El Niño, the natural weather phenomenon that added to global warming to make 2023 the hottest year ever measured, is coming to an end, paving the way for the likely return in a few months of the opposite cycle, La Niña, synonymous cooler global temperatures.
But this cooling could be very weak on average, climatologists warn, due to warming caused by humanity’s greenhouse gas emissions, already responsible for a rise in global temperatures of at least 1.2°. C on average compared to the end of the 19th centurye century.
Here’s how the El Niño-Southern Oscillation (ENSO) cycle influences global climate.
El Niño
El Niño refers to the cyclical warming of water in the central and eastern tropical Pacific Ocean, which influences precipitation, winds and ocean currents, and increases average global temperatures.
It occurs every two years at age seven and generally lasts nine to twelve months.
The latest El Niño, which began in June 2023, ranks among the five most intense ever measured, according to the World Meteorological Organization (WMO).
Normally, trade winds over the Pacific blow westward along the equator, carrying warm waters from South America toward Asia. To replace it, cold water rises from the depths. But during El Niño, these trade winds weaken and warm water is blown back toward America, fueling further warming of the atmosphere and the shift of the Pacific jet stream (or jet stream, powerful winds at the altitude where planes fly) towards the south.
This shift in the jet stream generally results in drier weather in Southeast Asia, Australia, South Africa and northern South America and, conversely, much drier conditions. humid areas in the Horn of Africa and the southern United States.
Climate change influences the ENSO cycle, but its effect still remains unclear, explains Michelle L’Heureux, specialist in the phenomenon at the American meteorological agency NOAA. Drier or wetter conditions caused by ENSO “may become amplified” because of global warming, she said.
The rise in global temperatures it causes also serves as a “window into the future” of climate change: “by giving a temporary boost, ENSO gives you a bit of a glimpse of what a warmer world is like.” hot,” says the meteorologist after an unprecedented year.
Neutral period
Although El Niño has weakened, the first four months of 2024 continued to break heat records, unsurprisingly since its cycle typically sends temperatures soaring the year after its onset.
ENSO does not work “like a switch,” explains Michelle L’Heureux. “It takes some time for global atmospheric circulation to adapt.”
According to the WMO, there is a 40% chance that the July-September quarter will remain a neutral period, between the two cycles, then 30% in August-November, with the probability of the start of La Niña increasing gradually.
La Nina
The La Niña phenomenon results in a cooling of the eastern Pacific Ocean for a period of approximately one to three years, producing the opposite effects of El Niño on the global climate.
La Niña brings generally wetter conditions to parts of Australia, Southeast Asia, India, southeast Africa and northern Brazil, but drier conditions in some regions of South America.
It may also contribute to a more severe hurricane season in the Atlantic. The American agency NOAA anticipates an exceptional 2024 season, with four to seven Category 3 or higher hurricanes possible.
A possible return of La Niña in the summer is not enough to nourish the hope of rapid relief for regions like Southeast Asia, hit by extreme heat waves since March.
“The planet is warming and ENSO only plays a secondary role in that,” insists Michelle L’Heureux. “Even with the potential development of La Nina as early as this year, we still expect 2024 to be among the five warmest years ever measured.”