22 November, 2003
This week we received the mid-November Sea Ice Report. Why should we care about what the ice is doing? Well, first of all, we are spending almost 99% of our time living and working on the ice. Secondly, we are studying an animal with a high affinity for ice cracks-the seals congregate where cracks provide easy access to the surface. This report provides relevant information for all who live on the ice of Erebus Bay, be they humans or seals.
At this point in time the sea ice has grown as thick as it will get for the year and has begun its transition towards becoming warmer and less stable. The colder the ambient air temperature, the more the ice grows. Sea ice strength is determined by the thickness and internal temperature; the colder the sea ice, the stronger its overall structure. Another important fact to consider is that while the sea ice thickens from the top down, it melts from the bottom up. Just looking at the surface of the ice will not disclose its strengths or weaknesses. The folks who produce the sea ice report use a number of gauging stations to determine ambient temperatures. The sea ice is now in what is known as a period 2 condition, where temperatures range from 14-23F. Period 1 temperatures are less than 14F, and the shift from period 1 to 2 is slow and gradual in comparison to the change from period 2 to period 3 (23-27F). Last season they recorded the change from period 2 to period 3 in the second week of December. Since seawater here freezes at around 28F, you can see how this subtle shift in temperature can affect its stability. Once the temperatures approach 27 or 28F you are definitely skating on thin ice!
While the air and ice is warming, it doesn't mean that we are all in danger of taking a big swim anytime soon. The first year ice near the water's edge is averaging 2m thick, with very little snow on its surface. The multi-year sea ice near us around the Dellbridge Islands and Cape Evans is about 2.3-2.5m thick, but is covered with 80-100cm of snow. This late season snow cover insulates the ice from solar heating which slows the melting process. On the other hand, a thick cover of snow can be heavy enough to depress the snow sufficiently to push it below the sea level, subjecting it to warmer ocean temperatures that might cause it to weaken. The ice here at camp is at least 2 meters thick-the drill we use for our wastewater holes is around 2 meters long-and we have yet to strike water.
So what are we actually seeing as we make our rounds of the seal colonies? The changes in the ice are subtle-it's a time to pay attention as we bounce across the sastrugi or wander on foot through the colonies. Most of the prominent cracks are filled with snow, making them harder to identify. Where the surface may have appeared solid last week, and supported the weight of a person walking or even driving a snowmobile, some areas now have large linear depressions or even gaps showing at some of the more substantial crack systems. The crack systems are also widening-where once you could easily snowmobile across a snow-filled crack 1/2 meter wide, there might now be a gap that is 2-4 meters wide to cross. When faced with the question of whether it is safe to cross, we will probe the crack with an ice axe to see if it is hard enough to support the weight of the snowmobile. If it's not hard enough, it's time to drive along the crack until you can find a spot that is either strong enough or narrow enough to provide support on the solid snow on each side as you cross. Misjudging the cracks probably would not mean a sudden conversion from snowmobile to jet ski as you hit the water below, but it would mean spending the rest of the day with a shovel and tow rope trying to extricate your snowmobile from the ditch. The basic message here is to pay attention and take the time to find a safe crossing if there is any doubt.
Sea ice is changing
The cracks are growing wider
Walk carefully now
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