An explainer on eddies and sea ice. Our paper on sea ice melting and floe size is out in GRL!

Our paper on the interactions between ocean eddies and sea ice floe size is now available online at GRL (here). If you can't access it, please email me and I will send over a pre-print. ​

Sea ice floes have a wide range of sizes. (L) 100 m floes around the Nathaniel Palmer in Antarctica. (R) 10 km floes in the Bering Strait

Sea ice is made up of a mosaic of individual pieces, known as floes. These floes span a wide range of sizes (see above), but generally, the horizontal size of these floes has not been considered a major part of the evolution of sea ice. Why? Consider the famous science experiment concerning melting ice cubes in a cup of water. When you smash up these cubes, you increase the total surface area they share with the water around them, and so they melt faster (and cool your drink quicker). Because of this, some companies (like Neve Ice, who made the cartoons on the right) manufacture "artisan" ice cubes, whose express purpose is to minimize the amount of ice surface area, so that drinks take longer to dilute. 

If you think of sea ice like you think of ice cubes, you quickly realize that the size of the pieces doesn't matter. Sea ice floes are much larger (hundreds of meters) than they are thick (1-5 meters). The surface area in contact with the water at the base of the sea ice (which doesn't care about how many pieces there are) is much larger than the surface area at their sides (which does), until you break the ice into pieces smaller than 30 meters or so. Most of the Earth's sea ice is bigger than this, and so, you might think, who cares!
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​Interactions between sea ice and the ocean change everything. When sea ice is present, it can develop a cold, fresh layer known as a lens underneath it. Outside of regions covered by sea ice the ocean can have any temperature or salinity. This contrast to something known as baroclinic instability, which occurs as the ocean tries to wipe out lateral variations in its density, and manifests itself as these beautiful eddies (like the ones in the heading of this blog). This happens at the sides of sea ice floes, too.
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We tested how this might work in an ocean model with sea ice floes in it. 
​Take a look at the videos on the left. Two simulations, one with 1 floe that is 50 km wide, and one with 16 floes that are 10 km wide are simulated in the conditions of ice melting. The scale of these floes is much larger  than the ice-cube theory of sea ice melting would consider important.  ​

​Lo and behold, eddies develop at the sides of the floes. These eddies form more where there is more ice perimeter, so that smaller floes (but still several kilometers across) melt much faster than larger ones. 
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​When considering a variety of different floe sizes, we found extremely large differences between simulations. The variation in the time to melt all the sea ice, for floes of 50 km diameter, and floes with 1 km diameter is over 4 months! 

Since most of the worlds sea ice is progressing towards being seasonal, the impact on floe size on climate may be significant. 

Curves of total ice volume for different floe sizes. From Horvat et al (2016)

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