تحلیل بی‌هنجاری‌های گردش‌های جوی تراز 500 هکتوپاسکال و تأثیر آن بر دوره‌های خشک و تر در ایران (1995-2024)

Extended Abstract

Introduction

Climate variability and change pose critical challenges for societies highly dependent on water resources, agriculture, and sensitive ecosystems. Iran, located within the arid and semi-arid belt of the Middle East, is particularly vulnerable to fluctuations in precipitation. Large parts of the country receive less than 250 mm of rainfall annually, making them highly sensitive to even minor shifts in atmospheric circulation patterns. Over recent decades, the frequency and intensity of droughts have increased markedly, exerting severe pressure on agricultural productivity, groundwater reserves, food security, and socio-economic stability. Understanding the atmospheric mechanisms controlling precipitation variability is therefore essential for sustainable water and resource management.

Among the key indicators of large-scale circulation, anomalies in 500-hPa geopotential heights serve as robust diagnostics for identifying synoptic-scale patterns associated with wet and dry periods. These anomalies capture the dynamics of mid-tropospheric troughs, ridges, and jet streams, which play central roles in regulating moisture transport and precipitation distribution over Iran. Furthermore, the ENSO–Modoki index has emerged as a novel teleconnection metric capable of influencing precipitation patterns across the Middle East, highlighting the need to integrate both mid-tropospheric diagnostics and teleconnection indices for improved hydroclimatic assessment.

Methodology

This study integrates long-term precipitation observations with large-scale atmospheric reanalysis data to examine monthly rainfall variability over Iran during 1995–2024. Monthly precipitation data from 179 synoptic stations were compiled, and for each month, the spatial mean across all stations was calculated as the national precipitation index. Monthly climatological means for the WMO reference period (1991–2020) were computed to derive precipitation anomalies and percentage changes.

Temporal variability was assessed using simple linear regression to estimate trends, standard errors, and coefficients of determination, while the statistical significance of trends was evaluated using the Mann–Kendall test. A ranked series of monthly percentage changes was used to identify the driest and wettest months, defined as the seven lowest and seven highest values, respectively.

Atmospheric circulation patterns were analyzed using monthly 500-hPa geopotential height fields from the NCEP/NCAR reanalysis. This level was chosen because it represents the mid-tropospheric flow that governs major synoptic systems affecting Iran. Composite anomaly maps were constructed for dry and wet months to assess shifts in the intensity and position of large-scale circulation features. Multi-month composites for October–May were also generated to highlight dominant dynamical signals.

To investigate teleconnections, correlations were calculated between precipitation, the ENSO-Modoki Index (EMI), and geopotential height anomalies over four key regions identified from spatial patterns of anomalous activity: the North Pacific, North Atlantic, Europe, and western Russia. These analyses provide a coherent framework for understanding the atmospheric drivers shaping rainfall variability across Iran.

Results and Discussion

Analysis indicates a strong linkage between mid-tropospheric anomalies and precipitation variability across Iran. During dry periods, subtropical high-pressure systems intensify while Mediterranean troughs weaken, producing persistent anticyclonic ridges over the Middle East that inhibit the eastward propagation of mid-latitude cyclones. As a result, subsidence dominates, convection is suppressed, and precipitation deficits occur. In contrast, wet periods are associated with deepened troughs and enhanced westerly flow, facilitating the intrusion of moisture-bearing systems from the Mediterranean and Red Seas, leading to frequent and intense rainfall events.

The results further show that El Niño–Modoki phases, through warm anomalies in the central Pacific, alter subtropical pressure distributions, deepen the Mediterranean trough, and enhance the propagation of Rossby waves toward Iran, creating favorable conditions for increased rainfall. Conversely, La Niña–Modoki strengthens subtropical ridges and reduces moisture transport, correlating with dry periods. Composite analyses reveal that 500-hPa geopotential anomalies over the North Pacific and North Atlantic act as key centers modulating Rossby wave patterns, thereby influencing precipitation variability in Iran.

Sub-monthly precipitation fluctuations are largely driven by baroclinic wave activity associated with horizontal temperature gradients, while intraseasonal variability is controlled by mid-latitude Rossby wave modulation. Teleconnection patterns indicate that anomalies in the North Pacific and Eurasian sectors strongly affect the Mediterranean trough’s intensity and eastward extension, thereby regulating moisture inflow toward Iran. Trend analysis further demonstrates significant long-term declines in winter precipitation, particularly in December, January, and March, suggesting increasing risks of prolonged droughts, reduced snowpack, and declining groundwater recharge.

Conclusion

This study highlights that precipitation variability in Iran is tightly linked to mid-tropospheric circulation at the 500-hPa level. The prevalence of anticyclonic ridges during dry months and intensified troughs during wet months emphasizes the central role of mid-tropospheric dynamics in shaping hydroclimatic extremes. ENSO–Modoki teleconnections modulate these patterns by altering subtropical pressures and Rossby wave propagation, with El Niño–Modoki promoting wet conditions and La Niña–Modoki enhancing dryness. Integrated analyses confirm that both 500-hPa geopotential anomalies and the ENSO–Modoki index are critical in explaining and predicting rainfall variability, with mid-tropospheric anomalies acting as intermediaries that transmit Modoki effects to the Mediterranean and Iranian regions.

The findings underscore the utility of combining reanalysis-based mid-tropospheric diagnostics with teleconnection indices to improve seasonal precipitation forecasts. Utilizing higher-resolution reanalysis datasets and numerical modeling in future studies is recommended to deepen mechanistic understanding and enhance predictive skill. These insights are crucial for strengthening climate monitoring, informing adaptive water resource management, and supporting proactive planning to mitigate hydroclimatic risks in Iran’s vulnerable arid and semi-arid regions.

Keywords: Atmospheric circulation anomalies; Precipitation variability; dry–wetness episodes; Iran