Temporal-spatial Variability of the Severest Dry Spells in the North-West of Iran

the variability of temperature and precipitation is regarded as one of the main characteristics of the climate. precipitation and its results, especially results such as droughts, vary on different temporal and spatial scales. the purpose of this paper is to determine the frequency of the inter-annual variability of the driest month in north-west iran. in order to obtain the best results, we used 11 synoptic weather stations and included 30 years of continuous data records 1977-2006 . initially, the driest months were determined using the standardized precipitation index spi ; thereafter, the harmonic method was applied for closer examination and demonstration of the frequency of precipitation. the results derived from the harmonic method showed that the highest percentage of variance distribution approximately 80% is found in the central sections of the region according to the first harmonic of the zanjan station. in these sections, the inter-annual changes of in the driest months typically fit a semi-annual pattern during the examined period. in the northern parts, higher harmonics are required to explain variance, which implies a higher variability and a lack of consistency in the occurrence of the driest month in these regions. the lowest pvr1 value nearly 28% was registered by the ardebil station. therefore, in zanjan station, the inter-annual changes of the examined component have more homogeneity owing to the high pvr value of the first harmonic, while the level of variability in the driest months was higher in stations like ardebil. higher harmonics are required to explain this variance.