La Niña is coming, increasing the chances of a dangerous Atlantic hurricane season – an atmospheric scientist explains this weather phenomenon

One of the big contributors to last year’s record global temperatures – El Niño – it’s almost finishingand its opposite, La Niña, is on the way.

Whether this is a relief or not depends in part on where you live. Above-normal temperatures still forecast in the USA in the summer of 2024. And if you live along the U.S. Atlantic or Gulf coasts, La Niña can contribute to the worst possible combination of weather conditions to fuel hurricanes.

Pedro DiNezio, an atmospheric and ocean scientist at the University of Colorado who studies El Niño and La Niña, explains why and what’s to come.

What is La Niña?

La Niña and El Niño are the two extremes of a recurring weather pattern that can affect the climate around the world.

Meteorologists know that La Niña has arrived when temperatures in the eastern Pacific Ocean, along the equator west of South America, cool by at least half a degree Celsius (0.9 Fahrenheit) below normal. During El Niño, the same region heats up.

These temperature fluctuations may seem small, but they can affect the atmosphere in ways that ripple across the planet.

The tropics have an atmospheric circulation pattern called Walker circulation, named after Sir Gilbert Walker, an English physicist of the early 20th century. The Walker Circulation basically consists of giant loops of air rising and falling in different parts of the tropics.

Typically, air rises over the Amazon and Indonesia because moisture from the rainforests makes the air more buoyant there, and occurs in East Africa and the eastern Pacific. During La Niña, these loops intensify, generating stormier conditions where they rise and drier conditions where they fall. During El Niño, ocean heat in the eastern Pacific alters these cycles so that the eastern Pacific becomes stormier.

EL Niño and La Niña also affect the jet streama strong air current that blows from west to east across the United States and other mid-latitude regions.

During El Niño, the jet stream tends to push storms toward the subtropics, making these normally dry areas wetter. On the other hand, mid-latitude regions that would normally receive storms become drier as the storms move away.

This year, meteorologists expect a rapid transition to La Niña – probably late summer. After a strong El Niño, like the one the world saw in late 2023 and early 2024, conditions tend to quickly swing toward La Niña. How long this will last is an open question. This cycle tends to oscillate from extreme to extreme every three to seven years on averagebut while El Niños tend to be short-lived, La Niñas can last two years or more.

How does La Niña affect hurricanes?

Temperatures in the tropical Pacific also control wind shear in large parts of the Atlantic Ocean.

Wind shear is a difference in wind speeds at different heights or directions. Hurricanes have a harder time maintaining the column’s structure during strong wind shear because the stronger winds higher up pull the column apart.

La Niña produces less wind shear, eliminating the brakes on hurricanes. That’s not good news for people who live in hurricane-prone regions like Florida. In 2020, during the last La Niña, the Atlantic saw a record 30 tropical storms and 14 hurricanes, and 2021 has had 21 tropical storms and seven hurricanes.

Meteorologists are already warning that this year’s Atlantic storm season it could rival 2021, due largely to La Niña. The tropical Atlantic has also been exceptionally warm, with sea ​​surface temperature breaking records for more than a year. This heat affects the atmosphere, causing more atmospheric movement over the Atlantic, fueling hurricanes.

Does La Niña mean the return of drought to the US Southwest?

The US Southwest’s water supply will likely be fine during the first year of La Niña due to all the rain last winter. But the second year tends to become problematic. A third year, as the region saw in 2022, could lead to severe water shortage.

Drier conditions also fuel more extreme fire seasons in the west, particularly in autumnwhen the winds increase.

What happens in the Southern Hemisphere during La Niña?

The impacts of El Niño and La Niña are almost a mirror image in the Southern Hemisphere.

Chile and Argentina tend to suffer droughts during La Niña, while the same phase leads to more rain in the Amazon. Australia had severe flooding during the last La Niña. La Niña too favors the Indian monsoon, meaning above-average rainfall. The effects are not immediate, however. In South Asia, for example, changes tend to appear a few months after the official appearance of La Niña.

La Nina is very bad for East Africawhere vulnerable communities already find themselves in a situation long-term drought.

Is climate change affecting the impact of La Niña?

El Niño and La Niña are now occurring on top of the effects of global warming. This could worsen temperatures, as the world saw in 2023, and precipitation could skyrocket.

Since the summer of 2023, the world has had 10 months in a row of record global temperatures. Much of this heat comes from the oceans, which are still in record temperatures.

La Niña was supposed to cool things down a bit, but greenhouse gas emissions that drive global warming continue to rise in the background. Therefore, although fluctuations between El Niño and La Niña can cause short-term temperature fluctuationsthe general trend is towards a warming world.

This article was republished from The conversationan independent, nonprofit news organization that brings you facts and analysis to help you understand our complex world.

It was written by: Pedro Di Nezio, University of Colorado at Boulder.

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Pedro DiNezio receives funding from the NSF.