Journal of Geographical Studies of Mountainous Areas

Journal of Geographical Studies of Mountainous Areas

Spatiotemporal Analysis of Temperature Extremes in the Past and Future over Western and Southwestern Iran Using Observational Data and CMIP Models

Document Type : Original Article

Authors
Sara Gholamrezaei 1
Mehran Zand 2
Seyyed Jamaluddin Daryabari 3
1 Department of Geography, SR.C., Islamic Azad Univercity, Tehran, Iran.
2 Soil conservation and watershed management research institude, agricultural research, education and extension organization(AREEO), Tehran, Iran.
3 Department of Geography, Semnan Branch, Islamic Azad University, Semnan, Iran.
10.22034/gsma.2025.2069915.1117
Abstract
1. Introduction
This study investigates changes in extreme temperature and precipitation events in western and southwestern Iran during the historical period (1988–2017) and the future period (2020–2060). The data used include daily observations from 30 synoptic stations provided by the Iranian Meteorological Organization, as well as outputs from CMIP6 climate models. Using 26 extreme climate indices based on CCL/CLIVAR recommendations, temporal trends were analyzed through the Mann-Kendall test.
The results for the historical period indicate an increase in warm temperature indices—such as warm nights, warm days, and the number of summer days—and a decrease in cold temperature indices—such as cold days, cold nights, and frost days—suggesting a regional warming trend. In terms of precipitation, total annual rainfall and related indices have declined, although these downward trends were not statistically significant at many stations.
The evaluation of the CNRM-CM6-1 model shows that it performs well in simulating daily temperatures but has certain limitations in forecasting precipitation. Future projections under the ssp126 and ssp585 scenarios suggest that the upward trends in warm temperature indices and the downward trends in cold indices will continue, whereas precipitation extremes do not show a clear pattern of change.
These findings carry important implications for water resource management, agriculture, and climate adaptation policymaking, emphasizing the need for flexible and adaptive planning to cope with future warming and drought conditions.
2. Methodology
In this study, to investigate changes in extreme climatic events in western and southwestern Iran, observational data on precipitation and temperature, along with outputs from CMIP model ensembles, were used to simulate these changes for the future period under two scenarios: RCP4.5 and RCP8.5. After collecting the data and conducting preliminary statistical analyses, extreme climate indices were calculated for both the baseline and future periods.
The trends of the indices were then assessed using the Mann-Kendall test and the Sen’s slope estimator. Finally, spatial maps of the intensity, duration, and frequency of the indices were generated.
3. Results
The analysis of temporal changes in warm event indices in the region showed that, for most stations, warm indices such as warm nights, warm days, the number of summer days, and the number of tropical nights generally exhibit an upward, increasing trend.
The temporal changes in cold event indices indicated that, at most stations, cold indices such as cold days, cold nights, and the number of frost days generally show a decreasing trend.
A key takeaway from the overall assessment of warm and cold extreme indices is the dominance of a warming trend and the expansion of a hot and dry climate type across a larger portion of Iran.
The results of analyzing extreme cold and warm temperature indices suggest that cold extremes in Iran are projected to decrease in the future, while warm extremes are expected to increase. The upward trend in warm extreme indices is statistically significant for most indices and across a considerable portion of the country.
4. Discussion
The characteristics of extreme events are among the major challenges identified by the World Climate Research Programme. Various studies have shown that the frequency and intensity of climate extremes have significantly increased over recent decades. As a result, growing attention has been paid to climate change in recent years due to the economic, social, and financial impacts associated with extreme weather events. This concern is particularly relevant for regions like Iran, which are located in mid-latitude zones with dry and semi-arid climates, making them inherently more prone to extreme temperature and precipitation events compared to other regions.
In light of these considerations, the present study aimed to investigate extreme temperature and precipitation events in western and southwestern Iran during both historical and future periods, based on projections from general circulation models (GCMs). To this end, 16 temperature-related extreme indices were used, including: the number of summer days (SU25), the number of tropical nights (TR20), warm days (TX90P), warm nights (TN90P), warm spell duration index (WSDI), monthly maximum of daily maximum temperature (TXx), monthly maximum of daily minimum temperature (TNx), number of frost days (FD), number of ice days (ID), cold days (TX10P), cold nights (TN10P), cold spell duration index (CSDI), monthly minimum of daily maximum temperature (TXn), monthly minimum of daily minimum temperature (TNn), diurnal temperature range (DTR), and growing season length (GSL).
Additionally, 11 precipitation-related extreme indices were analyzed, including: maximum 1-day precipitation (Rx1day), maximum 5-day precipitation (Rx5day), precipitation amount during the wettest 5 consecutive days in a month, simple daily intensity index (SDII), number of heavy precipitation days with daily rainfall ≥10 mm (R10mm), number of very heavy precipitation days with rainfall ≥20 mm (R20mm), number of extremely heavy precipitation days with rainfall ≥25 mm (R25mm), consecutive dry days (CDD), consecutive wet days (CWD), very wet days (R95p), extremely wet days (R99p), and total annual precipitation on wet days (PRCPTOT).
For this analysis, data were obtained from 29 synoptic stations across western and southwestern Iran with appropriate spatial distribution over a shared 30-year statistical period. Additionally, precipitation and temperature data from CMIP6 multi-model ensembles were used for the historical period (1988–2017) and for future projections (2020–2060). The non-parametric Mann-Kendall method was employed to identify trends in extreme event occurrences.
5. Conclusion
The analysis of temporal changes in warm indices in the study area showed that, for most stations, warm indices such as warm nights, warm days, the number of summer days, and the number of tropical nights exhibited an overall increasing trend. The behavior of these indices is influenced by the region’s geographic location, topography, and environmental conditions.
The temporal variation of cold indices indicated a general decreasing trend in indices such as cold days, cold nights, and the number of frost days at most stations. Changes in these indices across the region are largely affected by terrain and elevation.
A key conclusion drawn from the overall analysis of warm and cold extreme indices is the dominance of a warming trend and the expansion of hot and dry climate types in the southern and southwestern parts of Iran.
The analysis of the frequency of precipitation extreme indices revealed a decreasing trend in annual total precipitation and daily precipitation extremes across various stations in the region.
The results of precipitation extreme indices for the period 2020–2060 under the ssp126 and ssp585 scenarios indicated that, overall, no consistent trend is expected for these indices under either scenario.
The findings for extreme temperature indices showed that future cold extremes in the study area are projected to decrease, while warm extremes are expected to increase. This upward trend in warm extreme indices is statistically significant for most indices across a substantial portion of the region.

Author Contributions
In the preparation and writing of this article, all authors (first, second, and third) have contributed equally and jointly. All stages of the research, from study design and data collection to analysis of results and final writing of the article, are the result of collaboration and collective agreement of all authors.

Data Availability Statement
Data available on request from the authors.

Acknowledgements
We are very grateful to everyone who assisted us in conducting this research.

Ethical Considerations
All authors affirm that this research was conducted in accordance with ethical standards, with no data fabrication, falsification, or plagiarism.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflict of Interest
The authors declare no conflict of interest
Keywords
Extreme climatic events
CMIP6 models
Precipitation and temperature
Climate adaptation
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Journal of Geographical Studies of Mountainous Areas
Volume 7, Issue 1 - Serial Number 25
Spring
Spring 2026
Pages 233-249