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| Wed, 11th Jan 2023 16:18:00 |
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What are El Niño and La Niña, and how do they change the weather? |
New data shows that 2022 was the fifth hottest year for Europe since records began.
But scientists are warning that 2023 could be even warmer, as a climate phenomenon called La Niña - which has been suppressing global temperatures - comes to an end.
What is La Niña?
La Niña is part of a climate phenomenon called the El Niño Southern Oscillation (ENSO) system.
It has two opposite states - El Niño and La Niña - both of which significantly alter weather patterns across the globe.
For the last few years, the world has been in successive La Niña periods, which have lowered temperatures and brought heavy rains to Canada and Australia.
Winds blowing along the Equator above the Pacific Ocean - from South America in the east towards Asia in the west - were stronger than normal.
These "trade winds" piled warm water off the coast of Asia, raising the sea surface level. In the east, near the Americas, cold water flowed upwards to the surface.
During El Niño the opposite happens - weaker trade winds mean the warm water spreads out back towards the Americas, and less cold water rises towards the surface.
Winds blowing along the Equator above the Pacific Ocean - from South America in the east towards Asia in the west - were stronger than normal.
These "trade winds" piled warm water off the coast of Asia, raising the sea surface level. In the east, near the Americas, cold water flowed upwards to the surface.
During El Niño the opposite happens - weaker trade winds mean the warm water spreads out back towards the Americas, and less cold water rises towards the surface.
Changes to rainfall
During El Niño events, the warmer water pushes the Pacific jet stream's strong air currents further to the south and the east.
This brings wetter weather to southern US states and the Gulf of Mexico, while the north of the US and Canada remain drier.
Asia, Australia, and Central and Southern Africa typically experience drought.
In La Niña events the opposite is seen: drought in the southern US, and heavy rains in Canada and Asia.
In October 2022, Australia experienced record rainfall and flooding driven by La Niña.
Tropical storms
La Niña also generates more hurricanes in the Atlantic - affecting Florida and other southern states of the US - but fewer tropical storms in the Pacific.
The opposite is true of El Niño events.
How often do these episodes happen?
El Niño and La Niña episodes typically occur every two to seven years, and usually last nine to 12 months.
They don't necessarily alternate: La Niña events are less common than El Niño episodes.
How do these events affect us?
The extreme weather events caused by El Niño and La Niña affect infrastructure, food and energy systems around the world.
The drought in Canada and Asia caused by the 2014-16 El Niño phase resulted in crop failure and damaged the food security of more than 60m people, according to the UN Food and Agricultural Organisation.
During El Niño events, less cold water rises to the surface off the Americas which brings fewer nutrients from the bottom of the ocean.
That means there is less food available for marine species like squid and salmon, in turn reducing fish stocks for South American fishing communities.
Is climate change affecting El Niño/La Niña?
In 2021, the UN's climate scientists, the IPCC, said the ENSO events which have occurred since 1950 are stronger than those observed between 1850 and 1950.
However, it also said that historical evidence like tree rings, corals and sediment records shows that there have been variations in the frequency and strength of these episodes since the 1400s.
The IPCC concluded there is no clear evidence that climate change has affected El Niño or La Niña events.
Read original full article
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