Sea Ice: a story of cracks

By Harry Heorton

Ice, on a scale you can hold in your hands, is a brittle material. When it deforms it cracks into smaller pieces. Imagine hitting an ice cube with a hammer: lots of smaller pieces of ice. They don’t bend like rubber, they just crack. Sea ice floating in the ocean behaves in a similar fashion. In this case, there are many solid sea ice floes that are up to 100 km across. Instead of a hammer, another ice floe, pushed by wind and ocean currents, breaks these pieces up into smaller floes.  By assuming that the way in which the ice cracks in both cases is the same, we can make small-scale experiments to learn about large scale processes

In more controlled circumstances than the hammer case, a hydraulic press is used to crack a 10 cm cube of ice. Experiments like this are performed by Erland Schulson’s group at Dartmouth College. A good paper to read is Illescu and Schulson (2004).  The cube is compressed in two directions mimicking the forces present in the flat plane of the sea ice cover. The forces in the two directions are not equal and have a ratio R, called the confinement ratio. Due to the material properties of ice, for certain values of R, a pair of cracks form as shown in Figure 1 (typically R<0.3). The arrangement of the two cracks can be compared to satellite images of sea ice (see this video) where two sets of pairs of cracks combine to give diamond shaped ice floes. For higher R, a single crack forms across the plane. For large-scale sea ice this may correspond to pressure ridging, the break-up of sea ice that piles about itself into long (many km) ridges several metres high, like a mini Himalayan range crushed between the Indian and Asian plates.

2015 06 19 Harry Heorton Figure 1 alternate

Figure 1. Coulombic Shear Faulting (cracking) in a sea ice sample. Source: Illescu and Schulson (2004)

So there is a link between small scale cracks in a piece of ice and the patterns of cracks over 100s of km – why is this helpful?

Global climate
When considering sea ice as part of the global climate, we need to know how it moves when pushed around by the wind and ocean. Does it drift freely, bounce off neighbouring floes, or crack and form pressure ridges? Does Arctic sea ice drift into the north Atlantic where it is warm, or collect above Greenland where it is cold? Well, if we use the knowledge gained from our small ice sample we can track the likely direction of the diamond shaped floes. We use the applied forcing field (from the wind and ocean) to calculate the forces upon the sea ice and thus calculate R, the confinement ratio. If R is within a suitable range, diamonds form orientated in a particular direction. Now if we know that there are diamond-shaped floes in the ice cover, we can make predictions about how these floes slide past each other and resist the external forcing. The theory behind this method of prediction has been developed by my colleagues Danny Feltham, Alex Wilchinsky and Michel Tsamados and can be read about here and here. Recording the directional memory of a deforming medium in this style is called anisotropy, a word that makes us over-ambitious fluid dynamicists giddy with excitement.

Future Research
So what is the next step in this research? We know about cracks, we know about diamonds, and we can use them to predict the drift of sea ice. But this is sea ice, a constantly changing medium, and of course, the cracks can refreeze. Also, how fast do the cracks form? Thankfully there are scientists who venture to the frozen seas with chain saws and investigate cracks in sea ice. They watch how quickly the cracks refreeze and put jacks into them to split entire floes in half. Cracks also important for the formation of ice. When cracks open in the cold winter, the open ocean is exposed to bitterly cold atmosphere, causing the rapid production of ice in the water below. We have a recent study (very soon to be published) showing that this method may be responsible for a third of ice production in the central Arctic. My work is to explore these questions adding to our knowledge of sea ice cracks.

References
Iliescu, Daniel, and Erland M. Schulson, 2004. The Brittle Compressive Failure of Fresh-Water Columnar Ice Loaded Biaxially. Acta Materialia, 52, no. 20: 5723–35

Tsamados, Michel, Daniel L. Feltham, and Alexander V. Wilchinsky, 2012. Impact of a New Anisotropic Rheology on Simulations of Arctic Sea Ice. Journal of Geophysical Research

Wilchinsky, Alexander V., and Daniel L. Feltham, 2006. Modelling the Rheology of Sea Ice as a Collection of Diamond-Shaped Floes. Journal of Non-Newtonian Fluid Mechanics 138, no. 1 (September 2006): 22–32.

Wilckinsky, A., H. Heorton, D. Feltham, and P. Holland, 2014 (submitted). Study of the Impact of Ice Formation in Leads upon the Sea Ice Pack Mass Balance Using a New Frazil and Grease Ice Parameterisation. Submitted to Journal of Physical Oceanography

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