La Niňa

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0n 29 September 2020 the BOM declared a La Niňa event, which means wetter than average conditions in the next few months. How does that work?

It all depends on the Walker circulation. Gilbert Walker was a Cambridge University mathematician who became director general of observatories in India in 1904. He studied the Indian Ocean monsoon. What he found is that the temperature differences between land and sea powers the monsoon. We all know about the afternoon sea breeze. The air over land heats up and rises, this causes the air above the sea to come in and replace it. At high altitude, the air moves back from land to sea. This forms a loop which is indeed a mini Walker circulation.

Extend the idea to the Pacific tropical zone. The ocean surface is warm somewhere North of New Guinea, and cold in the East near the America coast. So you get the trade winds blowing from East to West at low altitude, and the other way around further up. This is the Walker Circulation. So over PNG and North Australia we have a lot of evaporation forming some big cumulonimbus and… rain!

In the neutral situation we get a just a nice amount of rain. But in the La Niňa situation, the Walker loop goes a bit crazy and everything gets amplified. So we get a lot of rain. Apparently, we should get some of this in the next few months, but not as bad as in 2010-2011. Fingers crossed.

Sources:

http://www.bom.gov.au/climate/enso/history/ln-2010-12/three-phases-of-ENSO.shtml

https://en.wikipedia.org/wiki/Walker_circulation#/media/File:LaNina.png

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