Overlap of North Atlantic Oscillation (NAO) periodicities with periodicities of droughts and wet years of Iran

In this research, in order to explain the drought and wet year periods of Iran and their time overlap with teleconnection cycles of North Atlantic Oscillation (NAO), two sets of data were used. The first set is the data related to monthly precipitation of 63 synoptic weather stations of Iran during 1988-2015, obtained from Iranian Meteorology Organization, and the second one is the values of North Atlantic Oscillation index for the same period obtained from National Center of Environmental/National Center for Atmospheric Research (NCEP/NCAR) affiliated with US National Oceanic and Atmospheric Administration (NASA). Moreover, Standardized Precipitation Index (SPI) was used to calculate and identify the droughts and wet years of studied stations. By calculation of SPI for all studied stations in a monthly scale, their time scales were prepared and set separately for all the studied months, in station scale and regional scale. In addition to these two time series, the North Atlantic Oscillations were also obtained for these months. Finally, in order to identify the dominant periods existing in the time series, the spectral analysis was done using Fourier method. The results of spectral analysis on NAO time series show that in each month, different periods from short periods to long periods are observable and the most dominance is related to short periods (less than 10 years). Investigation of the overlap of these periods with periods related to droughts and wet years of Iran in studied stations shows that except May, more than 65% of stations had at least one period identical to that of NAO. This can indicate the effectiveness of this climate fluctuation on conditions of droughts and wet years happening in Iran.