Analysis of observed and projected interannual variability in the summer season onset, length, and end dates across the Iran

• Published in: Theoretical and applied climatology Vol. 147; no. 1-2; pp. 549 - 558
• Main Authors: Sedaghat, Mahdi, Nazaripour, Hamid
• Format: Journal Article
• Language: English
• Published: Vienna Springer Vienna 2022; Springer; Springer Nature B.V
• Subjects: Analysis; Aquatic Pollution; Arid climates; Arid regions; Aridity; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; Climate change; Climate science; Climatology; Duration; Dynamic height; Earth and Environmental Science; Earth Sciences; Earth surface; Geopotential; Geopotential height; Global warming; Height; Interannual variability; Original Paper; Seasonal variability; Seasonal variation; Seasonal variations; Seasons; Semiarid climates; Statistical analysis; Summer; Surface temperature; Tropopause; Waste Water Technology; Water Management; Water Pollution Control; Iran
• ISSN: 0177-798X; 1434-4483
• DOI: 10.1007/s00704-021-03852-1

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 (PI), dynamic (DI), and synoptic (SI). 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, and 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 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% of the years after 1981 compared with less than 50% from 1948 to 1981.The multimodel ensemble from CMIP6 models projects an explicit and continuous increase in the duration of the summer season over Iran during the twenty-first century under three future scenarios, particularly based on PI.