Summary

Why Australia was not wet during spring 2020 despite La Niña

Scientific journal article

During the spring of 2020 much of Australia received near average to severely below average rainfall despite the occurrence of La Niña which typically leads to increased spring rainfall. La Niña is the positive phase of the El Niño Southern Oscillation and is associated with cooler than average sea surface temperatures (SSTs) in the central and eastern tropical Pacific Ocean and warmer than average SSTs in the western Pacific southeast Asia area. This SST gradient enhances stronger trade winds from the east. This condition is a predictable driver of wetter than normal conditions in eastern Australia. The La Niña of 2020 resulted in high confidence forecasts of above average rainfall using the seasonal climate model, ACCESS-S1 however, this rainfall never eventuated. Researchers from the Bureau of Meteorology and the University of Melbourne have investigated the causes for this over-prediction of rainfall.

Several factors contributed to the drier than expected conditions. The first is that ocean temperatures in southeast Asia were not as warm as would be expected based on historical La Niña observations and the long-term warming trend. Secondly, the negative phase of the Indian Ocean Dipole (IOD) deteriorated earlier than normal. A negative IOD indicates intensified equatorial westerly winds which results in a temperature gradient across the Indian Ocean with warmer than normal water in the east and cooler than normal water in the west. This state often accompanies La Niña and typically results in above-average winter–spring rainfall over parts of southern Australia as the warmer waters off northwest Australia provide more available moisture to weather systems crossing the country. Lastly, a third climate driver the Madden-Julian Oscillation (MJO) was close to the equator in November, reducing rainfall over northern and eastern Australia.

Despite ACCESS-S1 correctly predicting the strength of the La Niña, these other climate drivers led to the overprediction of sea surface temperatures north of Australia and an under prediction of the strength of the November MJO event. This led to the over-prediction of rainfall in Australia in spring, particularly November. These results highlight the need for more complete understanding of large-scale climate drivers and their interactions during spring when they exhibit the biggest changes. It also identifies possibilities to improve ACCESS-S1 forecasts.