Letting snow go with the wind

Jens Boldingh Debernard, Norwegian Meteorological Institute. Photo: Ellen Viste

If you have ever been in a blizzard, you have probably noticed how snow collects in huge heaps, while in other places the ground may be practically bare. Strong winds whirl snow into the air, making it drift off before it has time to settle. Over the sea ice in the Arctic and Antarctica, snow drifts off the ice and ends up in the leads between the floes. Does drifting snow lead to more or less sea ice?

Jens Boldingh Debernard’s runs the ice module of the Earth system model. From his office at the Norwegian Meteorological Institute, he turns snow drift on and off in the model. Some times he lets the snow settle where it lands, other times he lets it drift away with the wind.

“My goal is to make the model as physically correct as possible”, he says.

An Earth system model in which snow can drift freely with the wind, is a more correct reproduction of the real world than a model where the snow is stuck once it has landed. If drifting snow affects the amount of ice formed in the real world, this should obviously be included in the model.

The model is both a good tool for finding out how important snow drift is, and in itself a reason to try to find out.

Two reasons to care

Jens Boldingh Debernard has two reasons to care about snow.

In sharp sunlight, a field of snow can blind you. Snow is almost by definition white, and much lighter than ice. That is because it reflects more of the light that hits it, just like a reflector does. Snow returns more of the radiation from the sun back into space, absorbing less energy than ice.

Even though snow consists of ice crystals, intricate snowflakes can hardly be said to resemble solid ice. The second important characteristic of snow is that it contains a lot of air. There is air within and between the snow grains, and air is a terrible conductor of heat. Ice conducts heat about ten times as well as snow does.

In summary, snow absorbs less energy from the sun and isolates the ice beneath. This influences the amount of ice that forms in winter and how much that melts in summer.

Both more and less ice

When there is a lot of snow over the ice in spring, the ice will have less time to melt before winter comes. The snow does not only insulate and protect the ice beneath, but the snow will also have to melt before the sun can start working on the ice.

When snow drifts off the ice floes and into the ocean, the snow cover on the ice gets thinner. Can we then simply assume that snow drift leads to more melting and thinner ice?

«This is just half of the story», says Jens Boldingh Debernard.

In the polar winter, the air is colder than the sea. Snow drifting off the ice is like tearing out the insulation from the walls of a house. Both the ice and the water below will get colder. This causes more freezing under the ice, making it thicker. If the water remains cold, less ice will also melt during summer. This way, snow drift contributes to thicker ice.

Thinner ice because of more melting in summer and thicker ice due to less insulation and colder water – snowdrift does both. What will the result be?

“It depends on whether you are in the Arctic or in Antarctica”, Jens Boldingh Debernard replies.

The model experiments that include drifting snow produce more sea ice in the Arctic, but less in Antarctica. The physics is the same, the model is the same, so how can the results be opposite? The geography is not the same, the ice is not the same, and neither is the weather.

Left: Ice grows from below. Snow insulates the ice, reducing the heat loss from the warm ocean to the cold air. This way, snow makes the water less cold, and less ice will form. Middle: Snow drift causes more ice. When snow drifts off the ice and into the sea, the snow cover on the ice gets thinner. With less insulation on top, the ice will grow faster from below. Also, snow drifting into the sea cools the water, making more new ice form. Right: Snow drift causes less ice. With less snow on the ice in spring, less ice will make it through the summer. Ice absorbs more solar radiation than snow, heating up and melting faster. When meltwater ponds form, the ice will melt even more quickly. Ill.: Ellen Viste

Antarctic sea ice freezes from above

There is one, significant geographic distinction between the Arctic and Antarctica. While the South pole is on a continent surrounded by sea ice, the North pole is in an ice-covered hole between the continents. This means that the sea ice in Antarctica is much farther from the pole than the sea ice in the Arctic. The polar regions are dry in both hemispheres, but toward the equator lie the storm-tracks. More low pressure systems sail in over the Antarctic sea ice than into the Arctic Ocean, so Antarctic sea ice receives more snow. The snow insulates the ice and the water beneath, making the ice grow slowly in winter. In Antarctica, the sea ice is rarely more than a meter thick.

Still, it is not obvious that the sea ice around Antarctica will grow faster if the snow drifts off. Normally, ice will grow from below. In Antarctica, ice may also grow from the top. Thick layers of snow weigh down the ice, in some places so much that the ice floes are completely submerged. When water then freezes on top of the ice, the ice will grow from above. If enough snow drifts away from the ice, the ice stays above the water, ending ice formation on top of the ice. This way, snow drift contributes to making the ice in Antarctica thinner. Together with the increased summer melt caused by less snow, this effect is larger than the insulating effect of the snow. Snow drift makes the ice in Antarctica thinner.

In the Arctic, there is not much snow. The ice may be 1–2 meters thick, and the snow cover only 20–30 centimeter. Including snow drift in the model makes the sea ice in the Arctic thicker. With less snow on the ice, the extra amount of ice formed in winter is larger than the extra amount melted during summer.

Left: The ice in the Arctic lies far to the north, in dry regions without much snow. Only thin layers cover the ice. Through the ice, heat energy is quickly transferred from the ocean to the atmosphere, and more ice forms under the ice. Middle: The Antarctic sea ice lies farther from the dry, polar region than the ice in the north – closer to the storm tracks. As a result, the ice is covered by a thick layer of snow. The ice is thinner than in the Arctic, and the snow can weigh it down so much that it is flooded by sea water. When this water freezes, Antarctic sea ice will grow from above. Right: When snow drifts off the Antarctic sea ice, less snow will weight down the ice. Less ice will be flooded by sea water, and there will be less ice growth from above. Snow drift contributes to thinner ice around Antarctica. Ill. Ellen Viste

Antarctic sea ice melts from below

One other thing makes the ice around Antarctica thinner than in the Arctic, independently of snow drift. The water under the ice is colder in the Arctic. Big rivers like the Yenisej, Ob and Mackenzie dump enormous amounts of cold water from Siberia and Canada straight into the Polar Ocean. As this water is fresh, it is less dense than the salty sea water. The freshwater forms a cold and stable, protective layer above the warmer sea water.

Around Antarctica, the water is more well-mixed. As a result, warm sea water reaches the ice and may sometimes melt it from below.

Left: The water under the ice in Antarctica is well-mixed. This means that warm water from the north can melt the ice from below and from the sides. Right: In the Arctic, large rivers pour fresh water into the Polar Ocean. This water forms a cold layer above the warmer water entering from the Atlantic Ocean. The cold surface water shields the ice, causing less melting under the ice in the Arctic than in Antarctica. Ill. Ellen Viste

Sea ice from air and ocean

Any change – whether in snow drift, in the amount of sunlight that is reflected, in air temperature, ocean temperature or ocean currents – will influence how much sea ice that freezes or thaws. This is true in the real world as well as in the Earth system model. How much ice the sea ice module produces depends on the numbers transferred from the module for the ocean and the atmosphere. Small errors in the other modules may grow when further calculations are performed in the ice module.

“You cannot blame the ice modeler every time”, says Jens Boldingh Debernard with a smile.

Deviations in the ocean or the atmosphere have great consequences for the amount of sea ice. This is why it is so important that all parts of the Earth system model are as close representations of the real world as possible. Including snow drift is one of the steps toward an even better representation of our planet.