[By Jean-Louis Tison, Universite Libre du Bruxelles]
Hi everyone!…This is Jean-Louis Tison, one of the Belgians on board!..I thought I’d drop you a few words about our new exciting sight of yesterday, 28th of April. If you are a faithful PIPERS follower, you might have already seen pictures like the one shown here to the left, a breathtaking view of this peculiar type of sea ice, extending for miles around the ship. It does not require a lot of imagination (neither to be a fan of “Games of Throne”) to describe it, as a few of our predecessors already did, as “Dragon skin sea ice”…Indeed “a few”, since this is quite a rare sight, only seen twice by our Chief Scientist Steve (in his long career!) and once, more recently by Guy (Williams) during the Antarctic SIPEX I expedition. Vladimir, our “ice pilot”, as also seen it a few times in the Arctic. For me, it was a first! As you can see (overturned) on the side of the ship, it indeed consists of a thin layer of individual pancakes “stranded” on 60-80 cm of consolidated sea ice. As you know well, scientists are curious animals, and lively discussions quickly started to understand the origin of this “Dragon skin”. Not only scientists, by the way, and we were quickly pressed by our ASC friends and the crew to propose decent clues of interpretation!..
It quickly came to our mind that this peculiar type of sea ice might be the “signature” of the polynya activity, that we are here to better understand, and more specifically of the strong wind regimes that characterize it!…Let me try to explain you “how”, with the help of a little hand drawing here below (apologies, these are very busy times, and not much space is available to produce a digital equivalent!…I hope you will be able to read it through as I describe it!.. Don’t hesitate to blow it up to see the comments better!).
As you probably know now, polynyas (a Russian word) are permanent open water areas within the sea ice cover, generally located in coastal areas. They persist because of the frequent very strong and cold winds that flow down the slope of the ice sheets that we call “katabatic winds”. Our first polynya of interest will be the “Terra Nova Bay polynya”. Because of those strong katabatic winds, individual ice crystals (frazil ice) form in extremely large quantities at all times, and are quickly blown away to the open ocean, at least over a distance of a few tens of kilometers, where the katabatic winds lose their strength. During periods of quieter winds, however, frazil ice crystals will conglomerate and form thin pancakes. These will quickly be partly broken up by the next strong wind event (sometimes above 120 km an hour) and strongly pushed against and on top of the ice at the rim of the polynya, resulting in the so-called “dragon skin” sea ice.
Oceanographically, ice formation in the polynyas is an extremely important process!.. Since ice rejects a fair amount of salts (nearly 90 %) in the water below, the latter becomes more saline and denser, and will sink to become Antarctic Bottom Waters, a major component of the Global oceanic conveyor belt. Also, the ice formed in the polynya is entrained by wind and currents to form a vast “plume” of “fresh water” (compared to sea water), in our case to the North-East of the Terra Nova Bay polynya.
There is still some debate amongst us, as to the nature of the ice located at the rim of the polynya. It could be a) pre-existing sea ice drifting from the South of the Ross Sea (lower box A in my drawing); b) sustained loose frazil accumulation during continuous high-wind periods at the rim of the polynya (lower box B in my drawing) or c) piles of thin pancakes formed during previous cycles of high/llow winds, accumulated at the rim of the polynya, blocking the way to the “newcomers”” that override it. This is not a vain debate amongst enthusiast scientists, since it will partly control the efficiency of the polynya in exporting fresh-water ice to the open ocean. Option A seems somewhat unrealistic, , though, since it is difficult to explain the formation of the bands of dragon skin sea ice we have seen spreading over several tens of kilometers. Our first ice coring today under the dragon skin seems to favour option B…and we are therefore quite impatient, as you can imagine, to see what we will encounter as we penetrate further South, entering the immediate vicinity of the polynya…still two nights to wait :0)!..
I hope this has helped you to understand our excitement, In a way or another!…Stay focused on our new adventures in the coming days!..
Greetings from 74° South, Jean-Louis.