The Variation Characteristics Of The East Asian Subtropical Westerly Jet Stream In Spring And Its Relationship With Dust Weather In China

As an important circulation factor, subtropical upper westerly jet stream play an important role during the evolution of weather system in the Northern Hemisphere, the change of its location and intensity would affect the occurrence frequency of dust storms. In recent years, with the depth study of sandstorms, the role of high-altitude jet stream becomes important gradually. In this paper, the relationship among the evolution of the subtropical westerly jet’s characteristics in spring in China, the jet in sandy area of the north of China, the near-surface wind over the same period, and its impact on the occurrence frequency of the sand and dust storms in north of China have been analyzed, which provide a way to understand the cause of spring sandstorms in north of china from the wind dynamics angle.Firstly, in this paper, the ERA-Interim Global Atmospheric reanalysis data provided by ECMWF (European Centre for Medium-Range Weather Forecasts) have been used to analyze the changes of the characteristic parameters of the subtropical westerly jet in China. Studies have showed that regular changes in the latitude of the subtropical westerly jet axis, the longitude of jet center and the strength of the subtropical jet, and the latitude of the subtropical jet axis experienced three stages of dramatic change during a year, all these are the stage of stable location, the stage of moving northward and the stage of moving southward, the jet center located in the western Pacific in winter. After that, with the heat source effect of the plateau, the jet core moves westward rapidly, and reaches the most western position of the year in early August, then with the weakening of the solar radiation intensity, the jet center moves to the average position in winter. The relative strength is cosine distribution during the year, which reaches maximum in early February, and the smallest in late July. The subtropical westerly jet axis in China moves northward in winter, spring and summer, and it remains stable in autumn, meanwhile the jet center is in the east longitude, in addition to some individual years, it moves eastward in the spring, while in summer and autumn, it moves westward obviously. The relative strength of the jet becomes strengthen in the winter and spring, but in summer it becomes weaken, and it remains unchanged in fall.Secondly, by calculate the frequency of sandstorms in China, monthly average wind speed of each site in recent decades, and the relative strength of subtropical westerly jet stream over China. Significant correlation among them is found:on the whole, there is a significant negative correlation between China’s spring sandstorm days of all sites and the relative strength of the subtropical westerly jet over China, negative correlation is also showed between wind speed near the ground in the sand dust source area and the relative strength of subtropical westerly jet stream over China. For sites, negative correlation is showed between the number of the sandstorm of sites in most areas of Northwest China and the relative strength of subtropical westerly jet stream over China, negative correlation is also showed between the monthly mean wind speed of sites and the relative strength of subtropical westerly jet stream over China in most parts of Northern China. Therefore, the relationship between the subtropical westerly jet stream over China and occurrence frequency of sandstorm can be summarized as:when the relative strength of subtropical westerly jet stream over China enhanced (weakened), wind speed of dust source location in the north of jet is weakened (enhanced), which led to wind speed decreases (increases) in north area where there are rich dust. In the end, western and southern Inner-Mongolia and Alashan where have higher occurrence frequency of sandstorm decreased (increased).Finally, with the method of synthetic analysis, the difference of potential height and wind in middle and low-altitude of the subtropical westerly jet stream in spring between the multiple years that sandstorm happened and the less years have been compared. It showed that sandstorms happened in spring in the multiple years, during this period the East Asian Subtropical Westerly Jet is weaker, but in the sources location of dust such as most parts of the eastern Kazakhstan, Mongolia, Inner Mongolia and southern Xinjiang region, the zonal wind speed increases at 200hPa; the distribution of the difference of wind speed and geo-potential height at 500 and 850hPa is conducive to increase the ground wind speed in the source location of dust. In the source location of dust, westerly wind speed of high-level increased, the momentum was transferred to the low-level, which caused the gale on the ground, thus easy to produce dust storms.In summary, the East Asian Subtropical Westerly Jet affect the intensity of westerly wind over the sandy area which affect the near surface wind indirectly, leading to change in the frequency of sandstorms in north of China. This reveals another mechanism of remote correlation which causes the change of frequency of sandstorms in north of China.