A study of the impact of Indian Monsoon on summer climate of Iran

Summary: This research aimed to investigate the climatological effects (if any) of the Indian Monsoon on climate of Iran and to study the summer climate and summer rainfall in the east، south-east and center of the country. The study domain expanded between 0 to 60°N and 0 to 120°E، and the study period covered 61 years of 1950 to 2010، inclusive. The data consisted of the geopotential height، sea level pressure، temperature، meridional and zonal wind components، vertical velocity، precipitation، and the outgoing long-wave radiation (OLR) for three boreal summer months (June، July، August) taken from the NCEP/NCAR reanalysis monthly dataset. In a typical summer a low level thermal low is dominant over Iran and the north-western parts of the Indian Subcontinent are associated with convergent flows in the lower troposphere and divergent flows in the mid- to high levels. In the summer، relative humidity is the highest over the Monsoon region،including the Arabian Sea، Indian Subcontinent and over the north of the Gulf of Bengal، and is usually the lowest in the southern half of Iran. The tropical easterly jet is located in the tropics، while the subtropical westerly jet moves northwards to the mid-latitudes. At low levels، westerlies and south-westerlies are predominant in the Monsoon regions، but strong prevailing winds are northerly and northwesterly in Iran. Four dynamical and convectional indices were calculated to determine the intensity of the Indian Monsoon، and the correlation coefficients between meteorological quantities at different atmospheric levels over Iran and these indices were estimated. Our results showed that the correlation coefficients between the Monsoon intensity and the geopotential height were positive and significant at mid- to high levels. The correlation between the intensity of the Indian Monsoon and vertical velocity and zonal wind was negative and significant، indicating an increased downward velocity and strengthened easterlies over Iran with increased intensity of the Indian Monsoon. The magnitudes of the atmospheric variables were calculated separately for the years when the Indian Monsoon intensity was abnormally weak and strong. Our results showed a stronger convergence in the lower and a divergence in the upper levels associated with a stronger tropical easterly jet and subtropical westerly jet in the years that the Indian Monsoon was abnormally strong. Relative humidity was higher and low level westerliesand south-westerlies were stronger over the Monsoon region and near the equator in such years. The atmospheric patterns for the years with maximum summer precipitation in Iran were also compared with those with minimum summer precipitation. We found that the geopotential height was smaller in the lower levels and greater in the upper levels when the summer in Iran was relatively wet. In these years، the position and intensity of the tropical easterly jet and subtropical westerly jet changed، thereby vertical velocity، relative humidity and temperature increased in Iran، but decreased in Monsoon regions.