This research focuses on the large-scale atmospheric circulation over the southwest Asia (SWA) during boreal summer. The NCEP/NCAR daily reanalysis dataset is used to study the substantial characteristics and the nature of the large-scale atmospheric circulation over SWA. We also developed an index to determine the start time, ending time and the length of summer season as well as to study the intensity of the atmospheric circulation over SWA. First, we examined the sink and source of energy in regional scale. Then, zonal and meridional large-scale circulation was analyzed for April-October period over SWA and the dominant circulation patterns were derived. The results show that the monsoon circulation over south-southeast Asia is the main factor that controls the large-scale summer circulation over SWA in upper troposphere. The findings indicate that the onset of Asian monsoon is associated with the formation of a permanent westward flow in the upper troposphere. The corresponding maximum convergence and subsidence locations of the flow are the energy sinks over SWA and east of the Mediterranean Sea, respectively. The maintenance of large-scale atmospheric subsidence creates a transverse meridional gradient in the temperature field; therefore, the meridional large-scale circulation is changed and this in return creates a transverse Hadley circulation over the SWA. Investigating the nature of the summertime anticyclones over SWA, it is found that the anticyclones are dominant in all tropospheric levels. They are originally formed either as a result of the descending air which is basically originated from the Asian monsoon region or accumulated heat of the surface of plateaus due to vertical advection of heat over SWA. It is found that the Iran anticyclone as a thermally-driven anticyclonic circulation in mid-troposphere is established by the vertical heat advection induced by the existence of an elevated heat source in western Iran. On the other hand, the formation of the Turkmenistan anticyclone and the Arabian anticyclone has different mechanisms. Formation of the Turkmenistan anticyclone in the lower troposphere is mainly due to the convergence and subsidence of the air with monsoon origin in SWA in the levels below 600 hPa. In contrast, the Arabian anticyclone is strengthened by descending air originated from south Asia as well as affected by the surface heat forcing and the vertical heat advection from the highlands of the western Arabian Peninsula.

Keywords: Southwest Asia, Summer Atmospheric Circulation, Monsoon Circulation, Iran Anticyclone, Turkmenistan Anticyclone.