Abstract
The present study aimed at analyzing the synoptic pattern of atmospheric thickness of winter pervasive frosts in Iran. To this end, the data related to the daily minimum temperature of a 50-year period (1961–2010) were gathered from 451 synoptic and climatology stations. Then, the instances in which the temperature was below 0 °C for at least two consecutive days and this phenomenon covered at least 50% of the entirety of Iran were selected. Subsequently, the atmosphere thickness pattern was extracted for these days, with the representative day being identified and analyzed through cluster analysis. The results showed that the Siberian high pressure plays a significant role in the occurrence of pervasive frosts in Iran. In some other cases, the northeast–southwest direction of this pattern leads to its combination with the East Europe high pressure, causing widespread frosts in Iran. Furthermore, the interaction between counter clockwise currents in this system and the clockwise currents in the Azores high pressure tongue directs cold weather from northern parts of Europe toward Iran. The formation of blocking systems leads to the stagnation of cold weather over Iran, a phenomenon that results in significant reduction of temperature and severe frosts in these areas. In addition, the omega pattern (the fifth pattern) and Deep Eastern European trough and polar low pressure pattern (the fourth pattern) were the most dominant and severe frost patterns in Iran respectively.
Highlights
Changes in the frequency and intensity of climatic events are an indicator of climate change. Due to their severity and sudden occurrence, extreme frosts have a profound effect on ecosystems and human societies
In studies related to anomalies of climate variables in each area, the behavior of these variables is investigated in the light of their long-term average
Numerous studies of synoptic analysis of East Asian cold waves have indicated that the dominant pattern in these areas is the Siberian high pressure [4,5]
Summary
Changes in the frequency and intensity of climatic events are an indicator of climate change. Due to their severity and sudden occurrence, extreme frosts have a profound effect on ecosystems and human societies. They are the result of certain atmospheric conditions and atmospheric circulation [1]. In studies related to anomalies of climate variables in each area, the behavior of these variables is investigated in the light of their long-term average. Some others have investigated spatial-temporal anomalies of climate variables [9,10,11].
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