A23a: Iceberg That Broke Free In 1986 Still Spinning Today

The world's largest iceberg, A23a, has been spinning in its place just north of Antarctica for months instead of being swept along by Earth's most powerful ocean current. 

Scientists have discovered that the massive frozen block, over twice the size of Greater London (3,600 sq km), is trapped atop a huge rotating cylinder of water, known as a Taylor Column, despite being in the path of the powerful Antarctic Circumpolar Current (ACC).

“Usually you think of icebergs as being transient things; they fragment and melt away. But not this one,” stated polar expert Prof Mark Brandon from the Open University. “A23a is the iceberg that just refuses to die,” he told BBC News. 

A23a broke free from Antarctica in 1986, but got stuck in the Weddell Sea's bottom muds for 30 years, reports the BBC. It remained a static "ice island" until 2020 when it finally began to drift again.

Slowly at first, it then accelerated northward, moving towards warmer air and waters. It entered the ACC in April and this current was expected to propel the near-trillion-tonne iceberg into the South Atlantic, leading to its inevitable demise.

Instead, A23a has stayed put near the South Orkney Islands, rotating anti-clockwise by about 15 degrees a day. This rotation delays its decay and eventual melting. A23a now floats with at least a thousand metres of water beneath it, preventing it from becoming stuck on the seafloor again.

Oceanographers attribute this rare occurrence to the Taylor Column, a rotating cylinder of water first described by Sir GI Taylor, a physicist, in the 1920s. 

Mr Taylor, an expert in fluid dynamics, showed how a current encountering an obstruction on the seafloor can separate into two distinct flows, creating a full-depth mass of rotating water between them. In this case, the obstruction is a 100 km-wide bump on the ocean bottom known as Pirie Bank. The vortex sits on top of the bank, making A23a its prisoner. 

Prof Mike Meredith from the British Antarctic Survey said the “ocean is full of surprises, and this dynamical feature is one of the cutest you'll ever see.”

“Taylor Columns can also form in the air; you see them in the movement of clouds above mountains. They can be just a few centimetres across in an experimental laboratory tank or absolutely enormous as in this case where the column has a giant iceberg slap-bang in the middle of it,” he added. 

How long might A23a continue its spinning-top routine? It is uncertain, but when Mr Meredith placed a scientific buoy in a Taylor Column above another bump east of Pirie Bank, the instrument was still rotating in place four years later. So, for now, A23a remains an immovable giant, continuing its slow spin.