The “little girl” who makes the weather

The “little girl” who makes the weather

What exactly is La Niña? 

La Niña, meaning “the little girl”, is a large-scale cooling of ocean surface temperatures in the central and eastern equatorial Pacific Ocean. It’s a complex weather pattern that means changes to rainfall, winds and air pressure in the Pacific – usually more rain in Australia and eastern Asia, and drought conditions in coastal Peru and Chile – as well as other climatic effects over swathes of the Earth. This year’s La Niña event was confirmed by the World Meteorological Organisation in October, and it is expected to last until at least spring. It is the opposite of the better-known El Niño – a band of warmer water in the central and eastern Pacific. Together, these two make up what is known as the El Niño- Southern Oscillation (Enso) pattern.

Where do the names El Niño and La Niña come from? 

In 1891, the president of the Lima Geographical Society, Dr Luis Carranza, wrote an article about a warm counter-current that had been noticed by Peruvian anchovy fishermen. When the waters were warmer (and lacking in nutrients) the fishermen’s anchovy haul was greatly reduced. As the phenomenon always seemed to occur around Christmastime, the fishermen named it “El Niño de Navidad”, meaning “the Christmas Child”. La Niña was later adopted as the name for the opposite event.

How are these different from typical Pacific conditions? 

Normally, the trade winds that blow east to west across the Pacific push water warmed by equatorial sun along with them, so that it builds up on the western side of the ocean around Asia and Australia. At the same time, on the eastern side of the ocean, along the coast of South and Central America, the warm water that has been pushed west is replaced by cooler, nutrient-rich water from the depths of the ocean, a process called “upwelling”. During an El Niño – for reasons that are not fully understood – trade winds falter or even start moving west to east. Temperatures in the central Pacific increase; ocean currents shut down or reverse; and great masses of warm water move towards the Americas. During a La Niña, the effect is reversed: easterly trade winds strengthen, and most of the tropical Pacific is colder than usual: there’s an increase in upwelling of cold water, which moves across the ocean towards Asia and Australasia.

What effects do these have? 

Both La Niña and El Niño events change the odds of floods, droughts, heatwaves and extreme cold in a range of locations. During an El Niño, changing ocean conditions disrupt weather patterns and marine fisheries along the west coast of the Americas. Dry regions of Peru, Chile, Mexico and the southwest US are often deluged with rain and snow, while rainfall in Asia and Australia is greatly reduced. During the particularly intense El Niño event in the winter of 1997-1998, rain and mudslides in California and Peru left thousands homeless, while on the other side of the Pacific, Indonesia experienced severe drought, and Mongolia suffered a heatwave. An estimated 16% of the world’s coral reef systems died from heat stress, and after extreme rainfall in East Africa, there was a severe outbreak of Rift Valley fever. During a La Niña, by contrast, there is wetter weather in North America, Southeast Asia and Australia; and drier than usual conditions in western South America and East Africa.

Why do the impacts reach so far? 

Because the Pacific Ocean is so big – covering 40% of the equator – it affects the entire world. The El Niño/La Niña oscillation disrupts the atmospheric circulation patterns that connect the tropics with the middle latitudes, and also affects the jet streams – fast-flowing currents in the Earth’s atmosphere several miles above the surface. Within any given decade, the warmest years are usually El Niño ones, and the coldest are usually La Niña ones. El Niño years are one factor that can increase the risk of colder winters in the UK. It also decreases the likelihood of tropical storms in the North Atlantic, while La Niña can make them worse. The UN Food and Agriculture Organisation is currently warning that La Niña could cause drought from the Horn of Africa to Afghanistan and Iran; and flooding from southern Africa to Tonga and the Phillipines.

How was this great oscillation first discovered? 

A British mathematician called Gilbert Walker was working as the director general of observatories in India in 1904, and studying the Indian monsoon, when he noticed a see-sawing of atmospheric pressure between the Pacific Ocean and the Indian Ocean. He analysed meteorological data and published the first account of this oscillation. In 1969, Jacob Bjerknes, one of the founders of modern meteorology, realised that the atmosphere and ocean were coupled in one huge loop, which he called the Walker Circulation. The lower part of the loop flows east to west across the tropics near the ocean surface; the upper part flows west to east at higher altitudes. Rising air in the west and sinking air in the east connect the flow in one big, continuous circulation.

How often does a La Niña occur? 

The cycle is not easy to predict as it is not entirely understood, but the data shows that these Enso events happen roughly every 3-7 years. Often, an El Niño is followed immediately by a La Niña, as if the warm water is sloshing back and forth across the Pacific. La Niñas tend to last six months to two years.

Does climate change affect this? 

It may intensify some Enso effects, such as drought, just as La Niña tends to decelerate global warming, by lowering temperatures. But overall the relationship between the two is hard to describe or predict. One meteorologist, Tom Di Liberto, compares Enso to a light controlled by various dimmer switches, and climate change to “some bratty kid who goes into the room and fiddles with each switch differently”.

 

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