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

ABSTRACT
This study examines anomalies in 500 hPa atmospheric circulation and their influence on wet and dry periods in Iran during 1995–2024, with particular attention to the relationship between precipitation variability and the ENSO-Modoki index. Rainfall observations from 179 stations were integrated with reanalysis-based geopotential height fields to identify dry and wet periods and to construct composite anomaly patterns. The results indicate that the highest levels of annual precipitation occur in January, February, March, April, and December. A declining precipitation trend is observed in most months, with the most pronounced reductions occurring in December, January, and March, which poses a serious challenge to national water resources. Analysis of monthly precipitation changes reveals substantial interannual variability, including increases exceeding 100 percent (March 1996: +136 percent; December 2004: +143 percent) and decreases greater than 80 percent (January 2021: −87 percent; March 2008: −89 percent). These fluctuations are primarily associated with changes in large-scale atmospheric circulation patterns, particularly the configuration of mid-tropospheric troughs. Analysis of 500 hPa geopotential height anomalies indicates that circulation patterns over the North Pacific, North Atlantic, Europe, and western Russia exhibit distinct configurations during dry and wet periods, whereby negative anomalies are associated with dry conditions and positive anomalies with wet conditions in Iran. A statistically significant positive annual correlation is identified between the ENSO-Modoki index and national precipitation (r ≈ 0.51), accompanied by strong negative correlations between 500 hPa geopotential heights over major oceanic regions and precipitation across Iran. These findings suggest that the integration of emerging teleconnection indices.
Extended Abstract
Introduction
Climate variability and climate change pose significant challenges for societies that are highly dependent on water resources, agricultural systems, and environmentally sensitive ecosystems. Iran, situated within the arid and semi-arid belt of the Middle East, is particularly vulnerable to variability in precipitation patterns. Large areas of the country receive less than 250 mm of annual precipitation, rendering these regions highly sensitive to even minor changes in atmospheric circulation patterns. Over recent decades, both the frequency and intensity of drought events have increased markedly, placing substantial pressure on agricultural productivity, groundwater resources, food security, and socio-economic stability. Accordingly, a comprehensive understanding of the atmospheric mechanisms governing precipitation variability is essential for sustainable water and resource management.
Among the principal indicators of large-scale atmospheric circulation, anomalies in 500 hPa geopotential heights serve as robust diagnostic tools for identifying synoptic-scale patterns associated with wet and dry periods. These anomalies reflect the dynamics of mid-tropospheric troughs, ridges, and jet streams, which play a central role in regulating moisture transport pathways and the spatial distribution of precipitation over Iran. Furthermore, the ENSO–Modoki index has emerged as an important teleconnection indicator influencing precipitation variability across the Middle East, underscoring the need to integrate mid-tropospheric diagnostics with large-scale teleconnection indices for more robust hydroclimatic assessments.
 
Methodology
This study integrates long-term precipitation observations with large-scale atmospheric reanalysis data to examine monthly precipitation variability over Iran during the period 1995–2024. Monthly precipitation data from 179 synoptic stations were compiled, and the spatial mean across all stations was calculated for each month to derive a national precipitation index. Monthly climatological means for the WMO reference period (1991–2020) were calculated and used to derive precipitation anomalies and corresponding percentage changes.
Temporal variability was assessed using simple linear regression to estimate trends, standard errors, and coefficients of determination, while the statistical significance of these 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 respectively as the seven lowest and seven highest values.
Atmospheric circulation patterns were analyzed using monthly 500 hPa geopotential height fields derived from the NCEP/NCAR reanalysis dataset. This pressure level was selected because it represents mid-tropospheric flow patterns that govern the major synoptic systems influencing Iran. Composite anomaly maps were constructed for dry and wet months to assess shifts in the intensity and spatial configuration of large-scale circulation features. In addition, multi-month composites for the October to May period were generated to highlight dominant dynamical signals.
To investigate large-scale teleconnections, correlation analyses were conducted between precipitation, the ENSO–Modoki Index (EMI), and geopotential height anomalies over four key regions identified from spatial patterns of anomalous circulation, namely the North Pacific, North Atlantic, Europe, and western Russia. Together, these analyses provide a coherent framework for understanding the atmospheric drivers that shape precipitation variability across Iran.
 
Results and Discussion
The analysis indicates a strong linkage between mid-tropospheric circulation anomalies and precipitation variability across Iran. During dry periods, subtropical high-pressure systems intensify while Mediterranean troughs weaken, leading to the formation of persistent anticyclonic ridges over the Middle East that inhibit the eastward propagation of mid-latitude cyclones. As a result, large-scale subsidence dominates, convective activity is suppressed, and widespread precipitation deficits occur. In contrast, wet periods are associated with deepened troughs and strengthened westerly flow, which facilitate the intrusion of moisture-bearing systems from the Mediterranean and Red Seas and result in more frequent and intense precipitation events.
The results further indicate that El Niño–Modoki phases, through the development of warm sea surface temperature anomalies in the central Pacific, modify subtropical pressure distributions, deepen the Mediterranean trough, and enhance the propagation of Rossby waves toward Iran, thereby creating favorable conditions for increased precipitation. Conversely, La Niña–Modoki conditions strengthen subtropical ridges and reduce moisture transport toward the region, a pattern that is closely associated with dry periods. Composite analyses reveal that 500 hPa geopotential height anomalies over the North Pacific and North Atlantic function as key centers that modulate Rossby wave patterns, thereby exerting a strong influence on precipitation variability over Iran.
Sub-monthly precipitation fluctuations are largely driven by baroclinic wave activity associated with horizontal temperature gradients, whereas intraseasonal variability is primarily controlled by the modulation of mid-latitude Rossby waves. Teleconnection patterns indicate that circulation anomalies in the North Pacific and Eurasian sectors strongly influence the intensity and eastward extension of the Mediterranean trough, thereby regulating moisture transport toward Iran. Trend analysis further demonstrates significant long-term declines in winter precipitation, particularly during December, January, and March, suggesting an increasing risk of prolonged droughts, reduced snowpack accumulation, and declining groundwater recharge.
 
Conclusion
This study demonstrates that precipitation variability in Iran is closely linked to mid-tropospheric circulation patterns at the 500 hPa level. The prevalence of anticyclonic ridges during dry months and intensified troughs during wet months underscores the central role of mid-tropospheric dynamics in shaping hydroclimatic extremes. ENSO–Modoki teleconnections modulate these circulation patterns by altering subtropical pressure distributions and Rossby wave propagation, with El Niño–Modoki favoring wet conditions and La Niña–Modoki reinforcing dry conditions. Integrated analyses confirm that both 500 hPa geopotential height anomalies and the ENSO–Modoki index play critical roles in explaining and predicting precipitation variability, with mid-tropospheric anomalies acting as intermediaries that transmit Modoki-related influences to the Mediterranean and Iranian regions.
These findings underscore the value of combining reanalysis-based mid-tropospheric diagnostics with large-scale teleconnection indices to improve the skill of seasonal precipitation forecasts. Future studies are encouraged to employ higher-resolution reanalysis datasets and numerical modeling approaches to deepen mechanistic understanding and further enhance predictive capability.
 
Funding
There is no funding support.
 
Authors’ Contribution
Authors contributed equally to the conceptualization and writing of the article. All of the authors approved the content of the manuscript and agreed on all aspects of the work declaration of competing interest none.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.