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<p>The official 2024/25 Australian summer monsoon (ASM) onset occurred on 7 February, more than two weeks later than the previous record. Defined using Darwin winds, the official ASM typically arrives between mid-December and mid-January. Here, we investigate (1) why the ASM onset was record late, despite La Niña-like conditions, and (2) if the late ASM onset was consistent with other unofficial monsoon and rainy season onset definitions. Our results show earlier onsets when using wind and rainfall-based definitions, mainly due to strong synoptic-to-intraseasonal variability. Two onset definitions were triggered in December 2024 when the Madden–Julian Oscillation (MJO) contributed to deep convection and above-average rain over northern Australia. In late January, a monsoon trough formed over northeast and northwest Australia, but not over Darwin. With the MJO in a suppressed state, the Darwin region experienced record-breaking January temperatures. The ASM eventually arrived in early February, with the reason behind its historically late onset likely a result of weather noise and variability. Another location, ∼1200 km east of Darwin, did not experience such a late onset, using similar definitions. Relevant to monsoon systems globally, our study highlights the limitations of local onset or wind-only definitions, which may not necessarily capture the large-scale monsoonal conditions, and be amplified by short-term weather variability.</p>

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Why Was the Official Australian Monsoon Onset Unusually Late in 2024/25?

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年1月29日

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  • Tim Cowan,
  • Rajashree Naha,
  • Hanh Nguyen,
  • Hanna Heidemann,
  • Sugata Narsey,
  • Matthew C. Wheeler,
  • Corey Robinson,
  • Chris Lucas,
  • Andrew G. Marshall,
  • Lin Wang

摘要

The official 2024/25 Australian summer monsoon (ASM) onset occurred on 7 February, more than two weeks later than the previous record. Defined using Darwin winds, the official ASM typically arrives between mid-December and mid-January. Here, we investigate (1) why the ASM onset was record late, despite La Niña-like conditions, and (2) if the late ASM onset was consistent with other unofficial monsoon and rainy season onset definitions. Our results show earlier onsets when using wind and rainfall-based definitions, mainly due to strong synoptic-to-intraseasonal variability. Two onset definitions were triggered in December 2024 when the Madden–Julian Oscillation (MJO) contributed to deep convection and above-average rain over northern Australia. In late January, a monsoon trough formed over northeast and northwest Australia, but not over Darwin. With the MJO in a suppressed state, the Darwin region experienced record-breaking January temperatures. The ASM eventually arrived in early February, with the reason behind its historically late onset likely a result of weather noise and variability. Another location, ∼1200 km east of Darwin, did not experience such a late onset, using similar definitions. Relevant to monsoon systems globally, our study highlights the limitations of local onset or wind-only definitions, which may not necessarily capture the large-scale monsoonal conditions, and be amplified by short-term weather variability.