Question:
A C-17 transport plane could not land on Oct 15 the sea-ice on McMurdo because the sea-ice was too thin. A little over a week later, when Laurie and Anthony were flying in the sea ice has thickened enough so the US Air Force started landing on the Sea Ice Runway. How is it possible that the sea ice is still thickening even though the days are getting longer and the air is warming? The wind chill also decreases as the region enters the Austral summer.
Answer:
The key to this question is to know that water freezes only when its surroundings ( the atmosphere) is cold enough to remove the heat that is produced by the freezing. If there is already a layer of ice, this heat has to migrate through the ice, before the atmosphere can actually take it away. This migration of heat takes more and more time as the ice thickens, and the ice itself starts acting like a sink for the heat for quite some time even if the atmosphere stopped taking heat away, or even starts putting heat into the ice. So yes, the ice can still thicken when the Antarctic summer starts. And this can be quite some time (two weeks?) when the sea ice is very thick (e.g. 2m/6 feet)
Here are a few other things that are worth knowing in this context:
In early September 2012, at a time when the ice normally grows very fast, McMurdo had a week or two of very “balmy weather” that kept the ice from growing as it normally would. This also resulted in a delay for the establishment of the sea-ice runway.
When sea-ice freezes, the growing ice crystals are less salty (fresher) than the seawater itself. Consequently, the remaining water in between the sea-ice crystals is saltier than seawater. This is called brine, and this brine is much colder than seawater because it comes from the interior of the sea-ice, that is closer to the very cold sea-ice surface. As the sea-ice grows, it wants to float, as it is less dense than water. As it floats up, the ultra-cold brine sinks down and enters the underlying seawater, providing another heat-sink that allows the sea water to freeze more, at the bottom of the ice, or on the seafloor when these brines flow downslope. If you check out our dive pictures, you can see some amazing images of that freezing process, in the form of brinicles and anchor ice!
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