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Sea Ice North and South

By: Dr. Ricky Rood, 3:38 AM GMT on October 02, 2007

Sea Ice North and South

If you follow the links in this there is a lot of interesting reading!

There was a lot of discussion about melting sea ice in the Arctic and, perhaps, no melting sea ice in the Antarctic. In his blog, Jeff Masters has been regularly visiting the sea ice in the Northern Hemisphere and talking about its record decline. Some of you may recall that I had a series of blogs (Links at the end) on ice melting, and how when you consider the interaction of water with ice, ice can melt much faster than when it is just a dripping block. It’s like stirring your ice rather than just letting it sit there.

In 2004 when I went on sabbatical, I thought that sea ice and sea ice modeling must be simple. There is transport of ice and there is melting of ice. By coincidence, I started to diagnose an excess of sea ice in a new configuration of a climate model. I was surprised to learn the immense complexity of the physics of sea ice. Here is a link to one of the world’s most used sea ice models. CICE at Los Alamos

In this blog I will summarize some of the elements of the physics of sea ice. This is to get you thinking about the problem beyond whether it is just hot or cold. Here is a nice essay from NOAA on sea ice. Here is a link to a story about my friend David Barber, who does all sorts of things with sea ice and polar bears.

First, remember that by definition, sea ice exists at the edge. It exists at the edge of freezing and thawing. Sea-ice is made by the freezing of sea water. Intuitively, it is formed in the winter. What happens in the summer? There is in general a decline of sea ice in the summer; there is melting for various reasons. Since sea ice sits on the edge of a phase transition (freezing and thawing) its distribution is sensitive to small changes in the environment.

I was diligent to avoid, in the previous sentence, saying that the sea ice was sensitive to small changes in “temperature.” Sea ice depends on far more than temperature. Remember the sea is salty and all of us who sprinkle salt on our icy sidewalks know that salt melts ice. Therefore, 1) sea-ice forms a little below, about - 1.8 C, our normal idea of freezing (0 C, 32 F), and conversely, 2) sea ice is sensitive to the salinity of the water. To make it more complex as sea ice freezes and thaws it changes the salinity, and if the salinity is different, then its thaw-freeze temperature is different.

What about the temperature? The sea ice is sensitive to the air temperature and the water temperature. It is also sensitive to the absorption of solar and infrared energy. If the ice is dirty and dark, then it absorbs more solar energy in summer. If the ice is cracked and there is sea water in the cracks, then the water is dark and absorbs heat. If there is fresh white snow, sea ice is insulated from both air and the sun. And that air and water temperature: Sea ice does not just melt because the air temperature is warm or the sun is shining on it, sea ice is also melted from below by the unfrozen sea water. Sea ice exists on the edge in every direction, up, down, north, south, east and west.

Going back to those blogs on ice melting - once there are pools of water on the ice, it starts to flow and can accelerate the melting. Then there is that salinity again. The salinity changes the density of the sea water. Therefore, salinity helps to determine how the water circulates. Remember if more dense water sits on top of less dense water, it will sink, and other, perhaps warmer, water will come to the surface.

To make it even more complex – what happens when fresh water freezes and makes ice? It expands. What happens if water is heated towards boiling? It expands? Water is odd; it is expanding with both heating and cooling – at least close to freezing and boiling. Putting enough salt in water completely changes this behavior. And, this messes with the density, and then the dynamics.

A point: When you think about the northern and southern hemisphere, you have to think about much more than the atmospheric temperature. You have to think about the radiation budget, the cleanliness and the dirtiness of ice, snow on the ice surface, the ocean temperature and the ocean circulation. You have to think about how the wind blows the ice around – does it bunch it next to the coast or blow it into open warmish water. And, in many cases what might be most important is salinity, and the density changes and freezing temperatures associated with differing salinity. What if there was fresh water collecting around the sea ice? Where could fresh water come from?

Recent sea ice trends
Sea ice data

The End of Ice
Fast Ice 1
Warm Snow

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.