The Economist explains
What is La Niña?
The weather pattern is back for a second consecutive year
LAST SUMMER in Australia didn’t look like the postcards. In October 2020, giant hailstorms struck south-eastern Queensland, hammering some suburbs with stones the size of cricket balls. March brought the worst floods in half a century to New South Wales. At least two people died and over 20,000 were evacuated. Hundreds of homes in western Sydney were subsumed as rivers broke their banks. The violent conditions were fanned by La Niña. Last week, Australia’s Bureau of Meteorology (BOM) rained on Aussies’ summers by declaring that it was happening for the second year in a row. On November 30th, the World Meteorological Organisation confirmed it too. The BOM expects this La Niña to be weaker than the last, enduring “until the late southern hemisphere summer or early autumn 2022”. But what is La Niña?
Spanish for “little girl”, La Niña is the less famous counterpart of El Niño, a temporary warming pattern. Together they form one of the world’s most important weather-making phenomena. The El Niño-Southern Oscillation (ENSO) sloshes back and forth across the Pacific Ocean every few years, changing the temperature of surface waters and the state of the atmosphere as it does so. In “normal” years, trade winds blow west along the equator, pushing warm surface water from the Americas towards Asia. To replace it, cold currents surge upwards from the depths in the eastern Pacific. In El Niño years this is partly reversed, and warm water spreads back towards South America. La Niña years exaggerate the normal pattern. The winds blow harder than usual, causing warm water to pool around Asia, while surface temperatures across the rest of the Pacific fall. They must slip by 0.8°C below the average for Australia to deem it a La Niña. Other countries have different thresholds for declaring La Niña: America’s National Oceanic and Atmospheric Association (NOAA) said that one had developed in October.
The effects can be devastating to regions throughout the tropics. A strong La Niña brings destructive storms to Australia and parts of Asia, because heat and moisture rise off warm waters which pool around them, forming clouds, then rain. During La Niña in 1998, flooding in China killed thousands and displaced over 200m people, and over half of Bangladesh’s landmass was subsumed.
Outside Asia, La Niña has the opposite effect: it fuels droughts (as well as sometimes floods) in parts of Africa and the Americas. A collapse in food production triggered by La Niña in 2011 was followed by a famine in Somalia which killed 260,000 people and left 10m people in the Horn of Africa hungry. This year, wilting crops in the southern United States and Brazil have forced up global food prices; by reducing the amount of electricity generated by Brazilian hydroelectric dams La Niña also contributed to the spike in natural gas prices. La Niñas can wreak havoc on commodity markets, too. A cyclone which struck Australia in 2011 closed or restricted the output of 85% of Queensland’s coal mines and destroyed sugar-cane crops, sending prices of the sweet stuff to 30-year highs.
What of climate change? La Niñas temporarily cool global average temperatures and mask the warming caused by emissions of greenhouse gases. For instance, 2021 is likely to be between the fifth and seventh hottest year on record, but would have been warmer without the cooling effect of two back-to-back La Niña events. Climate change may also alter ENSO cycles. There is nothing unusual about consecutive La Niñas: of the 12 “first-year” examples recorded by the NOAA since 1950, eight were followed immediately by a second. However some scientists predict that La Niñas and El Niños will become more severe as global temperatures rise. Research published last year by the American Geophysical Union concluded that “under aggressive greenhouse gas emission scenarios” extreme Niñas and Niños may double in frequency.
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