Something extraordinary has happened to A23a, the world’s largest iceberg.
It has been spinning north of Antarctica for months, when in fact it should be moving along with the most powerful ocean current on Earth.
Scientists say the icy block, which is more than half the size of Puerto Rico or twice the area of Greater London, was trapped on top of a huge rotating cylinder of water.
It’s a phenomenon oceanographers call the Taylor column, and A23a may not escape its captor for years.
“Icebergs are usually thought of as transient things; they break up and melt. But this one doesn’t,” said polar expert Mark Brandon.
“A23a is the iceberg that refuses to die”the Open University researcher told BBC News.
The iceberg’s longevity is well documented. It broke off the Antarctic coast in 1986, but almost immediately became trapped on the bottom of the Weddell Sea.
For three decades it was a static “ice island”. It didn’t budge. It wasn’t until 2020 that it floated again and began drifting again, slowly at first, before heading north toward warmer air and waters.
In early April this year, the A23a entered the Antarctic Circumpolar Current (ACC), a monster that moves a hundred times more water around the planet than all the rivers on Earth combined.
This was supposed to propel the iceberg, weighing almost a trillion tons, into the South Atlantic, where it would be forgotten.
However, A23a didn’t go anywhere. It remains in place just north of the South Orkney Islands, rotating counter-clockwise about 15 degrees a day. And As long as it continues like this, it will escape decay and disappearance.
What has him trapped?
The A23a has not returned to land. There are at least a thousand metres of water between its underside and the seabed.
A type of vortex first described in the 1920s by a brilliant physicist, Geoffrey Ingram Taylor, has stopped in its tracks.
This Cambridge academic was a pioneer in the field of fluid dynamics and even participated in the Manhattan Project to model the likely stability of the world’s first atomic bomb test.
Professor Taylor demonstrated how a current encountering an obstacle on the seabed can, under the right circumstances, separate into two distinct flows and generate between them a rotating mass of water at all depths.
In this case, the obstruction is a 100-km-wide bulge on the ocean floor known as Pirie Bank. The vortex is at the top of the bank and for now, A23a is its prisoner..
“The ocean is full of surprises and this dynamic feature is one of the most beautiful you can see,” says Professor Mike Meredith of the British Antarctic Survey.
“Taylor plumes can also form in the air; they are seen in the movement of clouds over mountains. They can be just a few centimetres in diameter in an experimental laboratory tank or absolutely enormous, as in this case, where the plume has a giant iceberg in its middle.”
The importance of the seabed
It is not known how long the A23a will spend in that prison, but when Professor Meredith placed a scientific buoy on a Taylor column over another protuberance east of the Pirie Bank, The floating instrument was still spinning in place four years later.
Iceberg A23a once again perfectly illustrates the importance of understanding the shape of the seabed.
Mountains, canyons and underwater slopes profoundly influence the direction and mixing of waters.as well as in the distribution of nutrients that drive biological activity in the ocean.
And this influence also extends to the climate system: it is the movement of water masses that helps disperse thermal energy across the planet.
The behaviour of A23a can be explained because the ocean floor north of the South Orkneys is reasonably well studied.
This is not the case in most of the rest of the world.
Currently, Only a quarter of the Earth’s seafloor has been mapped at the best modern level.
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