Document Type : Research Article
Authors
Department of Physical Geography, Shahid Beheshti University, Tehran, Iran
Abstract
Subtropical jet streams are crucial upper-tropospheric synoptic systems that play a significant role in shaping regional precipitation through large-scale moisture transport and dynamic forcing. This study investigates the spatial characteristics, positioning, and elongation patterns of the subtropical jet stream during extremely wet years in northeastern Iran. Daily precipitation data from meteorological stations across North Khorasan and Razavi Khorasan provinces over a 33-year period, covering solar cycles 22 to 24, were used to identify extreme wet years based on the Standardized Precipitation Index (SPI) and DIP software. Upper-air variables, including zonal and meridional wind components, specific humidity, and vertical velocity (omega) at the 1000–500 hPa levels, were retrieved from the NCEP/NCAR and ECMWF reanalysis datasets. The findings indicate that during intense rainfall episodes, the jet stream core frequently shifts northward beyond 33°N latitude, enhancing atmospheric instability and promoting widespread precipitation. The most favorable jet core position is located between western Iran, the southern Caspian Sea, and the Semnan province. Additionally, among the dominant synoptic patterns identified, the Sudan Low was found to be the most frequent and influential system contributing to increased precipitation. These results underscore the dynamic influence of upper-level circulation patterns on the hydroclimate variability of northeastern Iran.
Extended Abstract
Introduction
The jet core is one of the most important factors in dynamic ascent; the position of the subtropical jet stream regulates and transforms atmospheric flows at all levels between the tropics and extratropics. The jet stream, which generally forms in the higher levels of the troposphere, plays a fundamental role in controlling the path of rain-producing systems, the timing of their entry and exit, determining precipitation periods, and influencing seasonal changes due to the intensification of divergence and convergence at its lower levels. Therefore, studying the behavior of the jet stream and determining the dominant synoptic pattern or patterns in extremely wet years is of particular importance for the proper management of water resources and the identification or prediction of possible risks, losses, and damages caused by floods.
Regarding the research topic, studies have been conducted on pressure systems or precipitation-generating systems in northeastern Iran, but no comprehensive research has been conducted on the location of the jet stream's establishment in relation to the creation of wet year conditions in this region, or on the dominant synoptic patterns responsible for creating the above conditions, which we address in this research. The aim of the present study is to determine the best location and elongation pattern of the subtropical jet under wet year conditions and to identify the dominant synoptic patterns in northeastern Iran.
Material and Methods
This research takes an environmental approach to circulation. In this regard, synoptic stations in northeastern Iran were selected for study over a 33-year statistical period (1986–2019), corresponding to the three most recent solar cycles (1986–1997; 1997–2008; 2008–2019). In the next step, precipitation data from the selected stations were obtained from the Meteorological Organization of Iran. Then, by using the SPI index with categories ranging from humid to extremely humid, and using DIP software, severely wet years in the region were determined.
The wet years identified are as follows:
First cycle: 1991–1992 and 1992–1993
Second cycle: 1997–1998
Third cycle: 2018–2019
Next, heavy and extremely heavy rainfall days were identified at the stations in the region. Out of the total rainfall days in northeastern Iran during the statistical interval, days equal to or above the 50th percentile were considered as heavy rainfall days. Atmospheric data for these days—including the zonal wind component, meridional wind component, geopotential height at 250 hPa, sea-level pressure (SLP), specific humidity, and omega—were retrieved from the NCEP/NCAR and ECMWF websites for the 1000, 925, 850, 700, and 500 hPa levels, across the region spanning 0° to 100° east longitude and 0° to 65° north latitude. To identify the most frequent jet stream pattern, a factor analysis, KMO criterion, and Bartlett’s test were conducted using SPSS24 software.
Results and Discussion
In this study, the spatial and temporal location of the zonal jet in northeastern Iran was investigated across three solar cycles (22, 23, and 24) at the 250 hPa level. On all days when heavy rainfall was recorded at the regional stations, the subtropical jet stream was observed at either lower southern latitudes or higher northern latitudes, sometimes extending over the northern half of Iran.
During the most severe wet years, the jet core locations were found within 48° to 60° east longitude and 33° to 40° north latitude—directly over the study area. The jet cores were concentrated and coherent, which had a significant impact on precipitation. The jet axes during these years were short, entirely meridional, and stretched from the Persian Gulf to Central Asia.
In the analysis of synoptic patterns, the Sudan low-pressure system emerged as the most influential in the severe wet years of northeastern Iran. This system was typically located in the lower troposphere, moving in a southwest–northeast direction over eastern and northeastern Iran. Simultaneously, the African High was positioned over the Mediterranean Sea, and cold advection from northern latitudes behind the Sudan system increased the temperature gradient on its western slope.
Moreover, the Tibetan High, extending over the warm Oman and Arabian seas, transported moisture from these sources into the Sudan system and subsequently toward northeastern Iran. At the 700 hPa level and above, a deep trough extending from northwest Iran to the southern Arabian Peninsula placed northeastern Iran on its right flank, enhancing precipitation. At the same time, the Arabian High, positioned over the Arabian Sea, further contributed to moisture transport through circulation ahead of the trough.
Conclusion
The results of this study showed that on the most intense rainy days in the region, the subtropical jet stream was often located above 33°N latitude and contributed to intense and widespread rainfall. The optimal position for the central cores of the subtropical jet, to create wet year conditions in northeastern Iran, lies between the western part of Iran, the southern Caspian Sea, and Semnan Province. Additionally, among the dominant synoptic patterns identified, the Sudan low-pressure system was the most frequent and influential pattern under extremely wet year conditions and had the greatest impact on rainfall in the region.
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