Polar Lows
A polar low can be described as an intense mesoscale area of low pressure that forms over the ocean polewards of the main polar front in both hemispheres. They typically last for a few days and have a diameter of 100 - 500 km. Polar lows have been descibed as "arctic hurricanes" because they share some features with their tropical cousins: they form over a warm sea surface (compared to a much colder atmosphere), and so can form as warm-cored systems which derive their energy from latent heat of condensation from the warm ocean. 

Due to their small size and short time-scales, polar lows were difficult to observe until the advent of satellite imagery in the 1960s, which revealed many small-scale cloud vortices at high latitudes in the winter. They can pose a threat to shipping and oil platforms in the polar oceans, due to the potential for storm force winds, high waves and heavy snowfall that they produce. Without a dense network of observations over the ocean and detailed satellite imagery, they can be difficult to accurately forecast.  
A Polar low over the Norwegian Sea, on 6th April 2007. A tropical cyclone-like "eye" is present.
Credit: Dundee Satellite Receiving Station 
Polar Low Formation

Although polar lows have a warm cored structure, they usually require other forms of insitability to develop. They generally develop in a surface baroclinic zone in a polar or arctic airmass, that provides the initial instability to develop an area of low pressure. Such a baroclinic zone can form between an ice field and open sea, or from an old occluded front. If very cold air overlays this baroclinic zone, the atmosphere becomes conditionally unstable (or absolutely unstable in the lowest layers), giving the potential for convection and thunderstorm development. The presence of forcing for ascent from other dynamical processes, such as                                             and                                         also provides more favourable conditions for polar low formation. 

The dynamical ascent and unstable atmosphere promote the formation of deep convection during the developing stage. Temperature differences between the ocean and mid-troposphere can exceed 50 ​˚C, and the resulting convection is fed by large latent heat fluxes from the warm ocean surface. The polar low is then able to intensify according to the         (Conditional Instability of the Second Kind) theroy, in a similar fashion to tropical cyclones, that results in a lowering of the central pressure. 

If atmospheric conditions are favourable and the sea surface is relatively warm, the intense convection can lead to the formation of a mesoscale warm core as a result of latent heat release. Polar lows with the most intense convection can form a cloud free eye with descending air, surrounded by a band of towering thunderstorms. The processes here are identical to those in tropical cyclones that occur over much warmer ocean waters. Such a storm structure can lead to a rapid and unexpected increase in the wind speed and precipitation that can pose a danger to shipping and oil platforms. 

positive vorticity advection
warm air advection
Simple schematic illustrating the "eye" of a polar low. 
From: http://rammb.cira.colostate.edu/wmovl/vrl/tutorials/satmanu-eumetsat/SatManu/CMs/PL/backgr.htm
A second type of polar low can also occur, which is driven more by baroclinic instability than convection. Some convection does occur, although the storm produces a comma-shaped cloud head that is more closely associated with an extratropical cyclone, and does not produce a hurricane-like eye. Large instability between the ocean and the mid-troposphere is still required for formation, however. 


Polar lows will generally decay rapidly after landfall. They start to fill fairly rapidly and their strong wind fields and heavy precipitation also diminish. This is due to three effects: 
  • The source of latent heat from the evaporation of seawater disappears after landfall, meaning that the warm core centre of the polar low cannot be maintained. 
  • The source of warmth from the sea surface is also lost. Land generally tends to be much colder than the sea in the winter, meaning the atmospheric instability is greatly reduced after landfall, leading to a decay of the convective thunderstorms.
  • Frictional convergence over the land is much greater than over the sea, and so the polar low will tend to fill repidly upon landfall. The net inflow results in an increase in the surface pressure. 

Polar lows that are driven more by baroclinic instability and upper-level divergence can maintain their intensity for longer after landfall, since they are not dependent on sensible and latent heat fluxes from the ocean. These will decay when dynamic forcing mechanisms are small or lead to descending air. 
A polar low displaying a comma-shaped cloud signature, associated with baroclinic instability. In the Greenland Sea, south-west of Svalbard, on 5th March 2013. 
Credit: Dundee Satellite Receiving station.