Iranian Journal of Geophysics

Iranian Journal of Geophysics

Dynamical analysis of rossby wave propagation, convergence, and jet stream variability during widespread winter dry periods in Iran

Articles in Press

Document Type : Research Article

Authors
Abbas Ranjbar SaadatAbadi 1
Faranak Bahrami 2
Ebrahim Fatahi 3
1 Professor, Atmospheric Hazard Early Warning Group, Research Institute of Meteorology and Atmospheric Science (RIMAS), Tehran, Iran
2 Ph.D. in Meteorology, Atmospheric Hazard Early Warning Group, Research Institute of Meteorology and Atmospheric Science (RIMAS), Tehran, Iran
3 Professor, Transportation Meteorology Research Group, Research Institute of Meteorology and Atmospheric Science(RIMAS), Tehran, Iran
10.30499/ijg.2026.574873.1753
Abstract
Understanding the dynamical mechanisms responsible for widespread winter droughts in Iran is essential for improving seasonal precipitation predictability and supporting effective drought risk management. Located downstream of the North Atlantic–Mediterranean wave corridor, Iran’s winter precipitation is highly sensitive to variations in Rossby wave propagation and upper-tropospheric jet stream dynamics. This study investigates how disruptions in Rossby wave energy transmission and changes in jet stream configuration contribute to the onset and persistence of widespread winter dry conditions across Iran.
    Rossby wave pathways, energy convergence, and intensity were examined using the wave activity flux (WAF) formulation of Takaya and Nakamura (2001). The spatial structure, intensity, and latitudinal displacement of the 250-hPa jet stream were analyzed to assess its role in modulating wave–mean flow interaction and large-scale vertical motion. The analysis focuses on December, January, and February.
    Observed monthly precipitation data from 179 stations across Iran were used to identify extreme dry conditions. For each winter month, four representative dry episodes were selected based on standardized precipitation Z-scores, enabling consistent comparison of Rossby wave behavior and circulation patterns. WAF analysis reveals that during all selected dry periods, downstream Rossby wave energy transfer from the North Atlantic and Mediterranean toward Iran was substantially disrupted. Two dominant mechanisms were identified. First, Rossby wave packets weakened and dissipated along their eastward path, reducing amplitude and dynamical influence before reaching West Asia. Second, over Iran, WAF vectors were weak, incoherent, and non-convergent, indicating insufficient wave energy accumulation to induce upper-level divergence, ascent, and persistent precipitation.
    In several cases, despite strong cyclonic activity and an intensified Rossby wave train over the North Atlantic, much of the energy converged over northern and western Europe. This diverted the main energy pathway poleward, positioning Iran downstream in a dynamically unfavorable “wave passage” regime rather than in active entrance or exit regions. An exception occurred in February 2016 when the eastern Mediterranean briefly acted as a secondary wave source; however, the dominant energy was directed south toward the northern Red Sea, leaving Iran decoupled from effective wave forcing.
Analysis of the 250-hPa jet stream shows a common pattern: a poleward displacement of the polar jet over the Atlantic–Europe and strengthening with meridional expansion of the subtropical jet over the Middle East and Iran. This configuration suppresses meridional variability, weakens Rossby wave amplification, limits constructive wave–mean flow interaction, reinforces upper-level stability, reduces vertical motion, and favors persistent winter droughts.
     Overall, widespread winter droughts in Iran primarily reflect a misalignment with Rossby wave energy pathways and jet stream structures. Combined WAF and jet stream analysis provides a physically consistent framework for diagnosing and anticipating winter droughts, supporting improved seasonal forecasts and climate risk management in water-limited regions.
Keywords
Rossby waves
jet stream
winter dry periods
Rossby wave activity flux

Subjects

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Iranian Journal of Geophysics

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Available Online from 05 April 2026