Imagine to be at sea in the high north winter time, and suddenly and unexpectedly be captured by strong winds, icing and heavy snowfall causing low visibility. Not the best conditions on a ship at sea! It is a scary and possible deadly situation, and unfortunately something mariners in polar regions have experienced in all times. There are many tales of storms appearing out of nowhere, of hazards to ships and coastal communities in the Arctic, of a whole fleet of fishermen never coming home again.
Modern satellite images have identified these storms as polar lows. They are Arctic hurricanes, short-lived and small compared to their tropical cousins, but still dangerous if you happen to be on their path. And the main problem: they have been – and still are – hard to forecast.
Polar lows are like hurricanes in the polar areas, like in the Norwegian Sea and the Barents Sea. They are storms with strong winds, heavy precipitation, snowfall and low visibility.
They are, however, small compared to the tropical hurricanes, both in extension and life time. Tropical hurricanes can develop within weeks and have a range of about 2000 km. A polar lows develops quickly and lasts normally not so much more than a day. The diameter is typically about 150-600 km.
Polar lows are formed in the winter when the difference between the sea surface temperature and the air temperature is at its greatest. The sea temperature is fairly constant through the year, but the air temperature varies greatly. It is this temperature contrast that causes the development, and they often appear at the margins of the Arctic sea ice. As cold air over the ice covered areas moves out over the open and relatively warmer ocean, a lot of energy is transferred from water and into the atmosphere, in a process known as a cold air outbreak.
In satellite images polar lows appear as comma clouds, with spiral cloud bands and a clear eye in the centre, like the tropical hurricanes. In the animation below Oskar Landgren has put together model results from one month. You can see three polar lows appear and burn out.
Model simulation of winds and clouds during one month. Three polar lows appear. Credit: Oskar Landgren
Polar lows are one of the few features of global warming where scientists expect a positive development for us humans. As the Arctic sea ice retreats, and global warming causes the Arctic air temperatures to rise quickly, the temperature difference between the ocean and the atmosphere decreases. Landgren and his colleagues expect fewer polar lows in the future.
At the same time, more human activity at sea in the high north, with new shipping routes, fisheries and oil activity going northwards, means that there are also more activity in the pathway for these sudden storms.
To investigate how polar lows may appear in the future, Oskar Landgren works on downscaling global models. The global climate models are too coarse to resolve such small scale phenomena as polar lows. The models he works with are very similar to weather forecasting models, with the same physics, thermodynamics and fluid dynamics. With these models, he can resolve the smaller scale structures.
Would you like to read more about polar lows and the theory behind them? Take a look here