Introduction

Anthropogenic climate change stands as one of the most formidable challenges facing contemporary civilization. While climatological research has traditionally prioritized the isolation of individual variables such as temperature or precipitation and their associated extremes (e.g., heatwaves or meteorological droughts), a growing body of scientific evidence suggests that the concurrent or sequential convergence of these hazards yields consequences far more devastating than the sum of their independent effects. Termed compound extremes, these phenomena emerge from intricate interactions between atmospheric physics and land-surface processes. Compound extremes are inherently multidimensional, arising from the interplay of several contributing variables; consequently, accurately characterizing these events necessitates an investigation into the joint and simultaneous behavior of the underlying factors. To illustrate, a compound hot-dry event is typically defined by the coincidence of distinct precipitation deficits alongside positive temperature anomalies or heatwaves. Of particular concern are compound hot-dry events, characterized by the coincidence of elevated temperatures and significant precipitation deficits, typically demarcated by the 80^thand 20^thpercentiles, respectively. These combined extremes possess a potent capacity to amplify environmental stress through positive feedback loops linking soil moisture and air temperature. Under drought-induced moisture deficits, a substantial fraction of incoming solar radiation rather than being dissipated through evapotranspiration is converted into sensible heat. This thermodynamic shift elevates surface temperatures and significantly intensifies heatwave dynamics. Recent global assessments indicate a rising trend in the frequency, intensity, and spatial extent of these compound extremes, particularly within arid and semi-arid zones. Accurately characterizing these events, however, necessitates the deployment of multivariate indices; conventional metrics focusing on singular climatic aspects fail to capture the synergistic nature of coupled hazards. Iran, with its vast climatic diversity and location within the global arid and semi-arid belt, is profoundly vulnerable to climate variability. In recent decades, severe precipitation fluctuations coupled with a prevalent warming trajectory have established ideal conditions for the genesis of compound hot-dry extremes. Such occurrences pose existential threats to the nation’s food security, water resources, public health, and biodiversity. Despite the critical implications, few inquiries in Iran have adopted a comprehensive approach using standardized compound metrics to analyze these coupled phenomena. To bridge this knowledge gap, the present study utilizes data from synoptic weather stations and the Standardized Compound Event Indicator (SCEI) to rigorously evaluate the spatiotemporal patterns of compound hot-dry events over the 1991–2020 period.



Materials and Methods

In pursuit of a rigorous spatiotemporal characterization of compound extreme events across Iran, we curated a dataset comprising daily temperature and precipitation records from 95 synoptic stations, selected for their optimal spatial representativeness. Despite Iran’s vast territory, its meteorological station lacks adequate coverage in key desert and mountainous regions. Nevertheless, the aim of this study is to analyze the data recorded by these existing stations to characterize the climatology and trends of compound hot-dry extremes over the last three decades. The study spans a 30-year climatological from 1991 to 2020. To ensure data integrity, the raw time series underwent comprehensive quality control and homogeneity testing prior to any statistical application. For the detection of compound warm-dry episodes, the 20th and 80th percentiles were established as critical thresholds; consequently, a compound event was defined by the concurrent incidence of heat and dry spells within a single calendar month. The methodological cornerstone of this study is the computation of the Standardized Compound Event Index (SCEI). This dimensionless metric is engineered to synthesize the joint anomalies of precipitation and temperature into a unified probabilistic framework. Within this scale, negative values quantify the intensity of warm-dry conditions exceeding the norm, whereas positive values denote the absence of such compound extremes. Based on standard statistical thresholds, the severity of these events was stratified into five distinct intensity classes: CE0 (abnormal), CE1 (moderate), CE2 (severe), CE3 (extreme), and CE4 (exceptional compound warm-dry events). Beyond the derivation of the index, we assessed the statistical significance of temporal trajectories over the past three decades utilizing the Modified Mann-Kendall test alongside Sen’s Slope estimator.



Results and Discussion

A concurrent analysis of temperature, precipitation, and the Compound Extreme Index reveals that the spatial distribution of hot-dry extremes across Iran is strictly modulated by the region’s topographical configuration. Specifically, the Alborz and Zagros mountain ranges act as orographic barriers, effectively blocking the intrusion of moist air masses from the west and north into the central plateau. These topographical shielding fosters arid and semi-arid climates characterized by intense thermal fluctuations within the interior basins. Consequently, the escalating trajectory of compound hazards observed in central and southern regions is not merely a function of global climate shifts; rather, it emerges from the complex interplay between local physiography and synoptic-scale atmospheric patterns. Dissecting the physical mechanisms governing these trends suggests that the observed variability is the product of intricate coupling between thermodynamic and dynamic variables. Notably, the asymmetric warming pattern where minimum temperatures rise more rapidly than maximums has curtailed the frequency of nocturnal frosts and extended the growing season. This shift inevitably amplifies evaporative demand and accelerates soil moisture depletion, creating a feedback loop that intensifies aridity.

Statistical inquiries into the data elucidate a significant upward trend in the frequency of compound hot-dry extremes across the majority of the country. Processing data from 95 synoptic observation stations uncovers substantial heterogeneity in the spatial distribution of these events. Modified Mann-Kendall tests indicate that 80.20% of the studied stations exhibit an increasing trend, with 26% of these shifts proving statistically significant at the 95% confidence level (p<0.05). Conversely, a mere 14.58% of stations displayed a decreasing tendency, none of which reached statistical significance at the 5% or 1% levels. These findings corroborate global studies reporting a surge in compound extreme events across mid-latitudes and arid zones. However, spatial analysis highlights a marked disparity in event distribution: regions within the Central Plateau, as well as Eastern and Southeastern Iran areas inherently predisposed to arid and desert climates have experienced the most pronounced escalation in compound hot-dry occurrences. In these locales, the strong negative covariance between temperature and precipitation (implying that heatwaves are predominantly dry) enhances the sensitivity of the SCEI (Standardized Compound Extreme Indicator) to anomalies. In contrast, while the Zagros and Alborz mountainous zones and the northern coastal strip generally follow an increasing trend, their patterns of compound extreme occurrences exhibit far greater complexity.

An examination of the SCEI frequency distribution across its five classification tiers reveals a significant inverse relationship between event intensity and spatial extent. The frequency percentage of SCEI classes over the 30-year study period indicates that the Moderate (10.4%) and Abnormal (6.73%) classes collectively account for over 17% of all recorded hot-dry extremes. This implies that for nearly one-fifth of the temporal scope, Iran’s climate possessed a high predisposition for simultaneous positive thermal anomalies and negative precipitation deviations. At the other end of the spectrum, the Severe, Extreme, and Exceptional classes comprise 3.1%, 2.52%, and 1.24% of occurrences, respectively. Although the cumulative frequency of these three high-intensity tiers is less than 7%, the statistical rarity of such tail-end events must not be conflated with negligible impact; their potential for catastrophic disruption remains disproportionately high.



Conclusion

Utilizing the multivariate Standardized Compound Event Indicator (SCEI) on a 30-year dataset from synoptic meteorological stations, this study reveals that Iran is undergoing a climate transition towards a regime where extreme events manifest not as isolated incidents, but as combined phenomena with escalating intensity. The significant upward trend in these occurrences serves as a stark warning to policymakers and natural resource managers. By integrating the interplay between temperature and precipitation, the SCEI provides a potent tool for the early and precise detection of environmental stresses.

The geographical footprint of these compound extremes is expanding, encroaching upon higher latitudes and the mountainous regions of the northwest. This development signals a fundamental shift in the climate patterns. Our findings demonstrate that compound extreme events are no longer transient, random fluctuations; they have consolidated into a structural and recurrent feature of Iran’s climate. To illustrate, severe to exceptional hot-dry compound events now account for approximately 7% of all recorded occurrences nationwide a critical alert for water resource and agricultural management.

This evolving reality suggests that current crisis management strategies, which are predicated on the probability of univariate events, may lack the efficacy to counteract the intensity and persistence of compound phenomena. This vulnerability is particularly acute in regions where the SCEI has registered markedly negative values (classes CE3 and CE4), as the resilience of both ecological and human systems to these compound pressures has been severely compromised.

Ultimately, our trend analysis corroborates this shift: while a mere 15% of stations exhibited a decreasing trend none of which were statistically significant a substantial 26% experienced a significant escalation in compound extreme events. Consequently, it is recommended that future research prioritize the dynamical modeling of interactions between atmospheric circulation patterns and Iran’s complex topography. A parallel focus should be on assessing the influence of climate change on the future intensity and frequency of these phenomena. Such efforts are indispensable for formulating more precise adaptation strategies tailored to the nation’s most vulnerable regions. This study furnishes policymakers and stakeholders with the essential insights needed to devise robust strategies for adapting to and mitigating the destructive consequences of compounded water and heat stress.



Keywords: Compound Extreme Events; Standardized Compound Event Indicator (SCEI); Dry-Hot; Iran.