Large patch of the Atlantic Ocean near the equator has been cooling at record speeds — and scientists can't figure out why

For a few months this summer, a large strip of Atlantic Ocean along the equator cooled at record speed. Though the cold patch is now warming its way back to normal, scientists are still baffled by what caused the dramatic cooling in the first place.

The anomalous cold patch, which is confined to a stretch of ocean spanning several degrees north and south of the equator, formed in early June following a monthslong streak of the warmest surface waters in more than 40 years. While that region is known to swing between cold and warm phases every few years, the rate at which it plunged from record high to low this time is "really unprecedented," Franz Tuchen, a postdoctoral research associate at the University of Miami in Florida who is tracking the event, told Live Science.

"We are still scratching our heads as to what's actually happening," Michael McPhaden, a senior scientist with the National Oceanic and Atmospheric Administration (NOAA) who oversees an array of buoys in the tropics that have been gathering real-time data of the cold patch, told Live Science. "It could be some transient feature that has developed from processes that we don't quite understand."

Sea surface temperatures in the eastern equatorial Atlantic were the hottest in February and March, when they exceeded 86 degrees Fahrenheit (30 degrees Celsius) — the warmest months on record since 1982. When June rolled in, temperatures began plummeting mysteriously, reaching their coolest in late July at 77 F (25 C), Tuchen recently wrote in a blog post.

Forecasts showed the cooling event may be on the verge of developing into an Atlantic Niña, a regional climate pattern which tends to increase rainfall over western Africa and decrease rainfall in northeastern Brazil as well as countries hugging the Gulf of Guinea, including Ghana, Nigeria and Cameroon. The phenomenon, which isn't as powerful as the La Niña counterpart in the Pacific, and hasn't occurred since 2013, would have been declared if the colder-than-average temperatures persisted for three months, until the end of August.

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However, the cold pocket of water has been warming in recent weeks, so "the verdict is already quite certain that it's not gonna be classified as Atlantic Niña," said Tuchen.