Climatology of atmospheric moisture transport to Western and North-Western Iran

The purpose of this study is to investigate the spatio-temporal variations in the atmospheric moisture budget (precipitation–evaporation, P-E) using ERA-Interim reanalysis data sets with a horizontal resolution of 0.75° in the west and northwest of Iran over a period of 36 years, from 1979 to 2014. To study the mechanisms that influence the hydrological cycle in the research area, the atmospheric moisture budget equation is divided into three components, including mean circulation dynamics (MCD), thermodynamic (TH), and transient eddy (TE). The time series of these components is also prepared to provide a quantitative analysis. As a result, the spatial distribution of the P-E values is affected by the direction of the Zagros Mountains. Negative and positive P-E values observed on the windward slopes (moisture source) and leeward slopes (moisture sink), respectively. The results revealed that the dynamic component was the main factor in the total changes of P-E. Changes in the other two components have also helped to strengthen or weaken the moisture source and the sink. As a result, the contribution of transient eddy strengthens the dynamic component. However, the trend of both components is decreasing, especially in recent decades, with changes in the dynamic component and transient eddy activity, respectively, with a change rate of 0.82 and 0.46 mm/day. These changes can be attributed to the reduction of cyclones in the region, which should be considered a major cause of the declining trend of the incoming moisture flux.