عنوان مقاله [English]
The 120-day winds of Sistan are considered as the most important and well-known climatic factors in eastern regions of Iran during hot period. They have various effects on the region. For example, these winds make dust storm, more evapotranspiration and sand prairie in this region. Generally, as these winds have great impacts on the environment and human life, they should be studied from different climatic aspects. Given the importance of the winds, this study aimed at evaluating the role of these winds in declining the temperature of the region. The results will prove one of the positive environmental aspects of these winds during hot period of year in eastern dessert regions of Iran. However most effects of these winds were negative and considered as one of the life limiting factors in the east Iran.
Materials and methods
The period used in this study is the 2480 days in 22 years (2012-1993) from May until end of September. The atmospheric circulation types have been extracted using daily mean of the 850 hPa geopotential height data. Then, the agglomerative hierarchical cluster analysis with the ward algorithm and Euclidean distance has been used to identify atmospheric circulation types over Iran in the mentioned period of years. Finally, 5 atmospheric circulation types have been identified in this period of years. We, then, have analyzed wind speed and direction, the wind thermal advection in levels of 1000, 925, 850 and 700 hPa, and also the thermal advection of atmospheric vertical profiles.
Results and discussion
The Position of Maximal Cores of Temperature (Thermal Equator)
As a matter of fact, maximal cores of temperature imply the position of earth thermal equator. Pattern 1, in which 120- day winds of Sistan cover east and southeast parts of Iran more intensely and more widely, reveals that maximal temperature covers Iraq and west part of Iran, while in the same latitude there is cool weather in east part of Iran. Pattern 2 in which the 120-days winds of Sistan have less intensity and expansion shows that thermal equator belt of east is penetrating into northern latitudes, even though lower temperature is still recorded in east and southeast Iran. This is compared with west parts of Iran and Iraq. Generally, these 2 synoptic patterns reveal that 120-day winds of Sistan with northern direction lead to a decrease in the temperature of east and southeast Iran. Besides it makes thermal equator belt move to southern latitudes. There are the patterns 3, 4, and 5. In these patterns, 120-day Sistan winds are not dominant in the area. This leads to an increase in the temperature of the area and a core formation of maximal temperature in east and southeast Iran. Unlike patterns 1 and 2, in these patterns eastern regions of Iran have higher temperature than Mesopotamia and west Iran. As a result, the advection of eastern winds in the region makes thermal equator penetrate northern altitudes as it covers east and southeast Iran. Due to this phenomenon, eastern regions record much higher temperature than the regions in Mesopotamia and west Iran, although they are in the same latitude.
Wind advection in eastern and southeastern regions of Iran during hot period of year is considered as one of the most important and most effective climatic phenomena having great impacts on environment and communities. There are two advection orders during this period of year, the advection of northern winds (120- day winds of Sistan) and the advection of eastern winds. Eastern winds mostly cover eastern and northeastern regions of Iran, while northern winds mostly cover eastern and southeastern regions. The calculation of thermal advection during the existence of each wind demonstrates that during the advection of northern winds, a core of negative thermal advection is made in east and southeast Iran. As these winds are intensified, the intensity of this negative thermal core is also increased. This phenomenon reveals that this is the heat transmission from the dominant regions of this negative thermal advection to surrounding regions which provide cool weather in east and southeast areas of the country. Besides, vertical profile of atmosphere also proves the altitudinal expansion of this core of negative thermal advection through higher levels. A core of negative thermal advection is made during the advection of eastern winds, although this core dominates less regions limiting to eastern regions of Iran. Besides, a core of positive thermal advection is made in southeast part of Iran. This phenomenon not only leads to heat aggregation, but also makes a core of maximal temperature in the region and transfers thermal equator to east and southeast of Iran. Moreover, temperature in eastern half of Iran is lower than that of the west and also that of Mesopotamia during the advection of 120- day winds, while this region shows higher temperature than west of Iran and Mesopotamia in the absence of 120- day winds. Therefore, the advection of northern winds (120- day winds of Sistan) makes thermal equator of the earth move to southern latitudes in southeast Iran to decrease the temperature of the region.