نوع مقاله : مقاله پژوهشی
نویسندگان
1 دانشجوی دکتری آب و هواشناسی ، دانشگاه تهران، تهران ، ایران
2 استاد آب و هواشناسی، دانشکده جغرافیا، دانشگاه تهران، ایران
3 دانشجوی دکتری آب و هواشناسی ، دانشگاه تربیت مدرس، تهران ، ایران
چکیده
کلیدواژه‌ها
عنوان مقاله [English]
نویسندگان [English]
Introduction
Identifying the patterns of the teleconnection and analyzing their effects on the dynamic components and circulation patterns of the atmosphere can be useful for better understanding and knowledge of climate systems and consequently their prediction (Mueller and Ambrizi, 2007). The Arctic Oscillation (AO) and the East Atlantic–West Russia teleconnection (EA–WR)are among the most important teleconnection in the Northern Hemisphere. The Arctic Oscillation is actually the anomaly of atmospheric pressure at the sea level in the north polar latitudes (55°N) and middle latitudes (45°N latitude), which was first discovered by two scientists named Thomson and Wallace in 1998 It was proposed as the main reason for the variability of the Subtropical regions of the Northern Hemisphere. In the positive phase of this index, the polar convection is stronger than normal and the jet at the top of the atmosphere is strengthened and moves in the form of a ring of strong westerly winds from around the North Pole, and this causes colder air to be limited in the polar regions. As a result, in this phase, the western winds are strengthened in the North Atlantic area and create warmer and more humid conditions than usual in Northern Europe. In the negative phase, the polar vortex ring and the upper level jet of the atmosphere take a meridional pattern and the western winds move towards lower latitudes (middle latitudes). Also, in this phase, the western winds are weakened in the northern part of the Atlantic and can penetrate to the areas located at low latitudes including southern Europe, the Mediterranean and the Middle East.East Atlantic-West Russia teleconnection EA-WR is obtained from the standardized pressure difference or geopotential height of East Atlantic Ocean from West Russia.This teleconnection was presented for the first time by Branston and Livesey (1987) using the method of principal components analysis in the form of a pattern of monthly changes in geopotential height of 700 hectopascals. This pattern includes four main centers of negative geopotential height anomaly along the orbits from the Atlantic Ocean to China. In the negative phase, this is reversed. In the negative (positive) phase, climate conditions are often drier (more humid) than the average situation over a large part of the Mediterranean region (Krichak et al., 2002).
Materials and methods
To investigate the mechanism of the Arctic Oscillation (AO) and the East Atlantic/West Russia Oscillation (EA-WR) in relation to the atmospheric circulation of the Middle East and Iran in two seasons: autumn (September, October, November) and winter (December, January, February), National Center for Environmental Prediction/Atmospheric Sciences (NCEP/NCAR) gridded data were used. The reason for using data with a spatial resolution of 2.5*2.5 degrees is that they are suitable for studying low-frequency phenomena on a planetary scale (Mohab al-Hajjah et al., 2015). Based on the purpose of the research, the data of geopotential height, sea level pressure, orbital component of wind and amount of precipitation for two seasons of winter (December, January, February) and autumn (September, October, November) were obtained between 1948-2020. Due to the importance of the level conditions of 200 and 500 hectopascals on the dynamic components of lower levels (Standel et al., 2021), these atmospheric levels were selected for investigation and analysis. After receiving the mentioned data and extracting the time series of these data, the map of the desired dynamic components was drawn and analyzed.Then the correlation maps between the mentioned teleconnection with parameters of geopotential height of 500 hectopascals, orbital wind component of 200 hectopascals, sea level pressure and precipitation amount In order to determine the long-term relationship of these links with the dynamic components - synoptic and precipitation of the region in the cold period of the year, it was drawn.
Results and Discussion
The pattern of correlation values between the meridinal wind speed level of 200 hPa and the AO teleconnection in the autumn season showed that a Rayleigh-shaped positive correlation line was drawn in the direction of the meridinals from the United States of America, the Atlantic Ocean, North Africa to the east of Iran.Both teleconnection have a strong positive correlation with the meridinal wind speed of 200 hectopascals in autumn and winter, Which means that, with the increase in the value of each of them in the autumn season, along with the increase in the meridinal wind speed, the tropical jet stream first has an meridinal - wave pattern in the direction of the Atlantic Ocean, north and East Africa, the Red Sea, and then it is formed by changing the shape with a meridian pattern from Iran to the west of Russia. Almost along the same path, there is a negative correlation between the mentioned teleconnections and the geopotential height of the middle level of the atmosphere in the two seasons of autumn and winter. By increasing the value of each of the mentioned links, the value of the geopotential height of the middle level decreases from the northwest side of Iran to the north of the Red Sea and the east of the Mediterranean, which indicates the penetration of cold polar air through the west of Russia to the areas located throughout the western half. and the northwest of Iran.
Conclusion
The results showed that the fluctuations of both teleconnections have a significant effect on the climate of the Middle East, especially Iran. In autumn and winter season, there is a positive correlation between the above-mentioned teleconnections with the meridinal wind speed of 200 hectopascals and a negative correlation with the geopotential height of the middle atmosphere in Iran and the Middle East. Therefore, the increase in the value of each of these teleconnections is associated with an increase in the meridinal wind speed at the upper level of the atmosphere and a decrease in the geopotential height of the middle level of the atmosphere in autumn and winter seasons. The East Atlas-West Russia teleconnections affects a wider area of Iran in autumn and winter.
Keywords Geopotential height, Iran, Anomaly, Correlation ،Meridinal Wind
کلیدواژه‌ها [English]
Yarahmadi, Azizi, Ghasem (2008). Multivariate Analysis of Relationship between Seasonal Rainfalls in Iran with Climate Indices. Georaphical Research Quarterly.
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