عنوان مقاله [English]
نویسندگان [English]چکیده [English]
In this research, temporal and spatial behavior of Siberian high (SH) was studied at the level of 1000 H.Pa. This study has been done using daily data of geo potential height at 12 GMT in NCEP/NCAR database with spatial resolution of 2.5Ã2.5 degree in a 55 years period including 20089 days from January, 1st, 1951(Dey, 11th, 1329) to December, 31st,2005(Dey, 10th,1384). At First, the mean geo potential height at 12 GMT is extracted for all days of solar calendar. Then, an agglomerative hierarchical cluster analysis on the basis of ward linkage method has been done in Matlab softâware. According to this analysis six circulation configurations (for cool and warm periods) in the studied level are recognized. The study of spatial behavior of SH in this patterns showed that only in cold (1-2(and pre-cold (1-1-2) circulation configurations, Westerly tongue of SH passed over north and northeast areas of Iran. The 1-2 circulation configuration which is coincided with the cold period of year, shows maximum activity of SH. The map analysis of circulation configuration of cold period illustrated that spatial configuration of SH depend on configuration of sub polar lows in on hand and Tibet and Pamir plateau on the other hand. On this basis, the role of SH in Iran climate at cold period of year not depend on intensity of this system but depend on intensity of sub polar low and spreading SH and its tongue inward Iran.
To identify temporal behavior of SH by selection of a suitable framework, mean of geo potential height at 1000 H.Pa. level is calculated and standardized for each day of solar calendar from 1330 to 1384. Thus, a matrix of size 19724Ã1 was obtained as Siberian high pressure standardized index (SSHI) that shows positive and negative anomalies (appearance and absence of this system) in each day. The analysis of time series of SSHI showed that index has twofold nature. Comparison between frequency of positive phase occurrence days and negative phase event days in the studied level showed that during recent 50 years the appearance and activity days of SH were definitely more than absence days of this system.