Abstract
Abstract. The response of the Australian monsoon to external forcings and related mechanisms during the Last Glacial Maximum (LGM) are investigated by multi-model experiments in CMIP5–PMIP3. Although the annual mean precipitation over the Australian monsoon region decreases, the annual range, or the monsoonality, is enhanced. The precipitation increases in early austral summer and decreases in austral winter, resulting in the amplified annual range, but the main contribution comes from the decreased precipitation in austral winter. The decreased winter precipitation is primarily caused by weakened upward motion, although reduced water vapor also has a moderate contribution. The weakened upward motion is induced by the enhanced land–sea thermal contrast, which intensifies the divergence over northern Australia. The increased Australian monsoon rainfall in early summer, however, is an integrated result of the positive effect of local dynamic processes (enhanced moisture convergence) and the negative effect of thermodynamics (reduced moisture content). The enhanced moisture convergence is caused by two factors: the strengthened northwest–southeast thermal contrast between the cooler Indochina–western Indonesia and the warmer northeastern Australia, and the east–west sea surface temperature gradients between the warmer western Pacific and cooler eastern Indian Ocean, both due to the alteration of land–sea configuration arising from the sea level drop. The enhanced Australian monsoonality in the LGM is not associated with global-scale circulation change such as the shift of the Intertropical Convergence Zone; rather, it is mainly due to the change of regional circulations around Australia arising from the changes in land–sea contrast and the east–west SST gradients over the Indian and western Pacific oceans. This finding should be taken into account when investigating its future change under global warming. Our findings may also explain why proxy records indicate different changes in Australian monsoon precipitation during the LGM.
Highlights
The changes of the Australian monsoon are crucial for human society and ecology in Australia (Reeves et al, 2013a), considering the socioeconomic importance of monsoon rainfall (Wang et al, 2017)
Note that the largest decrease in precipitation occurs from April to July, not exactly in austral winter, and the largest increase in precipitation occurs in November and December (ND), i.e., early austral summer
While the total annual precipitation decreases in the Last Glacial Maximum (LGM), the annual range of the Australian monsoon rainfall is amplified due to seasonal redistribution of the precipitation, especially the drying in austral autumn (April–May) and winter (JJA) over Australia
Summary
The changes of the Australian monsoon are crucial for human society and ecology in Australia (Reeves et al, 2013a), considering the socioeconomic importance of monsoon rainfall (Wang et al, 2017). M. Yan et al.: Understanding the Australian Monsoon change during the Last Glacial Maximum et al, 2016). It is important to understand how and why the Australian monsoon would change in response to global climate change. The Australian monsoon onset and variability during the post-glacial period, the late deglaciation period, and the Holocene have been studied using proxy datasets (Ayliffe et al, 2013; De Deckker et al, 2014; Kuhnt et al, 2015; Bayon et al, 2017). Due to the limitation of the scarce proxy datasets, the Australian monsoon change during the LGM is far from being clearly understood
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