Wildfires

Australian Bushfires May Have Caused Triple La Niña

Smoke from Australia’s 2019-20 black summer fires may have resulted in the rare “triple dip” La Niña that lasted from 2020 to 2022, research suggests.

Modelling from scientists at the US National Center for Atmospheric Research has found that smoke aerosols from the bushfires interacted with clouds to cool surface waters over the south-eastern subtropical Pacific Ocean. This created favourable conditions for a La Niña to form, the researchers believe.

Smoke particles resulted in smaller cloud droplets over the southern hemisphere. That makes them brighter, and it also makes them live longer [because they are less likely to rain out of the atmosphere]. The net effect of that is more sunlight gets reflected back to space.

The result was a phenomenon akin to what’s known as June gloom in the US, where you have very cold ocean conditions but very warm atmospheric conditions above this cloud layer – that’s referred to as an inversion.

Winds carried these anomalous conditions into the deep tropics, and that instigated the feedbacks that are commonly associated with La Niña. The winds became stronger as a consequence of the surface cooling off, and then the flow going into the tropics was also drier, because a cold surface means you have less humidity in the atmosphere.

Because of that, the whole band of precipitation that usually exists in the tropics move northwards, and that’s a critical component for getting a La Niña.