The frozen sea

sea ice

Sea ice forming out in the Laubeuf Fjord, trapping all of the icebergs in place. The lead of water is due to the wind which whistles down the fjord and ruffles the sea-surface, preventing ice from forming.

With the sun now hiding well below the horizon, the air temperature has fallen sharply over the last couple of weeks. This new chilly regime has been welcomed by all of us down here – after all, this is what we signed up for when we committed to stay over winter. However, one of the most exciting consequences of the cold weather is that the sea is now beginning to freeze over in earnest – our small part of Antarctica is starting to acquire to the shell of sea-ice that will encase it for a the next few months, imprisoning the icebergs and driving the wildlife further north.

sea ice

The view north from the base, with the sea completely ice-covered as far as you can see.

The formation of sea-ice is a tricky business. Some of the small bays around Rothera have been frozen over for some time now, as a result of floating chunks of loose brash ice blowing into the shoreline and aggregating into a messy crust. However, this is freshwater ice, the remnants of ancient glaciers from which all of the salt has leeched away over the centuries. True sea-ice requires a seawater temperature of less than -1.8 degrees Celcius, and a prolonged period of calm weather to limit the movement of water on the sea-surface.

Happily, these criteria are gradually coming together and large areas of the Laubeuf Fjord, the stretch of water between us and the mainland, are showing signs of freezing over. It is an interesting process to watch – first comes grease ice, which just looks like an oily sheen on the surface. This becomes porridge ice which is sludgier and more crystallised, but still not at all solid. If the surface remains calm enough, then the porridge aggregates into pancake ice, flat discs of ice that can be a few metres across, and are slightly ‘turned up’ round the edges where they constantly bump into one another. Eventually these start to weld together, and from that point onwards the covering gets steadily thicker and more solid.

sea ice

More ice. The narrow strips of newly-formed ice are created in the lee of icebergs, where the sea is sheltered from the prevailing wind.

Of course, the benefit of all of this to us is the opportunity to get out skiing and skidooing on the sea surface. Initially this will just be in the locality, but if the right conditions prevail then we can potentially make it over to some of the islands, or even the mainland itself (though this hasn’t been possiible for some years, but who knows?). It certainly opens up our limited horizons.

Despite all of the exciting possibilities, sea-ice travel can be a risky business and is treated very carefully by BAS as a consequence. The most obvious danger is that you or your skidoo can fall through the ice into the cold sea below – it is therefore a strict requirement to travel with a guide, who uses a standard rechargeable hand-drill with a big auger attached to drill holes at intervals and check that the thickness and consistency of the ice is adequate. The greater risk, though, comes from the wind – if there is a body of open water anywhere downwind of you then the entire ice sheet can break up and rapidly blow out into the open area, taking anyone or anything on the surface with it. The leads, or ribbons of open seawater, can open up between ice floes quickly enough that by the time you notice one forming, it may already be too wide to cross to get back to safety. There are numerous horror stories from Scott and every other Antarctic explorer since, describing how unwary travellers had to make desperate leaps from floe to floe as the ice broke up around them.

Tom

Not good conditions for sea-ice! This is Tom trying to take a photo on one of our more wintery days.

Fortunately, technology gives us an edge that Scott didn’t have – our meteorology computers and satellite links can forecast winds very accurately, and there is a complete ban on sea-ice travel in anything but flat calm conditions. We also get satellite pictures that show the current state of sea-ice over the whole sea area around us, giving clues about areas of open water that could lead to ice break-up if the wind speed increased. Fundamentally though, strict travel regulations, excellent training and common sense are the key to avoiding trouble. Fortunately, we have all of these in abundance – of course!

Still, all of that is some time away. At the moment we are just privileged to be witnessing the start of such an extraordinary change in the environment around us. It is nearly impossible to adequately the extent of the change on camera, but hopefully some of these pictures will give a hint about how dramatically our world is changing now.

Boat in ice

This is Nimrod, one of our dive boats, nudging the edge of the sea-ice. Pity the poor chilly divers, who were in the water at this point! It is a tricky time of year for the dive team – on this occasion they managed to sneak the boat out in a narrow lead of clear water, but this closed up again the next day. This prevents further diving until the ice solidifies properly, allowing them to skidoo out to the dive site and cut a hole in the ice to jump in through.

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