The Trend of Changes and Spatial Distribution of Extreme Cold Days in Northwest Iran

Over recent years, the extreme temperature events (cold and heat waves) have been a burning issue driving the scientific society to carry out extensive researches to investigate and discover the trend, frequency, intensity, and scope of their occurrence. The goal of this study is also the recognition of the trend and spatial analysis of the frequency and maximum of extreme cold days in northwest Iran. For this purpose, the data of minimum daily temperature from 21 synoptic stations of the country for the months of January, February, March, April, November, and December from 1986 to 2017 were used. By using the standard index coefficient, the cold days with an intensity of less than -1/2 were extracted as the extreme cold days for each station monthly. The Mann-Kendall test and spatial analysis with the Anselin Local Moran's I and  hotspot index were used to identify the behavior of extreme cold days. The results demonstrated that the presence of a decreasing trend of extreme cold days in most months (except for a few stations) had no statistical significance, and a significant decreasing trend occurred only in March in almost 50% of the stations. The spatial analysis of the trend of March indicated that the significant decreasing trend had created a cluster and a cold spot. Moreover, the results obtained from spatial analysis indexes in the northwest of the country demonstrated that the frequency and the maximum number of events of extreme cold days in the area in the studied months had formed a high cluster pattern (positive autocorrelation) and a low cluster pattern (negative autocorrelation), while more than 50% of the area of the studied zone had no spatial pattern. The cluster patterns in different months of the year formed in various places and the spatial arrangement of autocorrelation patterns and hotspots were not similar in the studied months. Furthermore, the obtained results based on the  index confirm the formation location of clusters in the Anselin Local Moran's I.