A New Approach for Definition of Summer Season in Iran

The climate of the summer season, dry and low rainfall region of the Middle East, has not been considered by atmosphere scientists for unclear reasons. The claim for the need to update the definition of seasons is due to the fact that the weather conditions are not considered in the meteorological and astronomical definitions. Almost all of the definitions given to identify the characteristics of the summer season follow three physical, dynamic and synthetic approaches. The physical index considers days with an average temperature above threshold as a summer day. Dynamically, more heat transfer in the summer leads to an increase in altitude and a reduction in the pressure of the tropopause. Synoptically, scholars have outlined the beginning of the summer to overcome the Persian trough over the region. In addition, with the sudden disappearance of the Persian trough in the middle of September, the summer season ends. With the assumption that global warming has accelerated the course of the summer period and significantly increased its intensity over the past decades, researchers have been reviewing the summer atmosphere in the Middle East, and especially in Iran, by designing new indicators. Materials and methods: In this study, using the reanalyzed data of temperature, tropopause pressure and geopotential height of the NCEP/NCAR database during 1948-2016, the characteristics of the summer season, including its start, peak, durability and ending in Iran has been evaluated. By using the script written in the GRADS software, the daily values of the data were extracted in the desired ranges. Then, in order to estimate the summer period based on the threshold of each indicator, the daily average of the data was calculated. The thresholds for physical, dynamical and synoptic indicators for estimating the start and end of the summer season and the number of summer days, respectively, were daily temperatures higher than 25 degrees, tropopause pressure less than 120, and geopotential heights less than 50 meters. The transit of daily data values from the threshold and their 10-day continuity indicates the beginning and end of the summer period. Finally, the summer weather variations in Iran were also studied based on the Kendal’s tau tests and the graphically Man-Kendal test. Results and discussion: the results of temperature-based index (physical index) with a threshold of 25 degrees Celsius and a 10-day durability shown that the beginning of the summer season in Iran is 1 June and its end is 27 September. Based on the variations of the tropopause level pressure (dynamic index) with a pressure threshold of less than 120hp and ten day duration, the beginning of the summer season is 4 June and the its end 1 October. The results of the assessment based on the index of geopotential height changes in 1000hp level (Synoptic index; Persian Trough Formation) with a threshold of 50 meters and ten days duration showed the beginning and end of the summer season in the Iranian region, 20 May and 15 September, respectively. The average annual summer season based on physical, dynamic and synoptic indexes was estimated, respectively, 134, 120 and 119 days. The results of trend analysis indicate that the frequency of summer days on the Iran based on the physical index has a significant upward trend. Accordingly, the summer atmosphere has a tendency towards early starters and late end-time. Dynamic and especially Synoptic indices the opposite of the physical index show the declining trend of summer atmosphere. Conclusion: The time series process of the temperature-based physical index indicates the upward trend in the number of summer days during the study period. The correlation coefficient of τ=0.39 and a significant trend in the time series of the mean time average of the summer temperatures of the 69-year period, also proving the hypothesis that climate change was happened. In terms of the physical index (temperature) over the last seven decades, the summer tended to start earlier and more sustained, while the synoptic index was opposite. In contrast, the time series of the synoptically index based on the period of the Persian trough formation over the Persian Gulf region indicates a decline in the number of annual summer days. Because of the differences in the results of the physical index and the similarity of the dynamical and synoptic indexes with previous studies, the authors emphasize the need for further research in this regard.