Synoptic analysis of relationship between subtropical high pressure at 600 hPa level and daily precipitation in Iran

In this research, temporal and spatial behavior of subtropical high pressure was studied at the level of 600 hPa. This study has been done using daily data of geopotential height at 12 GMT in NCEP/NCAR database with spatial resolution of 2.5×2.5 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 geopotential height at 12 GMT was extracted for all days of solar calendar to determine spatial behavior of subtropical high pressure and its relation to the contiguous patterns. Then, an agglomerative hierarchical cluster analysis on the basis of ward linkage method has been done in Matlab soft¬ware. According to this analysis four circulation patterns in the north hemisphere were recognized. To identify temporal behavior of subtropical high pressure by selection of a suitable framework, mean of geopotential height at 600 hPa level was calculated and standardized for each day of solar calendar from 1330 to 1384. Thus, a matrix of size 19724×1 was obtained that shows positive and negative anomalies (strengthening and weakening of subtropical high pressure) in each day. Comparison between frequency of positive phase occurrence days and negative phase event days in the studied level showed that during recent 50 years the aggravation days of subtropical high pressure were definitely more than weakening days of this dynamic system. Applying contingency table method and calculating χ 2 statistic during statistical period (1330-1383) showed that occurrence of precipitation in about 3.3% of studied stations(35 out of 1066) is independent of influence of subtropical high pressure. These stations are mainly encompass South coast of Caspian sea and north west of Iran. According to this analysis, the occurrence of precipitation in 96.7% of studied stations (1031 out of 1066) aren’t independent of influence of subtropical high pressure and are under the control of subtropical high pressure at the time of predominance of this system over Iran, intensively.