91av

Over the past three months, the shift from hot to cool temperatures in the equatorial Atlantic Ocean has happened at record speed. This emerging “Atlantic Niña” pattern comes just ahead of an expected transition to a cooler La Niña in the Pacific Ocean, and these back-to-back events could have ripple effects on weather worldwide.

The swing towards cooler temperatures in both oceans is a welcome change after more than a year of record heat at land and sea, largely driven by the rise in greenhouse gas emissions and a warm El Niño pattern in the tropical Pacific Ocean that developed in mid-2023.

“We are starting to see that the global mean ocean temperatures are going down a bit,” says at the University of Colorado Boulder. According to the US National Oceanic and Atmospheric Administration (NOAA), global sea surface temperatures this past July were slightly cooler than in July 2023 – ending a 15-month streak of record-high average ocean temperatures.

The Pacific El Niño faded in May, and between September and November, cooler-than-average La Niña conditions are likely to develop, according to the latest NOAA . This is driven in part by strengthening winds along the equator that allow colder water to emerge from the deeper ocean. El Niño, on the other hand, is associated with weaker so-called trade winds that reduce the upwelling of cooler water. This multi-year cycle is called the El Niño-Southern Oscillation (ENSO), and it is one of the main sources of natural variability within the global climate.

Though it has a much smaller influence on the climate, the equatorial Atlantic Ocean also fluctuates between warm “Niños” and cold “Niñas” every few years. Like ENSO, this oscillation is also often associated with the strength of trade winds.

As in the Pacific, the equatorial Atlantic saw unusually hot Niño conditions for much of 2023, and sea surface temperatures earlier this year were the warmest in decades. “It’s the latest episode in a string of events for a climate system that’s gone off the rails for a number of years,” says at NOAA.

Over the past three months, temperatures in that part of the Atlantic cooled off more rapidly than at any time in records extending back to 1982. This sudden shift is perplexing because the strong trade winds that normally drive such cooling have not developed, says at the University of Miami in Florida. “We’ve gone through the list of possible mechanisms, and nothing checks the box so far.”

If temperatures remain 0.5°C cooler than average for at least another month, it will officially be considered an “Atlantic Niña”, says Tuchen.

The two potential La Niñas are likely to influence weather patterns around the world due to their effects on temperature and humidity. A Pacific La Niña is generally associated with dry weather in the western US and wet weather in East Africa, while an Atlantic Niña tends to reduce precipitation in Africa’s Sahel region and boost it in parts of Brazil. The two La Niñas could also have opposing influences on the ongoing Atlantic hurricane season: the Pacific La Niña is expected to increase the likelihood of Atlantic hurricanes when it arrives in September, but the Atlantic La Niña may weaken certain conditions, such as atmospheric wave activity, required for hurricanes to form.

The cycles could also influence each other directly. Exactly how is challenging to predict, but there is reason to think the Atlantic La Niña could delay the development of La Niña in the Pacific, slowing its cooling effects across the global climate, says McPhaden. “There could be a tug of war between the Pacific trying to cool itself and the Atlantic trying to warm it.”