Remarkable Event for A23a: The World’s Biggest Iceberg
Something extraordinary has happened to A23a, the world’s largest iceberg. For several months now, it has been spinning in place just north of Antarctica, despite the fact that it should be racing along with Earth’s most powerful ocean current. Scientists believe the massive frozen block, which is more than twice the size of Greater London, has been captured on top of a vast rotating cylinder of water. This phenomenon, known as a Taylor Column, could potentially keep A23a trapped for years.
A23a: The Iceberg That Refuses to Die
“Usually, you think of icebergs as transient things; they fragment and melt away. But not this one,” observed polar expert Professor Mark Brandon. “A23a is the iceberg that just refuses to die,” the Open University researcher told BBC News.
This iceberg’s resilience is well-documented. After breaking free from the Antarctic coastline back in 1986, A23a almost immediately got stuck in the bottom-muds of the Weddell Sea. For three decades, it remained a static “ice island,” not budging until 2020, when it finally started drifting again—slowly at first—before charging north towards warmer air and waters.
A23a’s Unexpected Stagnation
In early April of this year, A23a entered the Antarctic Circumpolar Current (ACC), a massive ocean current that moves a hundred times more water around the globe than all of Earth’s rivers combined. This powerful current was expected to propel the nearly trillion-tonne iceberg into the South Atlantic, leading to its inevitable demise.
Instead, A23a went nowhere. It remains stationary just north of the South Orkney Islands, rotating counterclockwise by about 15 degrees a day. As long as it continues this spinning motion, the iceberg’s decay and eventual demise will be delayed.
The Science Behind A23a’s Captivity
Interestingly, A23a has not grounded again, as there is at least a thousand meters of water between its underside and the seafloor. Its motion has been halted by a type of vortex first described in the 1920s by physicist Sir G.I. (Geoffrey Ingram) Taylor. Taylor was a pioneer in fluid dynamics and even contributed to the Manhattan Project by modeling the stability of the world’s first atomic bomb test.
In this case, the vortex is created by a 100km-wide bump on the ocean floor known as Pirie Bank. The Taylor Column, a mass of rotating water, sits atop this bump, and A23a is now its prisoner.
The Ocean’s Surprising Dynamics
“The ocean is full of surprises, and this dynamical feature is one of the cutest you’ll ever see,” said Professor Mike Meredith from the British Antarctic Survey. Taylor Columns can also form in the air, seen in the movement of clouds above mountains. They can range in size from just a few centimeters in a laboratory tank to the enormous scale seen here, with a giant iceberg caught in the middle.
The Uncertain Future of A23a
How long will A23a continue its spinning-top routine? No one knows for sure. However, when Professor Meredith placed a scientific buoy in a Taylor Column above another bump east of Pirie Bank, the floating instrument was still rotating in place four years later.
The Importance of Understanding the Seafloor
A23a serves as a perfect illustration of the importance of understanding the shape of the seafloor. Submarine mountains, canyons, and slopes significantly influence the direction and mixing of ocean waters, as well as the distribution of nutrients that drive biological activity. This influence extends to the climate system, as the mass movement of water helps disperse heat energy around the globe.
A23a’s unusual behavior can be explained because the ocean bottom just north of South Orkney is relatively well surveyed. Unfortunately, this level of mapping does not extend to much of the rest of the world. Currently, only a quarter of Earth’s seafloor has been mapped to the best modern standard.
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