The world's largest iceberg is stuck twirling in an ocean vortex

Caption

A screenshot of the National Oceanic and Atmospheric Administration's satellite system, which monitored iceberg A23a in February as it drifted around the Weddell Sea near the Antarctic Peninsula. / National Oceanic and Atmospheric Administration's Joint Polar Satellite System

The world's largest iceberg, A23a, is stuck again.

For more than 30 years, the giant frozen block — equivalent to the size of Rhode Island — was grounded on the sea floor in Antarctic coastal waters. It finally freed itself in 2020 and started heading to the open ocean this past winter.

But a few months into A23a's journey, onlookers were stunned at what they saw: the iceberg spinning in circles.

Through satellite imagery, British Antarctic Survey noticed the mega iceberg rotating near the South Orkney Islands, about 375 miles from the Antarctic Peninsula, starting in January. According to the Survey, A32a maintains "a chill 15 degrees rotation per day."

Its dance moves likely are caused by a phenomenon in fluid mechanics known as a Taylor column. It's essentially a rotating cylinder that forms when there's an obstruction in a flow. In other words, A23a is trapped in a kind of ocean vortex.

Till Wagner, a professor at the University of Wisconsin-Madison who studies how ice interacts with climate, said he has never seen a real-life example of this phenomenon on such a massive scale.

"You know, you can make these Taylor columns quite easily in a rotating tank experiment in your lab. But to see it on a geophysical scale like this is really rare," he said.

There's a lot to be understood on how Taylor columns can affect a behemoth like A23a. It's unclear how often Taylor columns form in the ocean and how frequently icebergs get trapped in them.

In the case of A23a, it's anyone's guess how long it will spin in the vortex. The iceberg is also melting while spinning, and Wagner is curious to see how it will affect life in the surrounding ecosystem, like phytoplankton.

"It would be interesting to see whether next spring, we have like more active phytoplankton blooms in that spot," he said.

Iceberg A23a's origin goes back to 1986, when the Filchner Ice Shelf's leading edge broke off three icebergs. For decades, A23a was stuck to a sandbank in shallow waters. In 2020, the main bulk of A23a freed itself, and in November it finally started to float into the Southern Ocean.

It was expected to travel northward over the next year, reaching warmer waters, where it would rapidly melt and fragment. Now A23a's fate is a bit more uncertain.