عنوان مقاله [English]
نویسنده [English]چکیده [English]
Today, extra tropical cyclones are recognized not only for the important influence they exert on midlatitude weather conditions but also for their integral role in the earth’s climate system. (Gary luckmann, 2012). Extra tropical cyclones are fundamental meteorological features and play a key role in a broad range of weather phenomena. They are a central component maintaining the global atmospheric energy, moisture, and momentum budgets. They are on the one hand responsible for an important part of our water supply, and on the other are intimately linked with many natural hazards affecting the middle and high latitudes (wind damage, precipitation-related flooding, storm surges, and marine storminess). Thus, it is important to provide for society an accurate diagnosis of cyclone activity, which includes a baseline climatology of extra tropical storms (e.g., Hoskins and Hodges 2002). Identifying and tracking extra tropical cyclones might seem, superficially, to be a straightforward activity, but in reality it is very challenging (Urs neu and etal, 2013). One of the region of cyclogenesis is Mediterranean Sea in north hemisphere that positive vorticity and cyclogenesis occurred in west and east of this sea.
There are many study with numerical method about cyclogenesis in Mediterranean Sea and at first Petersen 1957, had identified the cyclone frequency in Mediterranean in north hemisphere study. Alpert and etal 1990, study the cyclogenesis in west and east of Mediterranean Sea and found that vorticity in Mediterranean Sea have main role in cyclogenesis in Mediterranean region, also lee ward cyclogenesis have main role in Geneva golf and cypress. One of the first study that performed about impact of cyclones of Mediterranean over climate of Iran is Alijani research, 1364, that he found that this feature have main impact on climate of Iran. Sedaghat and others 1378, had been studied the cyclone tracking of cyclone in middle east and they said that the most of cyclone are in west and North West of Iran. One of the most reason of this study is identify of spatial temporal change of Mediterranean cyclone. Because the climate change and global warming caused the change in atmospheric general circulation and change in atmospheric phenomena.
For numerical cyclone detection used the ERA-Enterim database, this is last reanalysis of global atmosphere by ECMWF by Dee and etal, 2011 which available in six hours interval and with resolution 0.5*0.5 longitude and latitude in duration of 1980 to 2013. The cyclone positions are defined by local minima of the geopotential height (z1000) of the 1000-hPa surface considering the neighborhood of eight grid points. Additionally, in order to locate intense vortices, the mean vorticity of a minimum point in 600 km radios required. (Blender and Etal, 1999) The threshold of mean vorticity is 5*〖10〗^(-5) s^(-1)because in this region there are shallow and thermal low and this is the best threshold for exclude of those. After calculation of cyclogenesis the spatial-temporal change of those identify with frequency sum and mean center for three duration of eleven years.
Spatial distribution cyclones shows in Figure 2 and include of 12180 cyclone in this region. As see in fig 2 the more cyclogenesis occurred in the west of Mediterranean and the most concentration of those are in west and east of Italia whatever we move to east of Mediterranean decrease from the cyclone concentrations the main center of cyclongenesis in east of Mediterranean are in cypress coast and Turkish west coast. It is considerable that in any time cyclone with more than one frequency are in Iran.
About of 14 percent of cyclones in Mediterranean occurred in January that mostly composed in west and east of Italia, in additional in this month cyclone concentration are in Turkish and Syria. The most frequency of cyclogenesis occurred in February and the lowest frequency of those occurred in the July. The trend of cyclogenesis in Mediterranean are positive and cyclones are increasing by 1.04 coefficient in each year. But the coefficient of determination in the regression equation are very low and nearest to the zero this shows that the regression equation couldn’t show the logic correlation between time and cyclone frequency. The trend month of cyclone in Mediterranean in May, July, august and October are negative and in others months are positive. But in month trend such as years trend the coefficient of determination of regression equation are low. The result show that the temporal trend of cyclogenesis in Mediterranean don’t change in this study area. The mean center of cyclogenesis show that the center of mean cyclone in the three duration have a shift to west and north latitude.