Synoptic analysis of Azores high pressure relationship with maximum temperatures of Iran

In this research, temporal and spatial behaviors of Azores high pressure was studied at the level of 700 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 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 on the basis of ward linkage method has been done in Mathlab soft¬ware. According to this analysis two circulation patterns in the North hemisphere are 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 event of daily temperature maximum in most parts of Iran. To identify temporal behavior of Azores high pressure by selection of a suitable framework, mean of geopotential height 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. 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 Azores high pressure were definitely more than weakening days of this dynamic system. Altogether, standardized Azores high pressure index(SAI) which defined and calculated in this research, illustrated that it can be used as a criterion to identify temporal behavior of this high pressure system. Applying contingency table method and calculating χ 2 statistic during statistical period(1340-1382) showed that at the 95% confidence level, daily temperature maximum in about 94.8% of studied stations(401 out of 423) isn’t independent of influence of Azores high pressure at level 700hPa.