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Eastern and western Tasmania rely on different weather systems for rainfall, new study shows

Tasmania’s rainfall is driven by a diverse range of weather systems, with new research revealing how these systems shape seasonal variability across the state.

The findings come from a study led by 21st Century Weather researchers based at the University of Tasmania, which examined the contributions of different weather systems to Tasmania’s rainfall.

The study shows that over 80% of Tasmania’s annual rainfall and more than 90% of moderate to heavy rain days are associated with extratropical cyclones, cold fronts, thunderstorms, and their co-occurrences.

However, the drivers of rainfall differ markedly between the east and west of the state.

In eastern Tasmania, where most people live and where rainfall is critical for agriculture, extratropical cyclones account for a large share of total rainfall and heavy rain days.

In contrast, western Tasmania’s very high rainfall is primarily driven by cold fronts, often intensified as they move across the state’s mountainous terrain. Cyclone-related systems can enhance rainfall further, particularly during anomalously wet years.

Analysing data from 1979 to 2023, the researchers also found a decline in warm-season (November-April) rainfall in western Tasmania, linked to changes in the frequency of key weather systems that bring moderate to heavy rainfall.

The research further highlights the important role of extratropical cyclones, including East Coast Lows, in shaping Tasmania’s rainfall patterns. The authors argue that systems extending as far south as 45°S should be considered when assessing the impacts of East Coast Lows on southeastern Australia, including Tasmania.

First author Moulik Mandal, a PhD student at the University of Tasmania, said: “Our study shows that long-term rainfall trends are largely driven by changes in how often these weather systems occur, while year-to-year swings between anomalously wet and dry seasons are more closely linked to how intense they are.”

Co-author Professor Neil Holbrook, a Chief Investigator with 21st Century Weather, added: “A better understanding of how these midlatitude weather systems may change in a warming climate is critical for Tasmania’s future water security, agriculture and renewable energy planning.”

The full study, ‘Contributions from different weather systems to rainfall in Tasmania, Australia’, is published in the Journal of Southern Hemisphere Earth Systems Science.