The pentagon-like shape ice takes while melting and capsizing multiple times. Credit: New York University’s Applied Mathematics Laboratory
The internet is littered with dramatic videos of icebergs capsizing. Giant chunks of ice suddenly overturn (sometimes dumping reckless climbers in the drink) to reveal startlingly blue subsurface structures.
But little is known about what triggers floating ice to flip.
In a paper in the journal Physical Review Fluids, researchers recreated icebergs in the lab to reveal the main physical mechanisms at play, with implications for modelling global ice melt.
“Our study contributes fundamental knowledge about ice physics, which is a vital factor in the health of our planet, and which needs to be understood to improve climate modelling and weather forecasting,” says senior author Leif Ristroph, an associate professor at New York University in the US.
“These results show how iceberg melting and capsizing are related in complicated ways. This information is crucial as ice melting can be considered the ‘canary in the coalmine’: the earliest warning of when the Earth is warming or otherwise out of its usual balance.”
Warming ocean temperatures speed up the melting of floating ice, resulting in less reflective polar regions, which in turn promotes further heating.
“The alarming depletion of the Earth’s ice reserves occurs predominantly through melting of floating ice which may move at the free surface, drift, break up, rotate, and overturn,” the authors write.
“Previous work on capsizing has emphasized its far-reaching consequences, including changes in global-scale ice dispersal, oceanic flows due to deposition of fresh water, and enormous energy release that may trigger destructive waves and induce collective capsize events.”
The researchers replicated floating icebergs by freezing water into bubble-free, cylindrical ice blocks 8cm in diameter and 18cm long. The shape was chosen so that the floating ice would flip only about the cylinder’s axis, like a wheel.
They placed the ice in a tank of room temperature fresh water and used cameras to capture its speed and movement for 30 minutes.
They were also able to visualise differences in the temperature and density of the water using a ‘colour schlieren’, a method which produces brightening, darkening and colour changes in an image due to changes in the way light moves through a medium.
“We found that melting gradually reshapes the ice, which then abruptly rotates or capsizes before settling into a new orientation,” says Ristroph.
“This process repeats over and over. We typically see about 10 to 15 capsize events during the 30 minutes it takes the ice to completely melt away.”
In an unexpected turn of events, they found that this process tended to reshape the ice to resemble a pentagon.
“This came as a total surprise, so we worked to explain the observations by developing a mathematical model that could account for how melting changes the shape of ice and how the evolving new shape can induce the ice to capsize,” says Ristroph.
“We learned that melting primarily happens along the wetted surface of the ice below the waterline while the ‘tip’ out of the water is almost unaffected, which eventually leaves the ice top heavy so that it loses gravitational stability in the water and rotates over.
“Surprisingly, it tends to rotate through a special angle corresponding to one-fifth of a complete revolution – and this relates to why the shape eventually has 5 sides.”
The flow of water beneath the surface of melting ice. Credit: New York University’s Applied Mathematics Laboratory
While the researchers acknowledge their laboratory model is too idealised to be directly relevant to icebergs and natural ice generally, they say the methods are versatile enough to address more natural situations.
“Our findings suggest interpretations to be tested and further effects to be investigated,” they add.
“The widely varying size scales, water temperatures, and salinity for natural melting are expected to substantially affect the flows, rates, and shape evolution.
“However, room-temperature fresh water has been viewed as geologically informative even for marine icebergs given knowledge of how environmental conditions map to bulk melting rates.
“The success of the uniform-rate melt model presented here suggests that the phenomena are dominantly caused by the stark difference in rate for portions above and below the waterline, a feature that can be expected generally.”