Mean Zonal and Meridional Variations of Westerlies in wet and dry periods in the western Iran

In this study, the relationship between mean zonal and meridional variations of westerlies with wet and dry periods in the western Iran was investigated. Therefore, the data cumulative precipitation values of the study area from Eropean Center for Medium-Range Weather Forecasts (ECMWF) version (ERA-Interim) with spatial resolution of 0.125*0.125 and also the geopotential height data of the version ERA5 with a spatial resolution of 0.25*0.25 degrees was received from the same center for the years 1979 to 2019.The Standard Precipitation Index (SPI) was used to determine the monthly wet and dry periods. So, to investigate the relationship between mean zonal and meridional variations of the westerlies with wet and dry periods in western Iran, the Middle East Synoptic Index (MESI) was defined based on variations of the westerlies from 25 to 45 degrees north latitude at atmospheric levels of 500, 600 and 700 hPa. The results indicated, that when wet periods occur in western Iran, MESI upper than the threshold and westerlies on the Middle East (Mediterranean and Red Seas) have a meridional flow. Whereas in dry periods, MESI is lower than the threshold and westerlies have a zonal flow. According to the results, no wet and dry periods was found, the value of which MESI was lower and upper than the threshold. Therefore, it seems that variations of the westerlies flow can be effective as one of the factors in formation wet and dry periods in the western Iran.