Abstract

Introduction According to the definition of World Meteorology Organization (WMO), as the speed of the air mass is more than 30 meters per second, the jetstream will arise. The core of the jetstream is with baroclinic atmosphere due to the large difference in temperature and wind speed. There are two west jetstreams in the northern hemisphere. The northern jet stream is called polar front jetstream and the southern is called subtropical jetstream. Polar Front Jet Stream produce intense temperature gradient by polar air mass and tropical Polar Front Jet Stream is produced by strong temperature gradient of polar and subtropical air. The subtropical jetstream is produced by temperature gradient in tropopause as limited to the upper troposphere. Subtropical jetstream situation is able to show a seasonal shift, this seasonal displacement of subtropical jet stream can cause the tropical and extra tropical alternative regimes on climate of Iran. The seasonal movement of extra tropical jetstream causes intermittent exposure regimes in tropical and extra tropical. When the jetstream is located over the south of Iran in the cold period of the year, extra tropical climatic factors enter into Iran such as westerliesand cyclones. While jet stream is in northern part of Iran in the warm period of the year, tropical climatic factors will enter Iran. Therefore, recognition of position of this phenomenon is necessary for detecting temporal changes of the other spatial atmospheric phenomena affecting the climate of Iran. On the other hand, possible behaviors in different situations can be recognized by accessing to the jet stream event. Studies have indicated that the mean latitude of the sub-tropical jet stream in both hemispheres have shifted toward the poles over the last few decades; while, the little changes in the jet stream position has huge effects on the distribution of temperature, precipitation and weather patterns. Therefore, it seems necessary to analysize the climate phenomenon, more than ever.   Materials and Methods In this present research, the related data to the zonal (U) and meridional (V) wind components of for a 60 years period (from 20-80 north degree and -10/-120 east degree and in 200 hap) has been derived from National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) in order to investigate the subtropical jet stream’s seasonal trends. To perform seasonal analysis, the data have been separated in seasonal scale by MATLAB. According to the placement of maximum speed in all of the seasons in 200 hap high level, the computation of related process is centralized on these high level data. The computation has been accomplished by liner regression and by the use of Least Square Error (LSE) method. Their result of spatial distribution has been drawn by SURFER software.   Results and Discussion The results of descriptive analysis in this research show that the maximum speed has been occured in winter and the minimum speed in the summer. The altitudinal range of this phenomenon in the winter is more than the summer. The results of review processing in 4 seasons have been exposed in the study area and the significant trend is 10, 2.9, 11, and 10 % for spring, summer, fall and winter, respectively. The interesting point in this result represents the maximum occurrence of significant equinox seasons. However, the researches were mainly conducted on winter season and there isn’t any research about equinox seasons.   Conclusion The results of this study have revealed that subtropical jet stream’s intensity is likely to be reduced in fall and spring in the future century. The results are in a confidence level of 95%. On the other hand, investigation about the zonal and meridional wind component trends of jet stream has displayed in the seasonal changes. The maximum percentage of Eastward changes has occurred in winter than any other season of the year. This has surrounded 7.4 percent of the area. These are occurred in conformity of the jet stream axis in east of Iran, Afghanistan, Pakistan, and India. Northward changes have represented the highest rate in the spring and fall with 10.6, 8.4 percents in the core of jet stream. In summer the trend is significant only in 2.5 percent of the study area. The major changes have occurred in the input and output of jet stream core, and eastward changes has occurred only in this season in winter, northward changes has occurred in 0.7 percent of the area and the eastward changes has occurred in 7.4 percent of the area. کلیدواژگان [English] jet stream, meridional component, trend, zonal component