2.3.1 Norwegian Coastal Waters

Information on occurrences of polar lows from all areas of European waters is scarce. By far the most comprehensive studies have been made concerning the Norwegian coastal region. A recently concluded study by Noer et al. (2003) broadly agrees with the trends found from an earlier one by Lystad (1986).
Only 49 cases were recorded in the four seasons 1999-2003, whereas the previous study found 79 cases in the three years 1982-1985 (Lystad 1986). The reduced frequency in the later study is a result of more restricted criteria for defining a polar low and the use of improved satellite data for verification. Many of the cases that now are classified as common troughs were classified as polar lows in the earlier study.

Frequency distribution for observed polar lows near Norway, based on combined results from two studies: 1971-1982 (Lystad, 1986) and 1999-2003 (Noer et al, 2003)

The graph shows the frequency of polar lows for each of the winter months based on the combined results of the two studies. The maximum frequency of polar lows was found to occur in January and March, with a secondary maximum in November. The minimum in February is probably due to a high occurrence of continental high pressure over central Scandinavia giving a southerly flow along the Norwegian coast. There is also large annual variation in the data from season to season. In the months of February to May 2003 the observed number of polar low events was low, which can be explained by the presence of long spells of southerly winds.

Although the statistical basis is thin, in the 49 cases recorded since 1999 there is a slight tendency to a monthly shift westward as the season progresses, this being consistent with the variability of the sea surface temperatures. The cases east of 40 deg. were all in October to January, while the April and May cases were all west of 10 deg.

Noer's study also noted that during the seasons 1999-2003 a polar low was found to develop in 27% of the cases where both a low-level cold outbreak and an upper cold trough were present, but no detailed examination of the upper trough was carried out before the prediction was made.

2.3.2 North Atlantic Distribution

Positions of fully developed polar lows in the seasons 1999–2003.

The map shows the positions of observed polar lows at a mature stage. The data is based on registrations and archived AVHRR imagery at the Norwegian Meteorological Institute in Tromsoe, Northern Norway from the seasons 1999 - 2003. The data covers the area from Iceland and eastwards to Novaya Zemlya, and north of 60° N. The area west of Island is not covered by these data, hence no conclusion of occurrance of polar lows in this area should be drawn from this picture.

The Norwegian registrations of data indicate that the polar lows most commonly appear east of the 0º meridian, up to 40° E, and occur during a northwesterly outbreak of arctic air. The majority are seen between 65 and 75° N, and become much scarcer farther south. Less frequently, polar lows are observed during arctic air outbreaks farther south and affect the UK and southern Scandinavia. These are generally a result of cold air outbreaks from the east, over the Scandinavian mainland. Although rarer, these events are seen to happen during most winter seasons, and the forecaster who provides services for any northern European coastline should be alert to the conditions that are favourable for a low to form.

References

Heinemann, G., 2003. Interaction of katabatic winds, mesocyclones and sea ice formation. Proc. of the 9th meeting of the EGS Polar Lows Working Group, Cambridge, UK, 2003.

Lystad, M., 1986: Polar lows in the Norwegian, Greenland and Barents Sea - Final Report. Norwegian Meteorological Institute, Oslo, Norway, 196 pp.

Noer, G. and M. Ovhed, 2003: Forecasting of polar lows in the Norwegian and the Barents Sea. Proc. of the 9th meeting of the EGS Polar Lows Working Group, Cambridge, UK, 2003.