A survey of Azores high pressure and its effect on precipitation of Iran

In this research, temporal and spatial behavior of Azores high pressure was studied at 700 hPa level. This study has been done using daily data of geopotential height at 12 GMT from 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, mean geopotential height at 12 GMT is extracted for all days of solar calendar to determine spatial behavior of Azores high pressure and its relation to the contiguous patterns. Then, an agglomerative hierarchical cluster analysis using ward linkage method applied to it. According to this analysis two circulation patterns in the Northern hemisphere were recognized. The first pattern which is coincided with the cold half of year, shows weakening of Azores high pressure and predominance of westerlies with Rossby waves.The second pattern which is coincided with the warm half of year shows maximum activity of Azores high pressure. Occurrence of this pattern is coincided with precipitation cease in most parts of Iran. To identify temporal behavior of Azores high pressure, by selecting a suitable framework, mean of geopotential height whit in this framework at 700 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 Azores high pressure) in each day. The mean intensity of Azores index is 0.84 during positive phase and -0.79 during negative phase. Applying contingency table method and calculating χ 2 statistic during statistical period(1330-1383) showed that occurrence of precipitation in about 4% of studied stations(40 out of 1066) is independent of influence of Azores high pressure. These stations are mainly encompass southrn coast of Caspian sea and North west of Iran. According to this analysis, the occurrence of precipitation in 96% of studied stations(1026 out of 1066) weren’t independent of influence of Azores high pressure and are under the control of Azores high pressure at the time of predominance of this system over Iran, intensively.