The association between regional and global atmospheric patterns and winter precipitation in Iran

This paper attempts to find the associations between the atmospheric circulation patterns ranging from regional to global scales and winter precipitation patterns over Iran. This study utilizes Principal Component Analysis (PCA), running correlations and composite maps for extremely dry and wet conditions. A PCA of winter precipitation reveals five components that account for 63% of the total variance. Analyses of SLP and GPH fields reveal five anomalous pressure centers over Kazakhstan, Syria, the Indian Peninsula, the North Sea and the northwestern African coast during wet conditions in Iran. These pressure centers make a special wind pattern, causing prevailing westerlies towards the Arabian Peninsula, that curve northward by an anticyclonic wind pattern over the Indian Peninsula. This pattern is destroyed in dry conditions and is replaced by the cold and dry northeasterly winds. The results also show that the zonal (meridional) wind during wet conditions is weaker (stronger) than the dry conditions. The five pressure centers are also found to be associated with some atmospheric indices. The strongest results are found for the North Sea-Caspian Pattern (NCP) and the Western Mediterranean Oscillation (WeMO) indices. These two indices have the most significant correlation with the PCs in Iran. Analyses of the results also reveal that the winter precipitation in Iran is mainly related to the Black Sea and the Mediterranean SLP. The best linkage is found between the Black Sea SLP and the PC4, expressed by correlation coefficients varying from 0.43 to 0.57. Our results also indicate that, the Red Sea trough under wet winter conditions in Iran is accompanied by a deep 850 and 500-hPa trough, located west of the Mediterranean. This trough is strongly weakened during dry conditions. We also found that, the winter precipitation PCs are controlled by some atmospheric indices. The best result is found for the PC1 that is affected by the WeMO and the Scandinavian Pattern.