Journal of Climate Research

Journal of Climate Research

"Interaction Between Surface Phenomena (ENSO) and Upper-Level Atmospheric Circulation (Jet Stream) in the Occurrence of Monthly Wet/Dry Periods Across Iran's Rainfall Regions"

Document Type : Original Article

Authors
Bahram Asefi 1
Ghasem Azizi 2
Mostafa Karimi 3
Masoumeh Moghbel 4
Faramarz Khoshakhlagh 5
1 PhD Student of Climatology, Department of Physical Geography, Faculty of Geography, University of Tehran, Iran.
2 2 Professor of Climatology, Department of Physical Geography, Faculty of Geography, University of Tehran, Iran.
3 Associate Professor of Climatology Department of Physical Geography Faculty of Geography, University of Tehran Vesal Shirazi Street, Enghelab Ave, Tehran, Iran
4 Associate Professor of Climatology, Department of Physical Geography, Faculty of Geography, University of Tehran, Iran.
5 5Associate Professor of Climatology, Department of Physical Geography, Faculty of Geography, University of Tehran, Iran
10.22034/jcr.2025.534094.1707
Abstract
Introduction

Numerous studies have shown that El Niño induces large-scale Rossby waves by altering the temperature and pressure distribution over the Pacific Ocean. These waves influence the position, intensity, and trajectory of the subtropical and polar jet streams. As the subtropical jet stream serves as a key pathway for Mediterranean precipitation systems, El Niño events may cause it to shift, intensify, weaken, or change its latitude. These alterations can impact the flow of moisture-laden low-pressure systems into Iran, sometimes diverting them northward or southward, or altering their intensity.

Understanding these interactions is crucial for seasonal forecasting and assessing climate impacts, as jet stream behavior during ENSO events plays significant role in shaping regional and global weather patterns. In domestic studies, the relationship between El Niño and the jet stream has been relatively understudied. This research identifies precipitation zones in Iran and examines the monthly correlation between each zone and ENSO teleconnections. High- and low-precipitation El Niño events were then selected for further synoptic analysis.



Materials and methods

SOI data, calculated and archived monthly by NOAA, were used to identify El Niño phases (negative SOI values). The monthly SOI data from 1987 to 2018 were paired with daily precipitation data from 98 synoptic stations across Iran. The precipitation data were aggregated monthly and averaged over 31 years.

Given the significance of altitude and latitude in precipitation patterns, data were weighted accordingly. Using SPSS software, the stations were spatially clustered into 10 zones via Ward’s hierarchical clustering method and squared Euclidean distance.

Next, the correlation between monthly SOI values and precipitation for each cluster was calculated, excluding the dry summer months (June, July, August). As some El Niño years showed increased precipitation and others showed decreases, the highest and lowest precipitation events per month were selected based on spatial extent and amount. Synoptic maps were plotted for each case, and key atmospheric variables including 500 hPa geopotential height, omega, temperature, specific humidity, sea-level pressure, and jet stream patterns were analyzed for the Middle East and Pacific regions.



Results and Discussion

In Iran, in terms of time, October, November and December, and in terms of space, the Alborz and Zagros mountain ranges and the central plateau in the areas adjacent to these mountain ranges have the highest rainfall correlation with the El Niño activity period. This correlation is lower in the southern and northern coasts of the Iranian plateau, northern Azerbaijan, Khuzestan, the Lut plain and the southeastern regions. In conditions of strong El Niño in Iran, the Pacific low pressure is stronger than in the month prior for the rainy months of October and December, and the high pressure adjacent to Chile is weaker, especially in December. In high-rainfall rainfall, the jet stream at 250 hpa is stronger over North Africa and Eastern Northt America and weaker in East Asia. Also, the 500 hp trough is located over the eastern Mediterranean instead of Central Asia and Central Europe, and the subtropical high has moved south. At the 850 to 500 hpa, moisture supply from the Red Sea and Africa is evident, and sea level pressure in Iran is lower, the Siberian high pressure is weaker, and its influence extends towards Europe and more northern latitudes instead of Central Asia and Iran.

Conclusion

The relationship between El Niño, the jet stream, and precipitation in Iran is complex, inconsistent, and often weak. Unlike other regions such as coastal South America or the southern U.S. where El Niño has a clear and predictable impact, Iran shows limited spatial and temporal consistency. This complexity is partly attributed to:

Iran’s geographic distance from the ENSO source region,

The strong influence of other teleconnection patterns like the North Atlantic Oscillation (NAO) and the Indian Ocean Dipole (IOD),

Local factors such as mountainous topography and proximity to the Caspian Sea.

These regional and global influences often override or modify the ENSO signal, making its impact on Iran more nuanced and less predictable. The main objective of this research was to identify and comprehensively analyze synoptic-dynamic atmospheric patterns affecting autumn precipitation (October, November, and December) in Iran and to distinguish the atmospheric mechanisms governing high and low precipitation conditions. This research, using a multi-scale approach, examined atmospheric systems from sea level to the upper atmosphere (250 hpa) and analyzed the interaction between different systems, including the Siberian high pressure, the zonal high pressure, the jet stream, the Mediterranean troughs, the Sudanese low pressure, and moisture sources. Finally, the development of an applicable seasonal forecasting system based on the identified patterns that can be practically used practically in water resources and agricultural management is one of the key suggestions for future studies.
Keywords
Southern Oscillation Index
Wet Months
Dry Months
Southwest Asia
Iran
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Journal of Climate Research
Volume 1404, Issue 63 - Serial Number 63
Vol. 16 , No. 63, Autumn 2025
Spring 2026
Pages 1-22