Something has suddenly cooled a large patch of the Atlantic Ocean

Over the summer, a vast stretch of the equatorial Atlantic Ocean experienced an unexpected and rapid cooling, setting new records.

Now, that unusually cold patch is slowly returning to normal temperatures, but the cause of the sudden chill remains a mystery to scientists.

Researchers are working to understand the reasons behind this abrupt cooling of the tropical Atlantic, yet they have found few clues. “Everyone is puzzled, trying to figure out what happened,” says Franz Tuchen, a postdoctoral research fellow at the University of Miami.

The cold anomaly appeared as a band of ocean extending a few degrees north and south of the equator in early June, following a record-breaking ocean heatwave that lasted 40 years.

Although the region naturally oscillates between cold and warm phases every few years, the speed of the transition from record highs to record lows was “unprecedented,” according to Tuchen.

Michael McFadden, a senior scientist at the National Oceanic and Atmospheric Administration (NOAA), who monitors real-time data from buoys in the region, suggests the anomaly might be a temporary phenomenon. However, he acknowledges that it likely arose from processes that researchers do not yet fully understand.

Observations indicate that surface temperatures in the eastern equatorial Atlantic reached a record 30 degrees Celsius in February and March. However, by June, temperatures had plummeted, hitting a low of 25 degrees Celsius in late July.

Researchers speculate that this cooling could be a precursor to the development of an Atlantic Niña, a regional climate pattern that typically brings increased rainfall to West Africa and decreased rainfall to northeastern Brazil and the Gulf of Guinea.

However, the Atlantic Niña is generally weaker than its Pacific counterpart, La Niña, and hasn’t been observed since 2013. For it to be officially declared, below-average temperatures would need to persist for at least three months, which hasn’t occurred—temperatures have been gradually warming in recent weeks, according to Tuchen.

Despite the absence of an official Atlantic Niña, scientists remain uncertain about what triggered the sharp cooling in the tropical Atlantic.

They have modeled several potential climate processes to explain the phenomenon, and while initial findings suggest it may not be directly linked to human-caused climate change, they are not yet ready to completely rule out this possibility.