identifying the controlling centers of the climate of iran and their pressure patterns in the 500 hpa. level during cold period of the year

the upper level pressure patterns control the climate of the surface. the identification of these patterns in the regional scale is useful for the study of the surface climate elements and their forecasting. in order to understand the pressure centers controlling the climate of iran during the cold period of the year, the daily 500 hpa. geopotential height of the grid points of 2.5 degrees distance within the windows of 10 to 70 north and 10 to 80 e. were obtained from the ncep data center for the 1948-2010 period. the action centers or the controlling points of the climate of iran were determined using the pca. each action centre controlled the climate of iran through a specific pressure pattern. the results showed that about eight controlling points affect the climate of iran during the cold period of the year. the northern siberia is the most important center in controlling the climate of iran. most of the controlling centers are located north of iran. only the monsoon of pakistan is located to the south of iran. most of the pressure patterns indicated a meridional orientation with low index cycle accompanied with cut-offs and blocking patterns. deep troughs and strong ridges are the frequent phenomena of the westerly winds over iran.