Are the antecedent drought conditions magnifying the summer hot extremes in Australia

The occurrence of extreme climate events such as droughts and heatwaves can have negative economic, environmental and social impacts. The simultaneous or sequential occurrence of these extreme events can increase these impacts, and their frequency is expected to increase in many regions of the world. Moreover, the occurrence of hot days/nights was shown to be correlated to drought conditions in Mediterranean areas. Recently the catastrophic fire seasons of 2019/2020 in Australia has been pointed out to be associated with a drought exacerbation of the summer hot conditions. Additionally, temperature trends In Australia since 1970 are positive in most of the territory, whereas annual precipitation presents a negative trend in the East and a positive trend in the West.

In this work, we propose to analyze the relation between summer hot days/nights and antecedent drought conditions in Australia. The Standardized Precipitation Evapotranspiration Index (SPEI) at time-scales of 1 to 6 months was used to assess drought conditions. The indices Number of Hot Days (NHD) and Number of Hot Nights (NHN) were computed as the number of days on each month that exceed the 90^th percentile of maximum and minimum temperatures, respectively. Data to compute these indices were retrieved from the ERA5 climate reanalysis dataset, and from the CRU TS4.03 dataset. Temperature data from the ACORN-SAT dataset was also used. A correlation analysis was performed between SPEI and NHD/NHH, using the concurrent months, and also the SPEI values on the previous 1 to 3 months. Significant negative correlations were obtained in southern regions. A probabilistic approach was also used, using copula functions, which allowed to estimate the joint probability of occurrence of dry and hot events.

Acknowledgements: This work was partially supported by projects FireCast (PCIF/GRF/0204/2017), and IMPECAF (PTDC/CTA-CLI/28902/2017). Andreia Ribeiro thanks FCT for the grant PD/BD/114481/2016.