Predicting the unprecedented: assessing contributions from large-scale modes of variability and climate change to Southeast Australia’s record spring rainfall in 2022

In September–November 2022, south-eastern Australia (SEA) experienced its wettest spring in the 126-year record, causing widespread and long-lasting flooding in the region. During this season, large-scale oceanic and atmospheric circulation conditions that generally drive SEA rainfall variations in spring were conducive to higher-than-normal rainfall over SEA: moderate La Niña and negative phase of the Indian Ocean Dipole (IOD) in the tropics and the positive phase of the Southern Annular Mode (SAM) in the Southern Hemisphere extratropics. The wet conditions occurred upon the background of long-term climate change in part due to increasing atmospheric greenhouse gases (GHGs). In this study, we focus on quantifying the portion of the 2022 SEA spring rainfall that can be explained by these large-scale climate drivers and anthropogenic global warming, using multiple linear regression and the Australian Bureau of Meteorology’s sub-seasonal to seasonal forecast system, ACCESS-S2, with modified initial conditions. Our statistical and dynamical modelling results show that the large-scale climate drivers and the GHG forcing could explain a substantial fraction of the record-high rainfall over SEA in spring 2022, though far from explaining the full amount. We have found that anthropogenic climate change contributed ~ 12% to the total rainfall and, while the tropical ocean conditions contributed to significantly wetter-than-average rainfall, the atmosphere played a key role in amplifying the extremity of the event, with: a zonal wavenumber one pattern featuring a high pressure anomaly south of Australia, record-strong thermal wind over Australia and more frequent rain-bearing mid-latitude weather systems during spring 2022.