Polar lows in the future

In September 2010, Nature published an interesting article on the future of polar lows. [Read and download it here]

A good friend of ours, Matthias Zahn, who like us did his Ph.D. on polar lows, performed an interesting experiment with Hans von Storch. Recognizing that climate models are unable to simulate actual polar lows (because their resolution is too low), they had to find another way to tell if the frequency and severity of polar lows will change in the future. They therefore took the results from one of the models and fed them into a high-resolution regional climate model (pretty much the same as a weather forecasting model), to “translate” the climate model output to more realistic weather.

In Zahn and von Storch’s simulations, the number of polar lows decreased quite a lot for the future scenarios that they used. The reason for this was that in a warmer future, the air heats up faster than the ocean. This makes cold air outbreaks occur less often (because the air isn’t so cold anymore), and as a consequence, you get fewer polar lows. Tom and I did a similar analysis using the same climate models a few years ago [download paper here], and came to the same conclusion: in the typical polar low regions the frequency of polar lows is going to decline in the future. But the other half of our conclusion was:

In the regions that are covered by sea ice in today’s climate, the number of polar lows will increase dramatically if the sea ice retreats.

I expect this to be true for many regions near today’s sea ice edge, such as the north-eastern Barents Sea, north of Russia (the Kara and Laptev and East Siberian Seas), north of the Bering Strait (the Chukchi Sea) and north of Alaska (the Beaufort Sea). The reason is that polar lows need open ocean and cold air, and when the sea ice disappears, they get just that.

This story was picked up by Reuters in 2009 [read the article here], and was published on many web sites (Scientific American, USA Today etc.). Zahn and von Storch’s paper got a lot of attention; see for instance this article in the Guardian.

Polar low south of Svalbard

A fine polar low under development off the Norwegian coast on 20 November 2008. Spitsbergen is seen at 12 o'clock, the Norwegian coast in the lower right quadrant and the Greenland coast and sea ice in the top left corner

This is an interesting case because I think Spitsbergen has an influence on the development. The elongated cloud stretching down from Spitsbergen’s north-west corner separates the cold Greenland air on the left from the warmer air on the right. The southern edge of the front starts twisting as it gets far enough south, and this is when the polar low starts developing. There is another development just to the east of the incipient polar low. The whiter shades of this second cloud indicates deep convection and high clouds. I’m going to try to find out just what happened in this case. I’ve seen quite a few of this kind of polar low, and it would be fun to see just how much Spitsbergen influences their developments.


Nice polar low near Iceland

A polar low embedded in a large cold air outbreak on 2 March 2009. Greenland in the top left corner and Iceland is partly covered by a cloud in the upper right quadrant

Here’s a pretty one. A large mid-latitude cyclone has moved in from the south and is positioned to the east of Iceland. On the left (west) side of the large cyclone, cold air is streaming down from the north-west. You can see the “cloud streets”, as they’re called. This kind of structure always means that cold air is flowing out over the ocean surface. What happens is that tremendous amounts of heat are transferred from the ocean to the air, much as what happens when you blow on hot soup. The clouds that form in such cold air outbreaks are normally Stratocumuli and Cumuli, with some Cumulonimbi. These are the larger dots, where there are probably isolated thunderstorms and heavy precipitation (snow).

This PL has a very nice spiral-like structure. It would be interesting to know what influence Greenland had on this one. When you have northerly winds to the east of Greenland, you tend to get very cold air damming up along the sides of the steep topography. In this case, there was also cold air streaming out of the fjords; see the nice clouds mirroring the shape of the narrow fjords.

The region that this PL formed in is one of the PL hot spots in the North Atlantic. The water is quite warm (eehhm – relative to the air, at least) and you often get the cold, northerly flow that we see in the picture. You can see why people got interested in PLs – they’re beautiful!


A historic polar low

A tiny, but historic polar low

This satellite image was taken on 8 January 2010, and as far as I’m aware, it’s the first polar low to have been spotted north of Spitsbergen (the biggest island in the Svalbard archipelago). It is also the first one I’ve seen to form north of 80N (the curve just below the PL). It’s not big, but it’s pretty.

Polar lows usually form much further south. The traditional hot spot is near the entrance to the Barents Sea, just north or north-west of the Norwegian mainland. But in recent years it has become clear that PLs occur quite often in other regions as well, such as in the Labrador Sea. Japan has its fair share of PLs each winter, too.

The PL north of Svalbard is interesting because I think PLs will become much more common in the high Arctic as the sea ice retreats. Polar lows can only form over open ocean, so there haven’t been any in the large parts of the Arctic that are covered with sea ice throughout the winter. Now that the Arctic sea ice is shrinking, there is suddenly a potential for PLs to form, and I think they will. All they need is some (in relative terms) warm water and freezing air. They have that in the Arctic.

In a paper that Tom and I wrote in 2007, we tried to find out how the potential for PLs was going to change in this century according to climate models. Here is the map that we compiled, using simulations from 13 models:

Projected changes to our marine cold air outbreak index during the 21 century

The details about what is shown in the figure can be found in the paper, but the red colours indicate large positive changes to the probability of PLs to form. (The negative, bluish colours indicate a declining likelihood, see also Zahn and von Storch, Nature, 2010)

What the red colours mean is that where the sea ice is retreating, there’s going to be a lot more polar low-like weather in the future. But there is a caveat. The climate models that we looked at had to much sea ice to begin with, so the projected changes along the southern rim of the Arctic Ocean are unrealistically large. However, I believe that in regions such as the north-eastern Barents Sea, the Beaufort Sea and the Chukchi Sea, where there is sea ice today, polar lows and similar weather phenomena are going to become more and more common in the future. This is probably bad news for the companies that plan to start using these regions for oil and gas exploration and transport.

To come back to the PL in the first picture now, it’s a small one, but it could be a taste of the future in the high Arctic. I plan to try to model it using a weather forecasting model to see what actually happened. More on that later.

Cheers, Erik