When icebergs break from an ice shelf or large glacier front, they ride the ocean’s currents, spin in its eddies, shift with the tides, and are blown by the wind. Occasionally the icy drifters become stuck, grounded on a shallow part of the seafloor or trapped in a rotating mass of ocean water. Iceberg A-23A did both.

After breaking from the Filchner-Ronne Ice Shelf in 1986, Iceberg A-23A spent decades stuck to the floor of the southern Weddell Sea. It began to wiggle loose in the early 2020s, and by March 2023 the Rhode Island-sized iceberg floated unencumbered. But such freedom lasted only a year. As it drifted northward in March 2024, the berg became ensnared by a rotating vortex of water, or Taylor column, caused by currents encountering a bump on the seafloor.

While every iceberg’s journey is unique, most follow the same general path. More than 90 percent of bergs around Antarctica enter the clockwise-flowing current of the Weddell Gyre off East Antarctica and eventually escape, shooting north along the Antarctic Peninsula and finally out across the Drake Passage into warmer South Atlantic waters—an ocean route known as “iceberg alley.”

But it’s not always a straight path to the Atlantic, even for giant bergs carrying a huge amount of momentum. For example, Iceberg A-68A, a similar mammoth-sized berg that broke from the Larsen C Ice Shelf in 2017, made some loops in the Drake Passage before continuing north and disintegrating in the Northern Scotia Sea near South Georgia island.

The motion of A-23A, however, appears quite out of the norm. For about eight months, the berg rotated tightly within the Taylor column about 200 kilometers (120 miles) north of the South Orkney Islands. According to Jan Lieser, an ice specialist with the Antarctic Meteorological Service who has been tracking the berg, A-23A made 15 revolutions between March and November 2024. “I am not aware of an iceberg that has been trapped in such a persistent manner in such a small area,” Lieser said.

The animation at the top of this page shows the iceberg between November 5 and December 16, 2024. Notice that by about mid-November, the berg appears to “spin out,” escaping the vortex and resuming its northeastward journey. Images for the animation were acquired by the MODIS (Moderate Resolution Imaging Spectroradiometer) and VIIRS (Visible Infrared Imaging Radiometer Suite) instruments on several NASA and NOAA satellites.

Christopher Shuman, a University of Maryland, Baltimore County, scientist based at NASA’s Goddard Space Flight Center, estimated that the berg drifted about 240 kilometers in one month since exiting the vortex. In other words, it traveled about 8 kilometers per day as it continued to rotate on its way to the northeast.

It is still unclear what might have nudged the berg from the vortex. “My hypothesis is that a random perturbation in the system might have triggered a slight variation of the ‘usual’ spin, such that the iceberg found an exit path,” Lieser said.

“This serves to remind us both of the mysteries of our oceans and the value of remote sensing data,” Shuman said. Cryospheric scientists will continue using satellites to observe changes to the ice in this remote part of the planet—including but not limited to icebergs.

NASA Earth Observatory images by Lauren Dauphin, using MODIS data from NASA EOSDIS LANCE and GIBS/Worldview, VIIRS data from NASA EOSDIS LANCE, GIBS/Worldview, and the Suomi National Polar-orbiting Partnership, and the Joint Polar Satellite System (JPSS). Map made using data from the Antarctic Iceberg Tracking Database and the U.S. National Ice Center(USNIC). Story by Kathryn Hansen.