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Sea ice minimum Feb 20th 2009Sea ice maximum Sept 4th 2008
Sea-ice formation

Sea ice seasonal variations. Sea ice around Antarctica varies from about 8 million square miles in September or October to about 1 million square miles in January or February.
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1/ What is pack-ice?
At the beginning of the austral winter starting around March, the loose pack ice that has spent the summer months circling Antarctica begins to drift northwards. 

Pack ice is old sea-ice, frozen sea water that is a year old or more, it froze and formed elsewhere and later floated off with the winds and currents.

Pack ice is heavy stuff and when it arrives somewhere it has the effect of steadying the ocean swell. The continuous rolling motion of the sea is stopped completely by a relatively narrow band of pack ice only 100m or so wide. The result is that where pack ice is present in reasonable quantity, the sea calms down sufficiently for low temperatures to freeze it easily - moving water cannot freeze as easily as static water.


2/ What is fast-ice?
This is sea-ice in the very early stages of formation. Sea-ice that forms in situ and attached to the coast is called "fast-ice", it is stuck fast. In this picture the surface of the sea is beginning to freeze as the temperature is dropping to -20C and below.

Pack ice has come near to the shore and so all movement of the sea has been killed completely allowing low temperatures to freeze the sea water. At this stage the ice is around an inch or 2.5cm thick but it has a spongy texture, you could poke a finger or certainly a fist through it relatively easily.

The patterned effect comes from the rise and fall of the tides. As the tide rises, so the surface of the sea enlarges slightly and so the ice cracks apart, as the tide falls, so the surface of the sea decreases slightly and so the slabs of ice overlap at the edges.


3/ Does the tide change the way the sea freezes?
Sea ice in the process of forming, the shore of the island in the distance is about 5 miles (8 kilometres) away and the whole of the sea surface in-between is made of forming fast ice.

Notice how the slabs of forming ice become larger further out to sea as there are less undulations of the coast to push the slabs together as the tide falls.




4/ What are ice pancakes?
The ice near to the shore here is known as "pancake-ice". This is formed when slabs of ice that are forming are jostled by the wind and / or movement of the sea.  

The pancakes of ice bash against each other around the edges and start to curl upwards at the edges

"Pancakes of submerged ice joined with others into great sheets, the rubbery green ice thickened, an ice foot fastened onto the shore, binding the sea with the land. Liquid became solid, solid was buried under crystals."
Annie Proulx - The Shipping News




5/ Do ice-bergs move around when the sea freezes?

A picture taken of consolidated pack-ice.  

The ice that you see is mainly pack-ice, last years ice that formed elsewhere, broke up and floated here. As the temperature dropped, then this ice became stuck together by fast-ice, sea-water frozen in situ and attached to the coast that acts as a glue for the loose bits of pack.  

The ice-bergs that you see have been frozen in position and will remain so until they are freed by the spring break-up of the surrounding sea-ice.



6/ What is a tide-crack?
Once fast ice (sea-ice frozen in situ) has become established, the patterns of the earlier pieces disappears. The tide still rises and falls however meaning that the sea surface expands and shrinks slightly as it does so.  

Tide cracks result from this (as ice is not known for its elastic properties!) that are formed when the ice moves apart, they close again when the tide falls. A tide crack is often many miles long, in this case stretching for around 5 miles (8 kilometres), but never more than about 18", 45cm wide between Signy and Coronation Islands in the South Orkneys group.

Tide cracks are valuable resources for wild-life as they provide a region where birds such as snow petrels can fish through for krill and also as a breathing hole for crabeater and Weddell seals.


7/ Does pack ice affect shipping?
This is pack-ice in the summer months around the Antarctic peninsula.  

The ice looks fairly continuous, but has quite a lot of open water between the pieces and so can be relatively easily pushed aside by an ice-strengthened ship, in this case HMS Endurance. Larger pieces such as this one that are hit by the bow of the ship crack up into smaller pieces.  

Proper Ice breakers have rounded hulls and rounded bows rather than being sharp and pointed. When breaking through very thick ice, the front of the ship rides up over the ice and the weight of the ship breaks through.

Passage is slow though, and heavy on fuel, but most of all, it takes an experienced and well informed ice-pilot to be confident in entering such ice so as not to be locked into the pack should the wind direction change and consolidate the ice.


8/ What happens to fast-ice in the summer?
At the end of the winter, rising oceanic swells and increasing temperatures cause the stable winter sea-ice to break-up and begin to drift away from where it formed.  

This years fast-ice therefore becomes next years pack-ice with a portion of it melting and disappearing completely. Here a small inflatable zodiac-like craft is (not entirely sensibly) negotiating quite close, but relatively light pack-ice.  

One person drives the boat, while another sits on the bow pushing the larger pieces of ice out of the way with his feet.

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