Temporal characteristics of cold waves hazard frequency in northwest of Iran and Arctic Oscillation and North Atlantic Oscillation impacts

In this study, cold waves (CW) were set and classified in different severity levels by applying standardized index (Z score) of minimum temperature (MT) in the period of 1951–2019, and also, the frequency of monthly, annual and decadal occurrence of CW was calculated and analyzed. The results showed that there was a decreasing linear trend in distribution of temporal changes of CW. The probability for annual occurrence of CW (which annually reaches 42 times or less) was 99%, and the probability of its occurrence for more than 42 waves was 1%. The trend analysis of time series component represented a significant reduction in the cold waves frequency (CWF) in the study area. Also the results of the trend analysis for the severity levels of occurrence of different cold waves showed that despite the significant reduction in the CWF, none of the classes had a significant trend except the reducing trend of annual cold waves frequency (ACWF). Investigating the decadal condition of CWF displays an increased incidence of CW from 70 s and stepwise reduction in the CWF from the 1981 to 2010, and consequently, the general trend of decadal occurrence of CW was decreasing. The twenty-first-century prediction of the CWF shows the CWF will decrease significantly compared to the long-term average of the twentieth century. The results obtained from the correlation and Morlet wavelet analysis (MWA) between the levels of CW severities with Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) teleconnections indicated that the correlation between CWF is in-phase, positive and incremental.