Ms. Aida Faroghi, Professor Manuchehr Farajzadeh, ,
Volume 25, Issue 78 (9-2025)
In this study, the frequency of merging events between the polar-front jet stream and the subtropical jet stream, along with their impact on precipitation patterns in western Iran, was analyzed over a ten-year statistical period (2010–2019). Utilizing coding in GrADS, 300 hPa jet stream maps were produced at six-hour intervals. Throughout the study period, the axes of these two jet streams merged on several occasions.
An examination of the frequency of merging indicated that, prior to 2015, the frequency of merging in December exhibited an increasing trend. However, this trend diminished in 2016 and 2017, only to experience a resurgence in 2018 and 2019. It is noteworthy that not all instances of jet stream merging resulted in significant precipitation events (e.g., December 2011, 2014, and 2017). For instance, in light of the substantial rainfall of 110 mm recorded at the Dehloran station, the period from December 12 to 15, 2010, was selected for detailed analysis to elucidate the atmospheric mechanisms responsible for the rainfall. From December 12 to 15, 2010, a decline in air temperature over Europe and Southwest Asia prompted a considerable meridional displacement of the polar-front jet stream, resulting in its merger with the subtropical jet stream. On December 12, 2010, as the polar-front jet stream underwent meridional movement and extended into tropical regions, its velocity core merged with that of the subtropical jet stream over the northern Arabian Peninsula, the Red Sea, and northeastern Africa. The convergence of these two jet streams led to a vertical expansion of the jet stream into lower atmospheric levels. At the mid-levels of the atmosphere, minimal meridional movement was observed. As a result, the Sudan low-pressure system migrated to higher latitudes, merging with the Mediterranean low-pressure system.
