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Why was La Niña late arriving this year? – the MJO was the culprit

5 Feb 2021 9:46 AM | Anonymous

by Iraphne Childs

In late September 2020, the Bureau of Meteorology (BOM) declared that conditions were right for a La Niña event, signalling a wet spring and summer for northern and eastern Australia, including for Southeast Queensland. So, we waited in joyful anticipation for the rain …. but November and December ended up being relatively dry in many locations across the north, east and southeast. What was going on?

The Madden-Julian Oscillation (MJO) was most likely the reason for the delay. This is a global-scale feature of the tropical atmosphere affecting the intensity and duration of rainfall. It is associated with weekly to monthly periods of both enhanced and suppressed rainfall especially over tropical Australia during summer. The MJO is characterised as an eastward moving 'pulse' of cloud and rainfall near the equator that typically recurs every 30 to 60 days. Its effects are most evident over the Indian Ocean and western equatorial Pacific, influencing the timing, development, and strength of the Indian and Australian monsoons. Although the MJO brings rainfall in its active phase, it suppresses rain before and after its arrival, when large-scale downward motion in the atmosphere prevents lift, keeping things hot and dry. In late 2020 the MJO appears to have blocked the lift and suppressed rainfall we would usually expect with La Niña conditions for northern and eastern Australia.

On 12 January 2021, BOM reported that the MJO had strengthened over the Indian Ocean and climate models indicated eastward movement. The northern Australian monsoon trough is now well established and active. BOM predicts that the MJO is likely to contribute to an increase in tropical rainfall and an above-average tropical cyclone risk around northern Australia in late January. BOM's outlook for this summer suggests there is a high likelihood of above-average rainfall for much of the country. So now, after a long wait the rain is finally falling, thanks to the MJO.

https://youtu.be/M1YBRSTdfnI

Source: BOM 2020

References:

BOM (2020) The Madden-Julian Oscillation (MJO) http://www.bom.gov.au/climate/mjo/

BOM (2021) Weekly Tropical Climate Note, 12 January http://www.bom.gov.au/climate/tropical-note/

https://youtu.be/UsWHHE_jkGE?list=PLbKuJrA7Vp7naJL31deES8QAV5E0q6U_H

Doyle, K. (ABC Weather 11 December 2020) Madden-Julian Oscillation: The bearer of tropical rain https://www.abc.net.au/news/2020-12-11/madden-julian-oscillation-mjo-the-bearer-of-tropical-rain/12961346



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