Research clarifies record-late monsoon onset, aiding northern Australian communities

Every year, Australia experiences a summer monsoon characterized by the reversal of winds, heavy rainfall and flooding. In 2024-2025 however, the Australian summer monsoon (ASM) was the latest on record since measurements began in 1957.

The monsoon's timely arrival is critical for Northern Australia. It dictates water security for communities, drives pasture growth for the vital cattle industry and signals the end of the high bushfire risk period.

The start of the ASM is defined by the change in prevailing dry southeasterly trade winds that occur most of the year to moist northwesterly winds that dominate during the wet season. These winds are measured above the city of Darwin, which is located in the northernmost part of Australia, and is often the first place in Australia to experience the onset of monsoonal conditions each season, usually around Christmas and the New Year.

Despite favorable large-scale climate conditions that would usually support an earlier onset, such as a La Niña-like state in the Pacific Ocean, record ocean warmth and strong ocean convection in December, the ASM onset occurred on 7 February 2025, more than five weeks later than usual and more than two weeks later than the previous record set in 1972-1973.

To better understand the reasons for the delayed ASM in the summer of 2024-2025, an international research group led by scientists from the University of Southern Queensland investigated monsoon criteria and local weather conditions that may have affected the onset of the ASM.

The team published their study on January 29 in Advances in Atmospheric Sciences as part of a special issue organized by the WMO's World Climate Research Programme monsoon panel.

“We… wanted to determine whether this record-late onset was a genuine feature of the broader monsoon system or a result of the local wind conditions at Darwin. To understand this further, we compared numerous monsoon onset definitions across northern Australia and investigated the atmospheric processes that occurred before and during the onset period,” said Tim Cowan, Associate Professor in the Centre for Applied Climate Sciences at the University of Southern Queensland in Toowoomba, Australia and first author of the research paper.

To determine if the late ASM was due to a true delay in the monsoon season or isolated conditions in Darwin, the research team applied multiple ASM- and rainy season (RS)-onset definitions and analyzed the atmospheric and oceanic conditions during the pre-monsoon and onset periods in Darwin and the surrounding area.

Importantly, the researchers observed that the conditions in Darwin during the summer of 2024-2025 did not necessarily reflect other parts of Australia. “The record-late ASM onset in 2024/25 was not entirely representative of the broader monsoon conditions across all of northern Australia but only specific to local wind conditions over Darwin. Other onset definitions showed much earlier onsets than the official ASM wind definition for Darwin, especially those that describe the onset of rainfall,” said Rajashree Naha, research fellow at the Centre for Applied Climate Sciences at the University of Southern Queensland in Toowoomba and co-author of the research paper.

The data suggests that local weather variability over Darwin prevented the upper-level winds from switching to easterlies, despite the presence of strong westerlies in December.

The team hopes their research will benefit weather communication with the general public. “Often people (and the media) confuse the official monsoon onset, based on local winds, with the onset of the first monsoonal rains. The release of our new study provides a great opportunity to improve the messaging around the monsoon arrival and what these impacts are on local communities,” said Cowan.

Ultimately, the research team emphasizes their work isn't just about wind definitions. It's about ensuring that communities who depend on the monsoon receive clear, actionable information. The team hopes the new study could help improve preparedness across the north Australia.

As the 2025-2026 wet season unfolds, communities and forecasters are once again watching Darwin's winds closely—a reminder of how critical clear and accurate monsoon onset information is for northern Australia each and every year.

Hanh Nguyen, Sugata Narsey, Matthew C. Wheeler and Chris Lucas from the Bureau of Meteorology in Melbourne, Australia; Hanna Heidemann from the University of Melbourne School of Geography, Earth and Atmospheric Sciences in Parkville, Australia and the Centre for Applied Climate Sciences at the University of Southern Queensland, in Toowoomba, Australia; Corey Robinson from the School of Earth, Atmosphere and Environment at Monash University in Clayton, Australia and the Australian Research Council Centre of Excellence for 21st Century Weather in Clayton, Australia; Andrew G. Marshall from the Centre for Applied Climate Sciences at the University of Southern Queensland in Toowoomba, Australia and the Bureau of Meteorology in Hobart, Australia; and Lin Wang from the Center for Monsoon System Research at the Institute of Atmospheric Physics (IAP) in the Chinese Academy of Sciences (CAS) in Beijing, China also contributed to this research.

This work was supported by Meat & Livestock Australia, the Queensland Government through the Drought and Climate Adaptation Program, and the University of Southern Queensland through the Northern Australia Climate Program (NACP).