Both in Arctic and Antarctic waters grease ice forms when open water starts to freeze over. Such grease ice is a mixture of small frazil ice crystals and sea water. The first sea ice formation in the autumn, or ice formation in leads during winter, is usually grease ice. When grease ice forms the surface temperature remains close to the water temperature. The cooling of the ocean therefore persists for much longer than when solid ice is forming, and more sea ice is formed.
In a new article in Annals of Glaciology Bjerknes researcher Lars H. Smedsrud first compares available measurements of grease with existing parameterizations. These are found to be too simple and partly wrong, compared to the new data from Svalbard. Smedsrud follows on to show that forcing from the ocean currents is necessary, in addition to wind forcing, to predict the grease ice thickness. A balance of the wind and current stresses, and the resistance force of the grease, is found. The solution is a nonlinear relation of the grease ice thickness. A typical wind speed of 10 m/s makes the grease ice about 0.3 m thick. If there are additional (tidal) ocean currents of 0.5 m/s present the predicted grease will be 0.8 m thick.
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| A grease ice layer in the Northern Barents Sea. The grease ice dampens ocean waves, in the same way as grease or oil, and this is the background for the English name (photo: Lars H. Smedsrud). |
A helping hand
The Arctic summer ice cover has decreased steadily over the last decades, and makes more open water present in the autumn. When these large areas of open water start to freeze over, a thick layer of grease ice will be produced. The grease will be effectively packed by the wind and currents, and in this way help out with re-growing some of the sea ice lost during summer. The future sea ice cover in the Arctic will thus be a more seasonal ice cover, with larger heat transfer from the ocean to the atmosphere, especially in October, November and December. Work is currently ongoing to add grease ice to the Climate models.