واکاوی آماری همدید اثر تاوه قطبی بر بارش‌های فرین و فراگیر غرب میانی ایران

Introduction

Many research have been made about of polar vortex (P.V) and its effects on precipitation in the world and in the Iran. Most of them emphasize the direct effect of the polar vortex on winter precipitation. Lashkari (1375) investigated the synoptic patterns of heavy rainfalls in the southwest of the Iran. The intensity, weakness and fluctuations of the subtropical high-pressure system restrain the climate of the mid-latitudes, including the Middle East and Iran. According to the related researches, in the cold season, when subtropical high pressure belt retreats the polar vortex goes forward and causing the transfer of energy and momentum and creates meridian waves. At the beginning of the warm period with the weakening and retreat of the polar vortex, the subtropical high pressure goes forward to north and penetrates to middle East and Iran and arid season is started.

Materials and methods

• Statistics

This paper has focused on the effects of polar vortex on the extreme and widespread precipitation in the Midwest of the Iran for a 30 years period (1986-2015). In the statistical section, after collecting the rainfall data above 1 mm and considering the minimum rainfall of 10 mm , three percentiles of 90, 95 and 98 were selected and based on this, 5 rainfall categories were introduced. By using the Mann-Kendall test, the average of heavy and super heavy rains was obtained, which showed that 50% of the stations had a decreasing and negative trend and 50% of them had an increasing and positive trend. The classification of heavy rains by the Mann-Kendall test also shows that among 18 stations 13 stations (72% of the data) had a positive and upward trend. In addition none of the stations had significant trends in rainfall. In the next stage, the correlation of the polar vortex passing curve from the region (5500 Gpm) with extreme precipitations has been investigated.

The geopotential values of this curve with the amounts of rainfall was tested statistically, and the average rainfall with the transition curve was found to be significant at the level of 0.05 (-0.120), which indicates the decrease of rainfall with the north displacement of the polar vortex curve. On the other hand, in the extreme rainfalls condition in the study area, the passing polar vortex curve over the study region at the 500 Hpa level has suffered a southward drift and geopotential height drop in historic period.

• synoptic

Synoptic patterns of heavy rainfalls at 850 and 500 Hpa levels are classified by using cluster analysis. In clustering level 850 after selection the representative days of each pattern, the synoptic maps in level of 850 Hpa and vorticity, relative humidity, wind vectors in level of 700 Hpa, and finally the maps of jet stream in 250 Hpa level were drawn. In these maps northern, central and southern representative curves of polar vortex, the location of jet stream, isopleths of vorticity and their relationship were drawn. Then, by classifying the data of representative days at the 500 Hpa level for both heavy and super heavy rainfalls, several patterns were obtained and in each map the wind above 30 m/s and the representative curves of vorticity are marked. It should be noted that the created maps are the result of the integration of manual and visual methods.

Conclusion

The results of this research showed that in most of the stations there is an inverse relationship between the criterion rainfalls and polar vortex curves. In fact, with the northward and southward movement of the central polar vortex curve, there are decrease and an increase in rainfall in the study area. According to other studies, there is a direct relationship between heavy and super heavy rainfalls with the curves of polar vortex. In fact by the southward movement of polar vortex curves the rainfall in the region has increased. Among the three curves that are considered for the polar vortex, the main one and the most impact is related to the central or 5500 Gpm curve. The correlation coefficient of the geopotential height of the transition curve and the amount of precipitation of each category is -0.12, which is significant at the 5% level. The mentioned number indicates the decrease of precipitation in the area that is related to the changes in the curve of the P.V at 500 Hpa level.

This precipitation's change can be the effect of climate change which has moved the area of subtropical high pressure to higher latitudes by about 4-5 degrees. These conditions cause the northward displacement of the polar vortex curves and the curve passing through the study region at the level of 500 Hpa and it has caused a decrease in rainfall (non-significant) during the study historical period. In the synoptic section, heavy and super heavy rains occur in relation to the synoptic weather patterns of 850, 700 and 500 Hpa levels were investigated. In the most of the stations and patterns that were obtained, precipitation indicates the presence of low pressure systems from the Arabian, Red and Mediterranean seas as premier cause of rainfall in the study region. By checking the position of the jet stream at the upper level and the moisture advection at the 700 Hpa level, it can be recognized that the most moisture that enters to the west of Iran comes from the Arabian and Red seas. According to these investigations, there is a direct link between temporal relationship of these rainfalls and the activity period of the polar vortex. These systems have entered to the country through displacement of polar vortex and have caused heavy and super heavy rains in the Midwest of Iran.