مؤسسة ژئوفیزیک دانشگاه تهران، صندوق پستی 6466- 14155
عنوان مقاله [English]
Using the NASA reanalysis data the thermodynamic structure of the troposphere over the Middle East region is investigated for a period of 14 years (1980-1993). The calculated quantities consist of the monthly mean of specific humidity and temperature, temperature and humidity fluxes for January and July at 850 and 700 hPa. In addition, the time series of specific humidity and temperature for Tehran/ Iran have also been studied for the 14-year time period. The area examined extends between 20˚ and 60˚ N and between 0 and 90˚ E.
The results show that under normal conditions three major sources of humidity which can affect the Middle East region are seen to the south of the domain here, located over North Africa, east of the Red Sea, and the western part of the Indian Ocean. The analysis of the monthly mean specific humidity distribution along with its fluxes suggests that the main part of the humidity over the south of Iran comes from the source located to the east of the Red Sea. The humidity source observed in North Africa exhibits remarkable annual changes and it disappears in some years, especially when the El-Nina phenomenon occurs.
The other humidity source and noticeable humidity fluxes that can affect the Middle East region and the central and northern parts of Iran is connected with the intense humidity fluxes over the east of the domain examined here. These humidity fluxes, which originate over the Atlantic Ocean, can be reinforced significantly as they move southward over the Mediterranean basin and then extend eastward over the Middle East. It is shown that the maximum specific humidity over Iran is connected with the existence of intense sources of humidity located over the east of the Red Sea and the western part of the Indian Ocean and seems to be related to the occurrence of intense the Indian monsoon. The results of time series of temperature and humidity during the whole period indicate that the seasonal changes of temperature are much greater in winter as compared with summer whereas the seasonal changes of specific humidity are much greater in summer as compared with winter. Our analysis suggest that the occurrences of El-Nino and La-Nina may have effects on the changes of the specific humidity in July and January, and La-Nina’s effect seems to be greater, as compared with El-Nino’s.
The monthly mean temperature distribution and its fluxes also show that three cold centers can be identified in the Northern Hemisphere, located over Greenland, Northern America, and the north of Asia. It seems the temperature distribution over the Middle East and Iran are affected mainly by the cold center over Greenland.