Abstract
It is very important to study the role of global warming on the variability of summer characteristics in arid and semi-arid climates such as Iran, because of its impact on natural and social systems.The aim of this study is to detect interannual variability in the onset, end and duration of the summer season in Iran. To achieve this goal, three indices are defined: physical, dynamic and synoptic. Daily data of the Earth's surface temperature (°C), tropopause level pressure (hPa) and geopotential height at 1000 hectopascals (m) are the basis of physical, dynamic and synoptic indices, respectively. Data were extracted from the reanalysis databases for 1948 to 2016 in different domains. A three-variable dataset using regional mean values allowed researchers to determine the start date (onset) of the summer based on the following thresholds: Temperatures above 25°C and duration of 10 days (physical index), tropopause level pressure less than 120 hPa and duration of 10 days (dynamic index), geopotential height at 1000 hPa less than 50 m and duration of 10 days (synoptic index). The opposite situation indicates the end of summer. The results showed that the start and end of summer based on physical, dynamic and synoptic indices are May 11 and September 21, June 4 and October 1, May 20 and September 15, respectively. The average length of summer is 134, 120 and 119 days, respectively. Statistically significant long-term trends have been observed for the start and end dates of the summer season. Seasonal variability has accelerated significantly since the late the twentieth century. In short, the summer season in Iran tends to start early and end late. As a result, summer lengths are longer in 80 percent of the years after 1981 compared with less than 50 percent from 1948 to 1981.