World’s biggest iceberg spins in ocean trap
Something remarkable has happened to A23a, the world’s largest iceberg.
For months now, it has been spinning in place just north of Antarctica, when it should be moving along with Earth’s most powerful ocean current.
Scientists explain that this enormous frozen block, more than twice the size of Greater London, has been caught atop a massive rotating cylinder of water.
This phenomenon, known to oceanographers as a Taylor Column, might keep A23a trapped for years.
“Usually, you think of icebergs as transient things; they fragment and melt away. But not this one,” noted polar expert Prof. Mark Brandon.
“A23a is the iceberg that just refuses to die,” the Open University researcher told BBC News.
The berg’s longevity is well documented. It broke free from the Antarctic coastline back in 1986 but quickly became stuck in the bottom muds of the Weddell Sea.
For three decades, it remained a static “ice island,” immobile until 2020, when it re-floated and began to drift again. Initially, it moved slowly, then accelerated northward towards warmer air and waters.
In early April this year, A23a entered the Antarctic Circumpolar Current (ACC) – a powerful current that moves a hundred times as much water around the globe as all Earth’s rivers combined.
This was supposed to propel the nearly trillion-tonne berg into the South Atlantic, leading to its certain destruction.
Instead, A23a stayed put. It remains just north of the South Orkney Islands, rotating counterclockwise by about 15 degrees a day. This rotation delays its decay and eventual demise.
A23a has not grounded again; there is at least a thousand meters of water between its underside and the seafloor.
The iceberg has been halted by a type of vortex first described in the 1920s by the brilliant physicist Sir G.I. (Geoffrey Ingram) Taylor.
The Cambridge academic, a pioneer in fluid dynamics, was even involved in the Manhattan Project to model the stability of the world’s first atomic bomb test.
Prof. Taylor demonstrated how a current encountering an obstruction on the seafloor can, under the right conditions, split into two distinct flows, creating a full-depth mass of rotating water between them.
In this case, the obstruction is a 100km-wide bump on the ocean bottom known as Pirie Bank. The vortex sits atop the bank, and for now, A23a is its prisoner.
“The ocean is full of surprises, and this dynamic feature is one of the cutest you’ll ever see,” said Prof Mike Meredith from the British Antarctic Survey.
“Taylor Columns can also form in the air; you see them in the movement of clouds above mountains. They can be just a few centimeters across in an experimental laboratory tank or enormous as in this case where the column has a giant iceberg slap-bang in the middle of it.”
How long might A23a continue to perform its spinning-top routine?
Who knows, but when Prof Meredith placed a scientific buoy in a Taylor Column above another bump to the east of Pirie Bank, the floating instrument was still rotating in place four years later.
A23a is a perfect illustration once again of the importance of understanding the shape of the seafloor.
Submarine mountains, canyons, and slopes have a profound influence on the direction and mixing of waters, and on the distribution of the nutrients that drive biological activity in the ocean.
And this influence extends also to the climate system: it’s the mass movement of water that helps disperse heat energy around the globe.
A23a’s behavior can be explained because the ocean bottom just north of South Orkney is reasonably well surveyed.
That’s not the case for much of the rest of the world.
Currently, only a quarter of Earth’s seafloor has been mapped to the best modern standard.
