عنوان مقاله [English]
نویسندگان [English]چکیده [English]
In this research, moisture sources and their transport over Iran and its relation to other influencing factors during different months for lower (1000-700 mb), middle (700-500) and upper (500-300 mb) layers of the atmosphere were examined. Using the National Center for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data of 0.5 degree horizontal resolution, described by Saha et al., 2010, several fields including the geopotential height, horizontal wind and temperature at several vertical levels along with the sea level pressure, precipitable water and rotational and divergent components of the moisture flux vector were presented and analyzed.
Results for the lower layer showed that during December-April period, the rotational component of the vertically integrated moisture flux vector with cyclonic curvature over the Mediterranean Sea and the moisture transport over the North-west of Iran were observed. The anti-cyclonic curvature of the rotational component of the moisture flux vector over the West of the Indian Ocean and the East of Arabian Peninsula and formation of the synoptic systems over the East of the Mediterranean and their combination with the north-eastern African low-pressure synoptic system are responsible for the moisture transport over the West and South-west of Iran. Due to the Indian Ocean general circulation associated with the Indian Ocean Dipole (IOD) during late May, the rotational component of the moisture flux vector over the Indian Ocean is anti-cyclonic, and hence the moisture transport from over the Arabian and Oman Seas into the Indian subcontinent and the south-east of Iran is observed.
The results for the middle and upper layers showed that during the December-April period, the subtropical high is shifted towards lower latitudes over the South of Iran and consequently the Mediterranean low causes the moisture to be transported from the East of the Mediterranean over the North-west and west of Iran. During the warm season (June-August), the subtropical high is extended from the South-west to the North-east of Iran and causes moisture to be transported from the North of the Indian subcontinent to the East of Iran.
Considering the divergent component of the moisture flux vector during December-April over the South of Indian Ocean, it is observed that the moisture flux vectors converge in the middle and lower layers and diverge in the upper layer. This means that in the lower and middle layers, the amount of evaporation is more than that of precipitation. The excess of evaporation in the lower and middle layers along with the vertical transport of moisture to the upper level causes the divergence of moist air in the upper layers. Over the north of the Indian Ocean, convergence of the moisture flux vector is observed in the upper layer and its divergence is observed in middle and lower layers. It means that there is a circulation of moist air at the meridional plane (45-70 E), between Oman Sea and the South of the Indian Ocean.
Over the Indian subcontinent, there is a convergence of moisture flux vector at the lower and middle layers and divergence of moisture flux vector is observed at the upper layer. On the other hand, over the East of Iran, convergence of moisture flux vector is observed at the upper layer along with divergence in the middle and lower layers. It means that along with the summer time Indian monsoon intensification during June-August, there is a circulation of moist air at the zonal plane (25-35N) between the Indian subcontinent and the east of Iran.