Since early 2023, severe to exceptional drought has developed in southern coastal Australia, with dam levels falling as stream flows plummet. The wet season, April to September, reflects the most equatorward position of the mid-latitude westerly wind regime that brings rain-bearing systems to southern coastal Australia. Climatologically, an upper-level tropospheric split-jet is present in the Australia–New Zealand region. This is evident in the subtropical jet (STJ) location when the 1965 to 1995 u-component of the 250 hPa wind anomaly, relative to 1991 to 2020, is located above northern tropical Australia, and the weaker polar-front jet (PFJ) branch anomaly spans the mid-latitudes south of Australia. Permutation testing revealed a statistically significant decrease in the 2016 to 2025 wet season mean precipitation across southern Australia. Compared with the 1965 to 1995 u-component wind anomaly at 250 hPa, the 2006 to 2015 decadal anomaly still shows the split jet with the STJ branch over northern tropical Australia and the PFJ in the mid-latitudes of the Australia–New Zealand region. However, there is a dramatic change in position and structure of the STJ branch of the split jet, between the 1965 to 2015 and the 2026 to 2025 anomalies. The split jet structure has shifted approximately 10° poleward, causing rain-producing systems to track south of the Australian continent. The reduced precipitation can generate more frequent and intense droughts, with greatly reduced stream flows and dam levels. Historically, the low precipitation warm season follows from October to March when heatwaves, combined with pre-existing dry conditions, often create catastrophic bushfire conditions.
Speer et al. (Tue,) studied this question.
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