Polar low trajectories in the Nordic Seas 1999-2013: a statistical analysis using kernel density methods

Abstract

Polar lows are mesoscale cyclones of high intensity, frequently occurring in the Nordic Seas during the winter season. They usually form in baroclinic zones, for instance in regions near ice edges, and they are often maintained by organized convection. Polar lows are characterized by severe weather conditions like heavy snowfall, strong winds, icing and large sea waves. In addition to forecasting complexity, polar lows thus represent potentially hazardous conditions for marine installations, ships and coastal communities.

In this thesis, we carried out a statistical analysis of polar low trajectories in the Nordic Seas during 1999-2013 by the use of kernel density estimation as the main method. This method provides smoothed estimates of the probability density function of the data, and the degree of smoothing can be adjusted by the choice of a bandwidth parameter. Our density estimates gave an accurate representation of the space-time distribution of polar lows trajectories, and displayed the main concentration located in coastal/marine areas close to Lofoten and Vesterålen. By segmenting the data set into time intervals, the main maximum was seen to shift spatially with time, displaying the space-time distribution as highly dynamic. A temporal shift in the densities towards the Barents Sea was proposed to be connected to climate change. Polar low formation and development was found to correlate well spatially with the Norwegian Atlantic Current and its branches in the Nordic Seas.

By investigating large scale wind flow, it was found that polar lows formed mainly within a northerly flow in the Norwegian Sea, and a north-easterly flow in the Barents Sea. We connected the combination of increased sea surface temperatures and cold air outbreaks from the Arctic which leads to a decrease in static stability, which in turn promotes favourable conditions for polar low formation and development. Polar low dissipation was found to relate well with systems being steered across land or sea ice, being deprived of their main energy source. A secondary analysis was made based on curve clustering of the polar low trajectories. This revealed how trajectories vary with regards to length, propagation direction and degree of curvature. Our results confirmed previous research on these characteristics connected to the distribution of genesis locations.