The long-term in the northwest region of Iran

The present research has been performed for the temporal-spatial reconstruction of an extreme cold episode (temperature anomaly - 15 °C) occurring over the northwest part of Iran for a 142-year period (1871-2012). In carrying out the following investigation, the NOAA Twentieth Century (V20) reanalysis data including mid-level and sea-surface-level data were used. In order to achieve the research goals and extract the sea-surface and mid-troposphere patterns, the methods of hierarchical clustering and factor analysis were utilized. The results showed that during the 142-year period, a total of 791 days were identified in which temperature of - 15 °C or lower occurred. For the synoptic analysis, 183 of these days during which the temperature anomaly was widely observed across the study area were used. The results demonstrated that for most of these days, a surface high pressure system over Asia Minor and northwest Iran contributed to extreme low temperature. These high-pressure systems, whether individually or as part of an extended episode of occurrences, affected the study area. Furthermore, the upper air level study demonstrated that the occurrence of atmospheric blocking or high latitude troughs located to the northwest of Iran (or upstream) and extending into the study area were the main causes for these low temperatures.