire season could be worsening in spite of no clear long-term trend in low precipitation. In other words, assuming that the temperature increases at the current rate, another extreme pIOD event may lead to potential bushfires similar to or much worse than the “Black Summer” of 2019 in the coming years.