One of the big contributors to the record-breaking global temperatures over the past year – El Nino – is nearly gone, and its opposite, La Nina, is on the way.
Whether that’s a relief or not depends in part on where you live. Above-normal temperatures are still forecast across the U.S. in summer 2024. And if you live along the U.S. Atlantic or Gulf coasts, La Nina can contribute to the worst possible combination of climate conditions for fueling hurricanes.
Pedro DiNezio, an atmosphere and ocean scientist at the University of Colorado who studies El Nino and La Nina, explains why and what’s ahead.
What is La Nina?
La Nina and El Nino are the two extremes of a recurring climate pattern that can affect weather around the world.
READ MORE: Zimbabwe declares state of disaster as El Nino-linked drought devastates southern Africa
Forecasters know La Nina 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 Nino, the same region warms instead.
Those temperature fluctuations might seem small, but they can affect the atmosphere in ways that ripple across the planet.
How La Nina and El Nino form. Video by NOAA
The tropics have an atmospheric circulation pattern called the Walker Circulation, named after Sir Gilbert Walker, an English physicist in the early 20th century. The Walker Circulation is basically giant loops of air rising and descending in different parts of the tropics.
Normally, air rises over the Amazon and Indonesia because moisture from the tropical forests makes the air more buoyant there, and it comes down in East Africa and the eastern Pacific. During La Nina, those loops intensify, generating stormier conditions where they rise and drier conditions where they descend. During El Nino, ocean heat in the eastern Pacific instead shifts those loops, so the eastern Pacific gets stormier.
During La Nina, the Walker Circulation intensifies, triggering stronger storms where the air rises. Graphic by Fiona Martin/NOAA via Climate.gov
During El Nino, the Walker Circulation shifts eastward, so more storms form off California as warm air rises over the warmer waters of the eastern Pacific. Graphic by Fiona Martin/NOAA via Climate.gov
El Nino and La Nina also affect the jet stream, a strong current of air that blows from west to east across the U.S. and other mid-latitude regions.
During El Nino, the jet stream tends to push storms toward the subtropics, making these typically dry areas wetter. Conversely, mid-latitude regions that normally would get the storms become drier because storms shift away.
This year, forecasters expect a fast transition to La Nina – likely by late summer. After a strong El Nino, like the world saw in late 2023 and early 2024, conditions tend to swing fairly quickly to La Nina. How long it will stick around is an open question. This cycle tends to swing from extreme to extreme every three to seven years on average, but while El Ninos tend to be short-lived, La Ninas can last two years or longer.
How does La Nina affect hurricanes?
Temperatures in the tropical Pacific also control wind shear over large parts of the Atlantic Ocean.
Wind shear is a difference in wind speeds at different heights or direction. Hurricanes have a harder time holding their column structure during strong wind shear because stronger winds higher up push the column apart.
READ MORE: This new satellite mission could change how we study hurricanes
La Nina produces less wind shear, removing a brake on hurricanes. That’s not good news for people living in hurricane-prone regions like Florida. In 2020, during the last La Nina, the Atlantic saw a record 30 tropical storms and 14 hurricanes, and 2021 had 21 tropical storms and seven hurricanes.
Forecasters are already warning that this year’s Atlantic storm season could rival 2021, due in large part to La Nina. The tropical Atlantic has also been exceptionally warm, with sea surface temperature-breaking records for over a year. That warmth affects the atmosphere, causing more atmospheric motion over the Atlantic, fueling hurricanes.
Does La Nina mean drought returns to the U.S. Southwest?
The U.S. Southwest’s water supplies will probably be OK for the first year of La Nina because of all the rain over the past winter. But the second year tends to become problematic. A third year, as the region saw in 2022, can lead to severe water shortages.
Drier conditions also fuel more extreme fire seasons in the West, particularly in the fall, when the winds pick up.
During La Nina, the jet stream tends to be farther north, causing drier conditions across the U.S. Southwest. Map provided by NOAA via Climate.gov
What happens in the Southern Hemisphere during La Nina?
The impacts of El Nino and La Nina are almost a mirror image in the Southern Hemisphere.
Chile and Argentina tend to get drought during La Nina, while the same phase leads to more rain in the Amazon. Australia had severe flooding during the last La Nina. La Nina also favors the Indian monsoon, meaning above-average rainfall. The effects aren’t immediate, however. In South Asia, for example, the changes tend to show up a few months after La Nina has officially appeared.
La Nina is quite bad for eastern Africa, where vulnerable communities are already in a long-term drought.
Typical La Nina climate impacts, though conditions aren’t always like this. Map provided by NOAA via Climate.gov
Is climate change affecting La Nina’s impact?
El Nino and La Nina are now happening on top of the effects of global warming. That can exacerbate temperatures, as the world saw in 2023, and precipitation can go off the charts.
Since summer 2023, the world has had 10 straight months of record-breaking global temperatures. A lot of that warmth is coming from the oceans, which are still at record-high temperatures.
La Nina should cool things a bit, but greenhouse gas emissions that drive global warming are still rising in the background. So while fluctuations between El Nino and La Nina can cause short-term temperature swings, the overall trend is toward a warming world.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
