Science1 publisher2 min readPublished
Collisions between ice floes account for the Arctic drift wind alone cannot explain
Arctic ice drifts and spreads in ways wind-driven models get wrong. A UC Riverside-led team reports that floe-on-floe collisions, with one weakly constraining fitted parameter, reproduce three of those measured discrepancies at once.
The Scientist · Science desk
What happened
- A UC Riverside-led study in Physical Review Letters attributes the odd drift and slow spreading of Arctic sea ice to collisions between the individual floes that make up the pack.
- Wind is the main force on the pack, but measured floe speeds vary in ways wind does not explain, and the ice spreads much more slowly than simple wind-driven models predict.
- The team's simulation treats floes like grains moving through a silo, except that the grains float, feel drag from the ocean, and are pushed by turbulent wind while colliding with each other.
- Model predictions were checked against sea ice measurements from the Fram Strait, the passage between Greenland and Svalbard through which Arctic ice exits toward the Atlantic.
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Why it matters
- capability Global climate models cannot follow the enormous number of small floes one by one, so a physics-based account of their collective behavior is the kind of term a modeler can put inside a grid cell.
- decision Collision frequency depends on how much ocean is ice-covered and how big the floes are, so a drift scheme whose state carries neither field cannot express the effect however carefully its wind drag is calibrated.
- constraint The paper supplies a way to ask whether looser ice disperses into warmer water and melts faster. It does not predict where warming will send Arctic ice.
Only one parameter in the simulation was fitted, and it had little influence on the outcome. Given measured local wind and ice conditions, the model reproduced three things at once: how quickly the ice spreads, the distribution of floe speeds, and how ice motion varies across timescales from hours to days [8]. The work was led by Bryan Shaddy, a former UC Riverside undergraduate now at the University of Southern California, with UCR materials scientist Alex Greaney and Bhargav Rallabandi, an associate professor of mechanical engineering at UCR [2].
This is a comparison between a simulation and field measurements, not an experiment with a control. Nobody can switch collisions off in the ocean and watch what changes, so the weight falls on the fact that one setting covered all three statistics [8].
The physics is a comparison of two rates. Where ice is concentrated, a floe runs into a neighbor much more often than the wind changes [9]. Each collision dissipates part of the energy the wind supplied and shortens the distance a floe travels freely before it meets the next piece of ice [10]. The colliding objects are not uniform, either: floes run from several meters to a few kilometers across [3]. Taking several meters as about five and a few kilometers as about three thousand, that is a factor of roughly 600 in width and about 360,000 in plan area [18].
"If you get a lot of ice floes together in the same place with some wind, they bump into each other and transfer energy to neighbors," Rallabandi said. "We showed that that's the only ingredient you need to explain these observations." [11] The claim is sufficiency: collisions alone can produce the measured statistics. Earlier work proposed unusual wind patterns, ocean eddies and fractures in the ice as explanations, and this study does not measure how much each of those contributes [5].
The paper points the same physics at avalanches, landslides, materials science and the particle-filled inks used in 3D printing [16]. "The model is not restricted to ice," Rallabandi said. "It just needs a noisy source of force and the things that are moving to experience collisions." [17]
What to watch
- Whether the same one-parameter model reproduces drift statistics in the central Arctic pack, where concentration and floe sizes differ from the Fram Strait.
- A test in loose marginal ice, where collisions should be rare, would separate the collision term from wind structure and ocean eddies.
- Whether any global or operational sea-ice model adopts a collision term and reports a measured change in drift skill.