Climate Pattern Shift from El Niño to La Niña Brings Wetter Conditions to Atlantic Shores
As El Niño winds down and its opposite phase, La Niña, takes hold, it could mean continued above-average temperatures in the U.S. in 2024 and a more active hurricane season along the Atlantic coast, according to The Conversation.
What is La Niña?
La Niña and El Niño are two extremes of a repeating climate pattern that can influence weather around the globe. Experts say La Niña occurs when the eastern Pacific Ocean near the equator off South America is cooler than normal by at least half a degree Celsius (0.9 degrees Fahrenheit). In contrast, El Niño is when this area is warmer. The temperature change may seem small, but it can influence the atmosphere in a ripple effect that spreads around the planet.
Atmospheric Circulation and Weather Patterns
The tropics have a pattern of atmospheric circulation known as the Walker circulation, named after early 20th-century British physicist Sir Gilbert Walker. The Walker circulation is a giant loop of air rising and sinking in different parts of the tropics. Normally, air rises over the Amazon and Indonesia because moisture from the rainforests makes the air lighter, and it sinks over East Africa and the eastern Pacific. In La Niña, this circulation is strengthened, leading to wetter conditions where the air rises and drier conditions where it sinks. During El Niño, warm ocean temperatures in the eastern Pacific shift the circulation so that there is more rainfall over the eastern Pacific.
Impact on Jet Streams and Hurricanes
El Niño and La Niña also affect the jet stream, a strong current of air that blows west to east across the U.S. and much of the mid-latitudes. During El Niño, the jet stream tends to push storms farther north, bringing wetter conditions to areas that are normally drier. Conversely, areas in the mid-latitudes that normally get storms may become drier.
Transition to La Niña and Hurricane Season
This year, experts predict a rapid transition to La Niña, likely in the late summer. After a strong El Niño, which the world has been experiencing in late 2023 and early 2024, it is typical for the climate pattern to switch relatively quickly to La Niña. How long it lasts is an open question. While El Niños tend to be relatively short, La Niñas can last for two years or more.
Impact on Wind Shear and Hurricane Strength
Temperatures in the tropical Pacific also control wind shear over large areas of the Atlantic. Wind shear is a difference in wind speed or direction at different heights or orientations. Hurricanes in the North Atlantic have a harder time maintaining their structure in high wind shear, because stronger winds aloft push the hurricane tower sideways. La Niña tends to produce less wind shear, taking away this brake on hurricanes. That's bad news for people living in hurricane-prone areas like Florida. In 2020, during the most recent La Niña, the Atlantic saw a record-breaking 30 tropical storms and 14 hurricanes, and in 2021 there were 21 tropical storms and 7 hurricanes.
Experts are warning that this year's Atlantic hurricane season could be as busy as 2021, largely because of La Niña. The tropical Atlantic is also unusually warm, with record-breaking sea surface temperatures that have persisted for more than a year. That warmth will influence the atmosphere, causing more upward motion in the atmosphere over the Atlantic, which gives hurricanes their energy.
Impacts Beyond the U.S.
Water supplies in the southwestern U.S. could improve during the first year of La Niña because of the winter rains, but the second year may be drier. By the third year, severe water shortages can occur. Drier conditions will also prime the West for a more severe wildfire season, especially in the fall.
The impacts of El Niño and La Niña are roughly opposite in the Southern Hemisphere. Chile and Argentina often see drought during La Niña, while the Amazon gets more rain. Australia experienced severe flooding during the most recent La Niña. La Niña also tends to bring above-average rainfall to India. However, the impacts are not always immediate. For example, in South Asia, the shifts typically occur a few months after the official onset of La Niña. La Niña also tends to worsen conditions in East Africa, where communities are already facing prolonged drought.
El Niño and La Niña occur against the backdrop of global warming. The world has seen global temperatures break records for 10 consecutive months, as of the summer of 2023. La Niña may bring a temporary cooling influence, but the long-term warming trend due to greenhouse gases continues. While the swings between El Niño and La Niña can cause temperature variations in the short term, the overall trend is toward a warmer planet.