Analysis of Seasonal Changes and Intensity Metrics of Extreme Climatic Heatwaves in Iran

Extended Abstract
Introduction
Heatwaves are among the most hazardous extreme climate phenomena, during which air temperatures persistently exceed reference climatic thresholds for several consecutive days. These events have significant impacts on agriculture, ecosystems, energy resources, and human health). The frequency and intensity of heatwaves have increased over recent decades across different regions of the world, with numerous events reported in Europe, North America, Asia, and Oceania. Consequently, understanding this phenomenon is essential for vulnerable regions. Numerous studies have investigated heatwaves both globally and in Iran. However, despite Iran's high vulnerability and the frequent occurrence of heatwaves, a comprehensive study that analyzes the seasonal intensity of extreme heatwaves using different indices is lacking. Therefore, the main objective of this study is to conduct a seasonal analysis of extreme heatwave intensity in Iran using four indices (HWMId, HEATcum, AVI, and AVA) and to compare their spatiotemporal patterns in order to identify the most suitable index for assessing heatwaves in arid and semi-arid regions of Iran. The results of this study can be applied in water and soil resource management and in planning for climate change adaptation.
Materials and Methods
This study used daily maximum temperature data from 112 synoptic stations across Iran for a 33-year period (1991–2023). Missing data were reconstructed following the World Meteorological Organization guidelines, and their quality was validated using the cross-validation method. A heatwave was defined based on the 90th percentile threshold of daily maximum temperatures within a 15-day moving window (7 days before, the day itself, and 7 days after), with a minimum duration of three consecutive days. Heatwave analysis was conducted separately for each season, and a 5-month window was considered for each season to account for cross-seasonal heatwaves. Four heatwave intensity indices were calculated: the daily Heat Wave Magnitude Index (HWMId), cumulative heat (HEATcum), average intensity (AVI), and average anomaly (AVA). HWMId was calculated by standardizing temperature anomalies using the 25th and 75th percentiles. HEATcum and AVA represent the sum and mean of temperature anomalies relative to the 90th percentile threshold, respectively, while AVI is based on the mean of absolute daily temperatures.
Results and Discussion
Seasonal Analysis of Maximum Temperature Distribution and Annual Average of Heatwaves
The results showed that the highest maximum temperatures occur in summer across central, southern, and southeastern regions, while the lowest occur in winter across northwestern and northern regions. The highest annual average of heatwaves is observed in the northwest of the country. The highest and lowest heatwave values belong to summer and autumn, respectively. The spatial distribution pattern of maximum temperature is influenced by local factors (latitude and altitude) and external factors (general atmospheric circulation elements).
Heatwave Intensity Indices
The HWMId index showed its highest values in spring at Rasht, Abadan, and Baft stations (mainly in 2023). Autumn had the highest heatwave intensity (Hamedan, 2019), while in summer and winter, northern regions exhibited the highest intensity, which is attributed to the percentile-based nature of this index. The HEATcum index showed its highest values in spring in northern regions (Gorgan, Babolsar, Rasht stations) between 2007 and 2011, with winter having the highest cumulative heat; areas with maximum values of this index are concentrated in northeastern, northern, and northwestern regions. The AVI index showed its highest values in spring in southeastern, central, and southwestern regions, and its lowest at Abali station (2019); it expanded across the entire country in summer and showed a significant increase in recent years (especially 2023); its distribution follows a similar pattern in spring, autumn, and winter. The AVA index showed its highest values in spring along the northern strip of the country (Babolsar, Nowshahr, Ramsar, Bandar Anzali) and Bandar Mahshahr between 2007 and 2011, with winter showing the highest values nationwide; the maximum of this index occurs at higher latitudes.
Comparison of Different Heatwave Intensity Indices
Comparison of the four indices revealed that the AVI index, based on absolute temperature, exhibits a completely different pattern from the three anomaly-based indices (HWMId, HEATcum, and AVA) and is unsuitable for comparing regions with different climates. HWMId and HEATcum showed similar patterns, both identifying northwestern and western Iran as areas with the highest heatwave intensity. Regarding the year of occurrence, HWMId and HEATcum had similar temporal patterns, with recent years recorded as the period of most intense heatwaves. Overall, anomaly-based indices are more suitable for identifying and analyzing heatwaves in Iran, and among them, cumulative-based indices (HWMId and HEATcum) are more effective for detecting the most intense heatwaves due to their greater sensitivity to short-lived, severe anomalies.
Conclusion
The present study aimed to conduct a seasonal analysis of extreme heatwave intensity in Iran using data from 112 synoptic stations and four indices (HWMId, HEATcum, AVI, and AVA) over the period 1991–2023. The results showed that the highest maximum temperatures are observed in southeastern, southern, and southwestern regions, while the highest annual average of heatwaves occurs in the northwest of the country. The highest and lowest heatwave values belong to summer and autumn, respectively. Due to its percentile-based nature, the HWMId index revealed a different pattern of heatwave distribution, with higher intensity in northern regions. The HEATcum index showed its highest values in northeastern, northern, and northwestern regions, particularly during winter and spring. The AVI index exhibited a similar pattern in spring, autumn, and winter and showed an increasing trend in recent years. The AVA index recorded its maximum at higher latitudes. Comparison of the four indices indicated that anomaly-based indices (HWMId, HEATcum, and AVA) are more suitable for identifying heatwaves in Iran than the absolute-temperature-based index (AVI). Among them, cumulative-based indices (HWMId and HEATcum) are more effective due to their greater sensitivity to short-lived, severe anomalies. It is recommended that future research on heatwaves in Iran examine quantitative and synoptic dimensions as well.