The role of transmitted moisture changes in occurrence of drought and wet year in Iran

Introduction :

Atmospheric moisture resources which affect rainfall in a region are one of the most important issues in atmospheric sciences. In addition, understanding the mechanism of origin and transfer of atmospheric moisture causes the knowing of water cycle in a region. The main components of water cycle are the evaporation from land surface and oceans, transfer of water from ocean to land, precipitation over oceans and lands and the recurrence of water from land to the ocean. Consequently, the percentage of moisture flow of a region and amount of evaporated water could be evaluated using atmospheric moisture flow and observed precipitation. Therefore, the effect of evaporation from water bodies and transfer of atmospheric moisture on precipitation could be determined by calculating them. The spatio-temporal variations of precipitation could be investigated by study of moisture fluctuation due to variations in atmospheric circulation in dry and wet patterns conditions. Therefore in recent study the difference in atmospheric patterns of drought and wetness conditions, moisture flux and water resources which affect the transfer of moisture over Iran in various atmospheric levels.

 Materials and Methods:

In this study, re-analysis data of the ERA interim center of Europe medium-term forecasts (ECMWF). With a spatial resolution of 1 ° * 1 ° of geographical latitude and longitude for a 1981-2011 period were used. In order to find the wet and dry patterns in Iran, precipitation data were used. Moreover, geo-potential height, specific humidity, zonal and Meridional components of wind in 00 and 12 UTC of 1000, 850, 700, 500 and 300 hPa for analysis of atmospheric moisture source were used. Standardized Z-Index was used for determining dry and wet patterns over Iran and the percentage of occurrence of the patterns worked out yearly and the year with maximum percentage of area of occurrence of dry and wet periods were selected for further analysis. In next step, precipitation periods of selected years with precipitation of more than 1 mm and minimum 30 percent of affected area were determined for estimating contribution of water bodies in the periods. The total vertical divergence of moisture flux were calculated in 1 ° * 1 ° of geographical latitude and longitude networks in lower, middle and upper and total atmospheric thickness over Iran for selected years. In the next step, the positive and negative values of moisture flux of each network were derived for Iran and water bodies out of Iran, respectively. In the last step, atmospheric circular and specific humidity maps of precipitation periods of wet and dry years were prepared and analyzed for 850, 700 and 500 hPa as a indicator of lower, middle and upper levels, respectively.

 Results and Discussion:

The results of calculation of Z-Index showed that the 1995-96 and 1999-2000 were determined as wet and dry years with 87.3 and 98.7 percents, respectively. The humidity flux and percentage of variations for all layers were more in wet year as compared to the dry year. The most contribution in transfer of moisture in wet and dry periods was related to Arabian Sea over Iran. The Mediterranean and red seas were in the second order, Persian Gulf was in the third order, and black Oman and Caspian seas were in the next orders in both wet and dry periods. The Arabian and Caspian seas had no impact on moisture due to the flow patterns in middle atmospheric levels. Therefore, the Mediterranean Sea had the most contribution for transfer of moisture over Iran. In upper levels, the contribution of water resources in transferring the moisture was the same as the middle levels. Although, the amount of moisture flux was changed. The results also revealed that the southern border of the country are the most important direction for the entering moisture, especially at atmospheric lower levels. It might be due to the fact that the southern water bodies and Arabian Sea are the main moisture sources at atmospheric lower levels. In overall, the atmospheric low levels had a significant role in transferring moisture in Iran at all directions and the atmospheric upper levels had lesser task in this regard and the southern and western directions had more contribution as compared to other directions. 

Conclusion:

 The results showed that the flow pattern of the region and water bodies had a remarkable role for imported moisture variations over Iran. The moisture flux at wet period was more at all levels in wet year. In addition, Arabian Sea at atmospheric low level and Mediterranean Sea at medium and upper levels had more contribution for transferring moisture content in Iran. Moreover, the results showed that southern border were in first priority and western borders in second priority for importing moisture at atmospheric lower, medium and upper levels over Iran.