Climate

How La Niña will shape heat and hurricanes this year


Rickshaw pullers are standing with umbrellas and caps on to get shade from the sun during the countrywide heat wave in Dhaka, Bangladesh, on April 29, 2024.
Heat waves have begun to take hold in Asia as El Nino begins to wane. | Kazi Salahuddin Razu/NurPhoto via Getty Images

Climate change and the outgoing El Niño will likely ignite more weather extremes.

The Pacific Ocean — Earth’s largest body of water — is an engine for weather around the planet, and it’s about to shift gears this year.

The warm phase of the Pacific Ocean’s temperature cycle, known as El Niño, is now winding down and is poised to move into its counterphase, La Niña. During an El Niño year, warm water starts to spread eastward across the surface of the equatorial Pacific. That warm water evaporates readily, adding moisture to the atmosphere and triggering a cascade that alters rainfall, heat waves, and drought patterns across the world.

The current El Niño is among the strongest humans have ever experienced.

It fueled wildfires, droughts, and floods in South America. It bent the jet stream, trapping heat over the southern United States last summer, and ended the year with the warmest winter on record for much of the country. It fueled both heavy rain and extreme dry conditions in southern Africa, killing crops and putting millions at risk of hunger. It heated the world’s oceans to the highest levels ever measured. It raised global temperatures to their tallest peaks scientists have ever recorded.

“The last year has been an amazing year in terms of records set around the world for extreme heat,” said Michael McPhaden, a senior scientist at the National Oceanic and Atmospheric Administration (NOAA).

The periodic swings between El Niño and La Niña, collectively known as the El Niño Southern Oscillation (ENSO), is a natural phenomenon cycling every three to seven years. Over the past year, the El Niño also synced with other natural patterns like the warm phase of the Atlantic Ocean’s temperature cycle, driving thermometers up further. But humanity’s relentless injection of heat-trapping gases into the atmosphere is pushing these changes to greater extremes.

Forecasters now expect that warm water across the Pacific to begin retreating westward, heralding a shift to La Niña. McPhaden said one of the most common definitions of La Niña is when surface water temperatures over a large area of the Pacific drop by at least 0.5 degrees Celsius below the historical average for three months or more. El Niño is typically defined when the same region is a half-degree Celsius hotter.

NOAA projects an 85 percent chance that the ENSO cycle will shift to its neutral phase between April and June 2024, and then a 60 percent chance a La Niña will develop between June and August 2024.

Historically, strong El Niños are followed by short neutral phases, about three to five months, before switching to La Niña. “The handwriting is on the wall with regard to this La Niña,” McPhaden said. “The question is exactly when will it come and how strong will it be?”

It also takes several months between when ENSO changes and when it starts to influence weather. So the warming impact of the outgoing El Niño is likely to persist and could raise global temperatures this year even higher than they were last year if the rising La Niña is weak or moderate. Heat waves are currently baking Southeast Asia, triggering school closures and health warnings.

When La Niña does set in, it will slow and reverse some of the intense weather patterns the world experienced over the past year. But it will also set the stage for more hurricanes in the Atlantic Ocean.

To make this all even more complicated, this is all occurring in a world that’s warmed to the highest levels humans have ever experienced, so it’s not clear yet how far some of these extremes will go.

How La Niña will likely play out in different parts of the world

Though they are on opposite sides of a cycle, the effects of El Niño and La Niña are not quite mirror images of each other. “It’s a bit more complicated than that,” said Pamela Knox, an agricultural climatologist at the University of Georgia Extension.

The specific types of weather impacts also vary by region, but looming shifts in the cycle can help forecasters calculate what kinds of heat, rain, and drought conditions are in store in the coming months. For instance, ENSO makes it easier to predict climate variability in the southeastern US, particularly in cooler months. “We have a pretty strong signal here compared to the central plains,” Knox said.

During a La Niña, the cooler waters in the equatorial Pacific soak up heat energy from the atmosphere while air currents deflect the jet stream — a narrow, high-altitude band of fast-moving air — pushing it northward.

Map of La Niña impacts on the United States
NOAA
La Niña tends to push the jet stream northward, leading to cooler weather to its north and drier conditions to the south.

That air current then tends to box in cold weather to its north in places like Canada and Alaska while trapping moisture in regions like the Pacific Northwest. States like Alabama, Georgia, and South Carolina tend to be warmer and drier during La Niña winters, while the Midwest tends to be cloudier, cooler, and wetter. (NOAA has published maps of the globe showing how these patterns typically play out around the world).

Mickey Glantz, director of the Consortium for Capacity Building at the University of Colorado Boulder, who studies the impacts of ENSO, noted that La Niña doesn’t just shift weather — it can also intensify existing rain and heat patterns in some regions. “La Niña, to me, is ‘extreme normal,’” Glantz said. “You have a wet season, it’s going to be really wet. If you have a dry season, the probability is it’s going to be really dry.”

La Niña may bring about a more severe hurricane season

One of the biggest consequences of a shift to La Niña is the higher likelihood of major hurricanes in the Atlantic Ocean. Hurricanes are built from several ingredients, but two parameters are especially important when it comes to ENSO: water temperature and air stability.

The ocean needs to be around 80 degrees Fahrenheit or hotter to form a hurricane, and the air above it needs to hold steady. El Niño years tend to heat up the Atlantic Ocean, but they also induce wind shear, where air rapidly changes speed and direction in the atmosphere, disrupting tropical storms before they can form. Still, the Atlantic was so abnormally hot last year that it fueled an above-average hurricane season.

The Atlantic Ocean is still startlingly hot, but now the looming La Niña is likely to stabilize the air above the sea — creating a foundation for more hurricanes.

The Weather Company and Atmospheric G2 projected that the 2024 hurricane season, which runs from June 1 to November 30, would see 24 named storms compared to an average of 14. They projected six hurricanes will reach above Category 3 strength, compared to just three in a typical year.

Researchers at Colorado State University expect 23 named storms. University of Pennsylvania scientists anticipate 33 named storms in the Atlantic this year, the highest count ever projected.

Why ENSO cycles are becoming harder to predict

The added difficulty in predicting how La Niña will play out is that people have heated up the planet. A “cool” La Niña year is now hotter than an El Niño year from 20 years ago. “It’s not the same climate regime that we forecasted the earlier [ENSO cycles] so it’s getting a bit harder to forecast,” Glantz said.

How will future climate change in turn affect ENSO? NOAA illustrated the answer with a helpful albeit highly technical schematic (bear with me):

Illustration of children on a swing set being pushed higher by climate change.
Anna Eshelman/NOAA
Climate change is likely to amplify the swings in the ENSO cycle.

The swings between the cool and warm phases of the ENSO are likely to get stronger if greenhouse gas emissions continue at the current pace. So many of the most densely populated parts of the world, like the Andean region in South America, sub-Saharan Africa, and Southeast Asia, are going to experience a more aggressive whiplash between wet and dry years, between calm and stormy summers, and between warmer and cooler winters.

For scientists, the rest of 2024 is going to be an important case study in the impacts of climate change and natural variability, sorting out where they diverge, where they intersect, and where they lead to more disasters. The world will be a real-world laboratory, showcasing severe weather that could become more typical as average temperatures continue to rise.

“It’s going to be a very interesting year,” McPhaden said. “We’ll have to wait and see and be ready for more extremes.”

You may also like...

Leave a Reply

Your email address will not be published. Required fields are marked *