Findings offer innovative ways to improve weather forecasting, measure climate-change impacts
Rising temperatures of the world's oceans threaten to accelerate the melting and splintering of glaciers-thereby potentially increasing the number of icebergs and, with it, the need to better understand more about their movement and impact. Through a series of experiments, a team of scientists has pinpointed some of the factors that cause icebergs to capsize, offering insights into how climate change may affect Earth's waters.
"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 modeling and weather forecasting," explains Leif Ristroph, an associate professor at New York University's Courant Institute of Mathematical Sciences and the senior author of the paper, which appears in the journal Physical Review Fluids. "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."
The researchers, who also included NYU's Bobae Johnson, Zihan Zhang, and Alison Kim as well as the Flatiron Institute's Scott Weady, conducted a series of experiments in the university's Applied Mathematics Laboratory that replicated floating icebergs.
To do so, they prepared ice blocks in the form of long cylinders-eliminating bubbles that would complicate the conditions-and placed them in a tank of room-temperature fresh water, which has proven to be an effective medium to gauge ice melt in past studies. The scientists then used cameras to capture the speed and movement of the model iceberg's melting and capsizing.
"We found that melting gradually reshapes the ice, which then abruptly rotates or capsizes before settling into a new orientation," explains 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."
During this process, the shape of the ice changes significantly, developing edges and corners to eventually resemble a pentagon-an unexpected outcome. The transformation is captured in this video of the experiments:
The video depicts how ice melts and then repeatedly capsizes while transforming from the shape of a cylinder into a pentagon before dissipating. Video courtesy of NYU's Applied Mathematics Laboratory
"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.
The mathematical model, developed by drawing from the experimental data, included the effects of the weight and buoyancy forces on the ice as well as the hydrodynamic forces due to its motion in the water-and illuminated the various factors driving shape change and capsizing.
The below video from the experiments shows the flows and temperature differences of the water beneath the ice:
The flow of water beneath the surface of melting ice. Video courtesy of NYU's Applied Mathematics Laboratory.
"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," recounts Ristroph. "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 five sides."
The research was supported by a grant from the National Science Foundation (DMS-2206573).
