In January of 2016, an area of roughly 300,000 square miles—twice the size of California—in Antarctica had an extreme melting event.

The cause of the melt was warm winds blowing over the continent forced by an especially strong El Niño.

The warm spell lasted more than two weeks, and during that time, satellite data revealed a mix of melted snow and ice over most of the Ross Ice Shelf, which channels about a third of the ice flowing from the West Antarctic Ice Sheet into the ocean.

While researchers have known for years that warm ocean water is melting Antarctic ice shelves from beneath, this is one of the first times they’ve seen warm air also causing widespread melting from above.

When the scientists became aware of a spell of unseasonably warm weather, they checked satellite data to see what was happening in the rest of West Antarctica. To their surprise, they saw the massive melting event.

Study lead author Julien Nicolas of the Byrd Polar and Climate Research Center at the Ohio State University, said, “What probably happened is that the surface snowpack was able to contain the meltwater, acting as a buffer and preventing the formation of melt ponds and streams that can be common on some Antarctic ice shelves,” he said, “but we cannot rule out the presence of standing water in many locations.”

Only a few weeks earlier, a research team had installed an advanced array of instruments to measure these processes in West Antarctica, as part of a study to understand how clouds affect the amount of energy that reaches the snow surface and influence the temperature at the surface.

“We were extraordinarily fortunate to be able to deploy state-of-the-art equipment to West Antarctica just before this large melt event occurred,” said study co-author Dan Lubin, a research physicist at the Scripps Institution of Oceanography. “These atmospheric measurements will help geophysical scientists develop better physical models for projecting how the Antarctic ice sheet might respond to a changing climate and influence sea level rise.”

The melt took place during one of the strongest El Niño events on record.

El Niños typically favor weather patterns that bring warm air to West Antarctica, but strong westerly winds that blow over the ocean to the north of Antarctica usually keep the warmer air away from the continent.

Melt events are likely to occur when those westerly winds are weak. But, as study co-author Aaron Wilson said, the January 2016 event is unusual because the warming occurred even though the westerly winds were strong. “Without strong westerlies, it’s likely there would have been much more melting,” he said.

Another study co-author, David Bromwich of Ohio State, said, “In West Antarctica, we have a tug-of-war going on between the influence of El Niños and the westerly winds, and it looks like the El Niños are winning. It’s a pattern that is emerging. And because we expect stronger, more frequent El Niños in the future with a warming climate, we can expect more major surface melt events in West Antarctica.”