Abstract
Periods of unusually high or low temperatures have a significant impact on both nature and society at all times of the year. A possible change in frequency or persistence of such events would impact the wildlife as well as the economy and people’s everyday life. In Norway, a major factor determining temperature anomalies is the large-scale atmospheric circulation. It is therefore of interest to examine frequency and length of both cold/warm spells and certain circulation patterns in order to find a possible connection between trends. In the following study, it is done by analyzing daily minimum and maximum temperature from 1979 to 2018 measured at selected stations owned by the Norwegian Meteorological Institute and applying different classification methods included in cost733class software on daily mean sea level pressure from ERA5 global reanalysis for the same time period. The results show that both frequency and length of cold and warm spells during winter (October to March) and summer (April to September), defined as a number of consecutive days with minimum/maximum temperature below the 25th/above the 75th percentile, have changed during the last four decades at all locations, with an overall increase in both measures for warm spells during both seasons and an overall decrease in both measures for cold spells during both seasons, with exception of the length of cold spells in winter which are varying between locations. By looking at relative frequency of different circulation types during cold and warm spells, as well as average temperatures measured during each circulation type, circulation types associated with these spells are identified. Warm spells are generally associated with air flow from southerly directions, while cold spells are associated with air flow from northerly directions. Air masses of continental origin in the east contribute to warm spells in summer and cold spells in winter, while the opposite is true for maritime air masses to the west. Further analysis indicates that circulation types responsible for cold spells have got slightly less frequent, while circulation types resulting in warm spells have got slightly more frequent, which would contribute to the change in frequency of observed cold and warm spells. Still, the magnitude of change is larger for the actual spells than for the atmospheric circulation, so a dominating factor would be that the temperature has increased everywhere for almost every circulation type, increasing the probability of the occurrence of warm spells and lowering it for cold spells.