Summary

Climatology and variability of the evaporative stress index (ESI) and its suitability as a tool to monitor Australian drought

The Evaporative Stress Index (ESI) is a measure of the vegetation moisture stress based on several factors including rainfall, temperature, humidity, and wind speed. Comparisons between the ESI and other indices confirm the ESI is a robust drought index, performing much better than an indicator based solely on rainfall. This work provides a more complete understanding of drought in Australia and demonstrates the usefulness of ESI in real time drought monitoring and forecasting.

Trends in ESI since 1975 largely follow rainfall but suggest increasing temperatures are exacerbating dry conditions. Decreases in ESI since 1975 were observed in east-central Australia in winter, the southern half of Western Australia in winter and spring, the south-southeast in spring and all of Tasmania year-round. Increasing ESI over that period was most widespread in summer and autumn including much of the west and the southeast coast strip with increases in northwest Australia in winter and spring. Definitions of drought, based on ESI, were developed, and applied to NRM regions from 1975 to 2020. The results showed that over all regions, there was an average of 2.5 droughts per decade with an average duration of 11 months.

ESI was more strongly correlated with rainfall than temperature and this correlation between ESI and rainfall was significant across much of the country. ESI was also correlated with phases of major climate drivers associated with drought, including El Niño, a positive Indian Ocean Dipole (IOD), and a negative Southern Annual Mode (SAM). The strongest correlations were associated with El Niño conditions in the north. Correlations with the positive IOD occurred in the southwest in winter and the southeast in spring. Correlations with the negative phase SAM were strongest in spring with a 1-month lag between SAM and the ESI, suggesting a potential opportunity for early warning of drought intensification.