Decadal variation in precipitation regime in northwest of Iran

Previous studies revealed that the interannual variation of precipitation in the northwest of Iran (NWI) was controlled mainly by the Mediterranean cyclones and local mechanisms. Drawing upon the literature, the aim of this study is twofold: examining precipitation variability resulting from each of the abovementioned mechanism and investigating variation in precipitation regime in the NWI during 1961-2010. To provide a better understanding of the features of precipitation regime in the NWI, in the present study, I put the spotlight on the changes in precipitation patterns occurring over five decades in a context experiencing climate change. Accordingly, the annual, semiannual, and triple cycle of precipitation data from 1961 to 2010, which were obtained from 250 stations and by which a gridded dataset of monthly precipitation for the period above was created, were analyzed using harmonic analysis. The results showed that the regional average of precipitation during five successive decades decreased from 385.8 mm in the first decade to 297.3 mm in the last decade. The differences in the precipitation values during the five decades under study can be due to the slight positive trends in precipitation in summer and considerable negative trends in other seasons. Moreover, global warming and changes in the path of storm tracks can be considered as the possible reasons behind the observed changes in the precipitation in the NWI.