Document Type : Research Article
Authors
Department of Physical Geography, Shahid Beheshti University, Tehran, Iran
10.22067/geoeh.2025.91689.1542
Abstract
Abstract
Jets are one of the synoptic systems that affect the climate of a region. By examining and identifying their interactions in the upper levels of the troposphere and determining their location, can be identified weather events, as well as positive and negative effects. The aim of the present study is to determine the best location and elongation pattern of the subtropical jet in wet year conditions and to identify the dominant synoptic patterns in the northeastern of Iran. Accordingly, daily precipitation data from selected stations in the northeastern of Iran (North Khorasan and Razavi provinces) in a 33-year statistical period corresponding to three solar cycles (22, 23, and 24) were extracted from the Iranian Meteorological Organization. Based on, the annual average precipitation and SPI index, and using DIP software, severe wet year were extracted, in which at least half of the stations were involved in wet year. After identifying severe wet years, atmospheric data including orbital and meridional wind speed component data, specific humidity, and omega levels of 1000 to 500 hpa were recieved from the NCEP/NCAR and ECMWF websites. Then, using factor analysis and repeated visual inspections, the most frequent patterns were selected for synoptic analysis. The results of the study showed that the subtropical jet streams on the most intense rainfall days were located in some cases above 33 degree latitude, and in other cases, they passed through lower latitudes, causing heavy and widespread rainfall in the region. In examining the location of the subtropical jet streams to create suitable dynamic conditions and severe instabilities in the region, the best location of the jet stream was observed in the areas of Kuwait, the Persian Gulf, and Bushehr and Fars provinces. Also, in examining the dominant synoptic patterns identified in the region, the Sudan low-pressure pattern was observed in more cases, and this system had a greater impact on the region's precipitation.
Keywords: Subtropical Jet, location, synoptic, wet year, the northeast
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 time of their entry and exit, determining precipitation periods, and seasonal changes due to the intensification of divergence and convergence of its lower levels. Therefore, studying the behavior of the jet stream and determining the dominant synoptic pattern or patterns in severe wet year is 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, research has been conducted on pressure systems or precipitation-generating systems in the northeastern of Iran, but no comprehensive research has been conducted on the location of the jet stream's establishment on the creation of wet year conditions in this region, as well as the dominant synoptic patterns in creating the above conditions, which we will address in this research. The aim of the present study is to determine the best location and elongation pattern of the subtropical jet in wet year conditions and to identify the dominant synoptic patterns in the northeastern of Iran.
Methodology
This research has an environmental approach to circulation. In this regard, first, synoptic stations in the northeastern of Iran were selected for study over a 33-year statistical period (1986-2019) corresponding to the three recent solar cycles (1986-1997; 1997-2008; 2008-2019). In the next step, precipitation data from selected stations was received from the Meteorological Organization of Iran. Then, by using the SPI index with a degree of almost (humid) to (extremely humid) and using the DIP software was determined severe wet year in the region. The wet years are in the first cycle: 1991-1992 and 1992-1993; the wet years of the second cycle: 1997-1998 and the wet years of the third cycle: 2018-2019. In the next step, heavy and extremely heavy rainfalls were identified at the stations in the region. Accordingly, out of the total rainfall days in the northeastern of Iran in the statistical interval, days equal to or above the 50th percentile were considered as days with heavy rainfall. Then, the atmospheric data of these days, including the orbital wind component, meridional wind component, geopotential height at the level of 250 hpa, slp, specific humidity, and omega, were received from the websites of NCEP/NCAR and ECMWF for levels of 1000, 925, 850, 700, and 500 hpa in the range of longitude 0 to 100 degrees east and latitude 0 to 65 degrees north. To identify the most frequent jet stream pattern, factor analysis method, KMO criterion, and Bartlett test were performed in SPSS24 software.
Discussion
In the present study, the spatial and temporal location of the zonal jet in the northeastern region of Iran was investigated in three solar cycles (22, 23, and 24) and at a level of 250 hpa. On all days when heavy rainfall was recorded at the stations in the region, in some cases the subtropical jet streams were located at lower and southern latitudes, and in other cases they were located at higher latitudes and over the northern half of Iran. The location of the jet cores in the most severe wet years was in the range of 48° to 60° east longitude and 33° to 40° north latitude and completely over the study area. The cores were concentrated and not scattered, which had a great impact on precipitation. The jet axes in these wet years were short and completely meridional and extended from the Persian Gulf to Central Asia. In the study and analysis of synoptic patterns, among the synoptic patterns identified in the severe wet years of the northeastern Iran, the Sudan low-pressure pattern had a greater impact on the rainfall in the region. The structure of the systems in this pattern was such as that the Sudan low-pressure system was located in the lower levels of the troposphere, moving in the southwest-northeast direction, over the east and northeast of Iran; and at the same time, the Africa High was located over the Mediterranean Sea and, while the cold advection of northern latitudes behind the Sudan system, increased the temperature gradient in the western slope of the Sudan low-pressure system. On the other hand, the Tibetan High, by expanding over the warm Oman and Arabian seas, advects the moisture of these seas into the Sudan system and then to northeastern Iran. At the level of 700 hpa and above, the existence of a deep trough from northwest Iran to the south of the Arabian Peninsula has caused northeastern Iran to be located on the right arm of the trough; at the same time, the Arabia High is established over the Arabian Sea and, with the circulation, the moisture of this sea is brought to the front of the trough;
Conclusion
The results of the study showed that the subtropical jet streams on the most intense rainy days in the region were often located above 33 degrees latitude and caused intense and widespread rainfall in the region. The best location for the central cores of the subtropical jet, for createing wet year conditions in the northeastern of Iran, is the western part of Iran to the south of the Caspian Sea and Semnan Province. Also, in examining the dominant synoptic patterns identified in the region, the Sudan low-pressure pattern was the most frequent pattern identified in intense wet year conditions and had a greater impact on the rainfall in the northeast.
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