عنوان مقاله [English]
One of the main elements of the general circulation in mid latitudes are the fast and narrow flow maxmia called jet streams whose speed is usually more than 30 meters per second. They are one of the dominant features of upper level weather maps but changing through time, space and layers of atmosphere. The jetstreams cause vertical motion underneath through which produce stability and instability over the earth surface. It should be mentioned that they are much known for their instability production. Jet cores are one of the main components of the general circulation and their location and displacement are controlled by the elements of the circulation such as the Arctic Oscillation. According to the researches, the location of the jet streams is very important in climate events. Therefore, this research tried to identify and present the location and speed of jet streams in Middle East. There was no comprehensive study in this scale so far.
Materials and methods
In order to study the jet streams, the six hourly (00, 06, 12 and 18 GMT) speed of U and V components of the winds at the 700, 600, 500, 400, and 300 hPa levels were obtained for the window of 0 to 120 E and 0 to 80 N during 1965-2014 period. In total, 20 time series were produced from the combination of these hour and level scales for each pixel with the size of 2.5 by 2.5 degrees. In each time series the wind maxima of 30 m/s and higher were extracted. At the final stage, the mean monthly speed and monthly frequency of jet streams of all pixels were mapped for the study area.
Results and discussion
The frequency and speed of jet cores were mapped and are described here in monthly, seasonal and annual scales. According to annual frequency of jet core locations, the highest speed maxima in 300 hPa level are located over the North Africa in 40 percent of times, depicting the main track of the jet cores. Given the fact that jet cores enter the study region only in the cold period of year, this temporal frequency of 40 percent is not a low value. The location of speed maxima is the same in 500 hPa level but in about 20 percent of the time. This low value is reasonable for this level. As we know, the synoptic systems of this level control most of the time the weather and climate of the surface and these surface systems are not very frequent during a normal year. In this level, the bifurcation of westerlies is obvious. The southern branch is very influential in the Middle East. This bifurcation indicates the presence of a blocking high in the region, which most of the times prevent the entering of jet cores and hence an unstable conditions over the region. There are no speed maxima in the region in 700 hpa level. Since in the annual scale only in the 500 and 300 hPa levels showed the jet cores, in the seasonal scale we look only for these levels. The frequency of jet cores in winter is more than the other seasons and show two separate belts. The jet core affecting the climate of Iran is passing over the Persian Gulf which extends from North Africa towards China. Jet cores pass through this belt all of the winter season. However, their frequency decreases toward the north over Iran and from there extends toward the other maxima over the latitude 50 N to 60 N. The jet frequencies have decreased in spring reaching to about 52 percent over the area (Figure 3B). This rate of decrease indicates the sudden and rapid change from winter to the spring conditions over Iran. Its northward shift is also obvious over Iran. In this season, the jet maxima travel in a ridge like track over Iran. It indicates an unstable weather over the west part of Iran while dominating the stable conditions over the east part of the country.
Jet streams are very important instability factors in the atmosphere. Their spatial location and speeds control the tracks of pressure systems and surface climate. For this reason, this study tried to understand their speed and spatial variations in the Middle East. Their speed and frequencies were studied at the pressure levels from 700 hPa to 300 hPa in the monthly, seasonal and annual scales. The result showed that in all levels and scales, two speed maxima tracks were established over the latitude belts of 20N – 30N and 50N – 60N which is confirmed by findings of Li et al (2004); Zhang et al ( 2006); Li and Wettstein( 2012) and Pena-Ortiz et al (2013). The most important findings of the research is the coincidence of speed maxima with frequency maxima of jet cores in these latitude belts. During the warm period, both the frequency and speed of the jet cores decreased over the Middle East. The jet cores were absent at the levels lower than 500 hPa, and in 300 hPa all winds experienced speeds higher than jet threshold which is in accordance with Geer et al (1996). In brief, we can conclude that through the fall season the jet cores move southward to dominate over the Middle East and Iran during winter. Through the spring season, they began backward movement to the northern latitudes so that in May no jet core could be found in the area. This research demonstrated the importance of the wind patterns of 500 hPa on the climate and weather conditions of the Middle East as well as Iran.