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

Document Type : Full length article

Authors

1 Assistant Professor of Climatology, Faculty of Geography, University of Tehran, Tehran, Iran

2 PhD Student of Climatology, University of Shahid Beheshti, Tehran, Iran

3 Associate Professor of Climatology, Faculty of Geography, University of Tehran, Tehran, Iran

Abstract

Introduction
Atmospheric moisture resources affecting rainfall in a region are one of the most important issues in atmospheric sciences. In addition, understanding the mechanism of initiation and transfer of atmospheric moisture help know water cycle in a region. The main components of water cycle are the evaporation from land surface and oceans, transfer of water vapor 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 the amount of evaporated water could be assessed 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 mechanism. The spatio-temporal variations of precipitation could be investigated by study of moisture fluctuation due to variations in atmospheric circulation in drought and wet patterns conditions. Therefore, in recent study the difference in atmospheric patterns of drought and wetness conditions, moisture flux and water resources can affect the transfer of moisture over Iran in various atmospheric levels.
Materials and methods
In this study, we have analyzed the data from the ERA interim center of Europe in medium-term forecasts (ECMWF) with a spatial resolution of 1 ° * 1 ° of geographical latitude and longitude for the period 1981-2011.
Precipitation data have also been used in order to find the wet and drought patterns in Iran. 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 have been used for analysis of atmospheric moisture source.
Standardized Z-Index has been used to determine drought and wet patterns over Iran and the percentage of occurrence of the patterns worked out yearly. The year with maximum percentage of occurrence of drought and wet periods is selected for further analysis. In next step, precipitation periods of selected years with precipitation values 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 levels with 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.
Finally, atmospheric circulation and specific humidity maps of precipitation periods of wet and drought 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 drought 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 drought year.
The transfer of moisture in wet and drought periods was related to Arabian Sea over Iran. The Mediterranean and red seas were in the second order, Persian Gulf in the third order, and black Oman and Caspian seas in the next orders in both wet and drought 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 is the most important direction for the entering moisture, especially at lower atmospheric 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 played a significant role in transferring moisture in Iran at all directions and the atmospheric upper levels. Hence, the southern and western directions had more contribution to the process as compared to other directions.  
Conclusion
The results showed that the air flow pattern over the region and position of water bodies play a remarkable role for imported moisture variations over Iran. The moisture flux at wet period was more at all levels. 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.

Keywords

References

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Physical Geography Research
Volume 51, Issue 4
January 2020
Pages 545-562

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History

  • Receive Date: 25 October 2017
  • Revise Date: 22 May 2019
  • Accept Date: 22 May 2019

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