This post is motivated by a lesson doled out to us by nature over the last few weeks on the dynamic, ever changing nature of sea-ice in Antarctica.
To cut a long story short, we stalwart mariners on the good ship James Clarke Ross found ourselves in a little bit of difficulty about 2 weeks ago. Attempting to get into the far south of the Weddell Sea, I awoke one "morning" (really 11:30pm, when my shift working the CTD starts) to find myself not floating on an ocean but instead staring at something that looked like the planet Hoth from The Empire Strikes Back, but with the big-walky-robot-things replaced by a gang of slack-jawed penguins (no big-walky-robot-things? Antarctica can be disappointing sometimes). Essentially, the crack in the ice we had used to get to our current location had closed up behind us, leaving us surrounded on all sides by thick pack ice, with nobody around save a bastard of a crabeater seal who would repeatedly pop his head up from a small hole in the ice near the ship, call us a foul name (which I will not repeat for fear of causing offence) and then disappear. We were, well, just a little bit stuck.
Eventually, the we cranked up some AC/DC (hey, I'm Australian, I'm allowed to indulge in some of the best examples of my national culture), put the pedal-to-the-metal and, with the help of two very large engines, a lot of diesel and some choice swear words, managed to bang, ram and smash our way out of the ice and to freedom, FREEDOM I SAY! Finally clear of the heavy ice, safe and sound, we then proceeded to prove that we scientists, as skilled as we are with the manipulation of Greek-symbols, lack just about any common sense. We immediately began negotiations with the ship's captain and officers to try to convince them to turn around and head straight back into the ice field. Yes, the very same ice field that nearly trapped us forever, leaving us at the mercy of the roving bands of man-eating penguins that are known to roam these here parts.
Fast forward one week and, with the captain's grudging blessing, we headed back to the southern Weddell Sea and towards our goal of the Flichner Ice Shelf. In doing so, we went straight past the place where we had previously been stuck. The difference in the local environment was, well, striking, as you may be able to tell from the photos below.
So from the planet Hoth to that planet from the movie Waterworld (wait a minute.... Himalayan mountains... sunken petrol tankers... that was our planet! You blew it up! You maniacs! Damn you all to hell!) in a week. As a famous philosopher once mused: what gives?
In my last post, I gave a bit of an overview of sea ice. Mostly, I blabbered on about where sea-ice exists, why it's important, and how we measure it from ships. Today, as promised, I'll go into a little bit of detail on the physics of sea-ice. Specifically, I'll try to give you an idea of how sea-ice forms.
Sea Ice Growth and Formation
There is one main ingredient to the formation of sea ice from ocean water: it's got to be cold. However, ocean water is constantly in motion and is full of impurities like salt and dissolved gasses, all of which make the formation of sea ice a far more complicated process than simply putting an ice cube tray in the freezer and leaving it for a few hours before dropping it into a sweet, sweet glass of Bourbon (oh yes, brownest of the brown liquors....). Sea ice, much like a butterfly or the music tastes of an adolescent, has several life stages.
Let's take some ocean water and cool it down. We'll cool it down from the top by blowing really cold air over the top, so that the surface layer, that teensy, tiny part of the water that is actually in contact with the air, cools down the fastest. It's here, in the surface layer that tiny ice crystals will form when the water temperature drops below the freezing temperature. These ice crystals will be small, 1mm across or less, and, because as water freezes it expands, will generally float on the ocean surface.
What happens next depends a lot on the state of the ocean. In calm conditions, where the ice crystals aren't sloshed around, the crystals will grow outward and join up with their buddies next door, eventually matting together to form a thin sheet of ice on the ocean surface. With nowhere to expand outward, the sheet of ice will begin to grow downwards, thickening up to a layer about 5 to 10cm thick. This ice is called Nilas. It's elastic (that is it bends easily on waves and goes back to its original shape after the wave has passed), crystal clear and breaks when you throw a snow-ball at it (yes, I am a child. Yes, I pretend not to care that you are silently judging me). There's a nifty little series of diagrams I drew below where I try to explain the process.
When conditions aren't calm but more sea-sickness inducing, the action of waves and turbulence breaks up the little ice crystals. These broken ice crystals get mashed up together with other broken up ice crystals to form little congregations of ice called frazil. Frazil ice crystals grow up to about 2 or 3 mm, bounce around in the waves and turbulence until enough of them are bound together that they can float to the surface. There, they continue to get sloshed around, colliding with other blobs of ice to form superblobs called pancakes. It might surprise you to know that pancake ice is called pancake ice due to oceanographers, once again, having no sense of creativity in their naming conventions.
Here you go, some more nifty diagrams will hopefully make what I've said easier to follow.
Most Arctic sea ice gets formed through congelation processes - that is the growth of ice in calm conditions. Winds are generally weaker in the Arctic and, being surrounded by large land barriers like, well, Canada and Russia, there limited ocean swells propagating into the ice from outside. Near the Antarctic however, ice is formed primarily by frazil congregation: being surrounded on all sides by the Southern Ocean, swells and high winds mean that Antarctic waters are rarely very calm.
Whether formed in nice sheets of nilas, or big lumpy pancakes, we have now got a thin sheet of ice covering the ocean surface which has direct influence on how the ice continues to grow and evolve. For one, the ice is now insulating the ocean water below it, which slows down dramatically the rate at which the ocean can be cooled by the cold wind above it, and hence, how quickly we can freeze more water and create more ice. Secondly, there is now a surface for ice-crystals to latch onto.
Although sheet ice does insulate the ocean below it, ice is not a 100 percent perfect insulator. Ice does conduct heat, so eventually, if you cool the surface of the ice in contact with the atmosphere enough, the bottom of the ice, the bit in contact with the water, will cool down enough to so that the water will freeze and the sheet ice will get bigger, growing from the bottom down. However, as the ice gets thicker, its ability to insulate the water improves, so the rate at which we can freeze water and create more ice drops further still. Basically, the thicker the piece of ice, the slower it will grow. In fact, there's elegant mathematical theories that you're probably not interested in, that show that the growth rate of a piece of sea ice is inversely proportional to its thickness.
Once sea ice reaches a thickness of about 10cm, it graduates from the nilas/pancake stage and is upgraded to what we call young ice. This pubescent ice is sufficiently thick that its rate of growth through the freezing of additional sea-water becomes slow enough that it is of secondary importance. Instead, the ice will grow by either accumulating the snow that falls on it, and, much like an adolescent human-being, by smashing into other piece of ice and congregating together. Ice will continue to grow by a combination of these mechanisms until it reaches the equivalent of its 20s: first year ice, that can be anywhere between 30cm and 2 metres thick. This ice has graduated from ice university, yet like a 20-something, still hasn't really found its place in the world.
Ice that's been around the block, accumulated some experience, some battle scars, learned a little, loved, laughed, lost and all those other coming of age metaphors that I promise I'll stop using gets called multiyear ice. This is ice with personality, and by personality I mean it's nasty, hard stuff that is quite happy to smash up an ice-breaker that's bitten more than it can chew.
More than half of the Arctic ice-pack is made up of multiyear ice. However, down south, only a very small fraction of the sea ice has survived a summer, mostly in the western Weddell Sea and parts of the Ross Sea. What this means is that the Antarctic ice pack has a much larger seasonal variation than the Arctic ice: much more of the Antarctic ice melts away during the summer than in the Arctic. There are a few reasons for this, but primarily the cause is that the Antarctic continent gets in the way. While the Arctic ocean spans most of the region from about 70N all the way to the geographic north pole, in Antarctic water, sea ice can only get to about 75 degrees south, before the land gets in the way.
So, there you have it. The birth of ice, through its awkward teenage years and into middle age. Now you know.
For my next post, I'll talk a little about how sea ice moves around, and why it's movements are so hard to predict.
I'm a physical oceanographer at LOCEAN Paris. My research concentrates on the Southern Ocean and its role in the climate system.