Picture: Tree silhouette at sunset. Credit: Peter Robinson.

La Nina, which was active during the austral spring of 2020, is the most predictable climate driver of Australian springtime rainfall. Consistent with the 2020 La Niña, the Bureau of Meteorology’s dynamical sub-seasonal to seasonal forecast system, ACCESS-S1, made very confident predictions of wetter-than-normal conditions over central and eastern Australia for spring when those forecasts were initialised in July 2020 and thereafter.

However, many areas of Australia received near average to severely below-average rainfall, particularly during November. CLEX researchers found that several compounding factors contributed to the drier-than anticipated spring conditions.

Firstly, although the ocean surface to the north of Australia was warmer than normal, which would have led to increased rainfall over northern Australia, it was not as warm as expected from its historical relationship with La Niña and its long-term warming trend.

Secondly, a negative phase of the Indian Ocean Dipole, which normally acts to increase spring rainfall in southern Australia, decayed earlier than normal in October.

Finally, the Madden–Julian Oscillation (MJO) activity over the equatorial Indian Ocean acted to reduce rainfall across northern and eastern Australia during November.

ACCESS-S1 accurately predicted the strength of La Niña over the Niño3.4 region, however, it over-predicted the ocean warming to the north of Australia and under-predicted the strength of the November MJO event, leading to an over-prediction of the Australian spring rainfall and especially the November-mean rainfall.

  • Paper: Lim, E.-P., D. Hudson, M.C. Wheeler, A.G. Marshall, A. King, H. Zhu, H.H. Hendon, C. de Burgh-Day, B. Trewin, M. Griffiths, A. Ramchurn, and G. Young. 2021. Why Australia was not wet during spring 2020 despite La Niña. Scientific Reports, 11, 18423, https://doi.org/10.1038/s41598-021-97690-w.