Atmospheric circulation patterns of the summertime rainfalls of southeastern Iran during July 1994



A synoptic method based on stream analysis is applied to understand the governing mechanism of summer rainfall occurrence in the south of Iran. Based on this, the rainfall data of 152 stations of Iran were analyzed for a 33-year period (1970-2002). Investigating the spatial and temporal distribution of summer rainfall, a typical rainy region is recognized in the south east of Iran. It is found that July 1994 had the greatest and most widespread rainfall during the study period. In order to understand the circulation patterns causing the summer rainfalls in the south east of Iran, 6-hour, daily and monthly mean values of geopotential height, specific humidity and zonal and meridional wind components of the different atmospheric levels were obtained from NCEP/NCAR reanalysis dataset. Using the abovementioned dataset, the geopotential height, streamline, vector wind and relative vorticity composite maps, the hovmoller diagrams and vorticity cross-sections are produced and analyzed. Also, the monsoon depressions track is drawn for July 1994 and summer monsoon intensity is determined for the study period by using the WY, OLR and AISMR indices. Finally, their relationship with the summer rainfall of the south east of Iran is analyzed.
The result showed that there is a triangular area in the south east of the country located to the east of 58:30 E and south of 28:30 N which has rain almost every summer. It is also found that the monthly and interannual variation of summer rainfall over the south east of Iran is in close relationship with the variation of summer monsoon intensity over India. The most humid period in southeast Iran, July 1994, is associated with the increased summer monsoon intensity, increased monsoon depression numbers over west India and the positive rainfall anomalies over India. Additionally, the upper and middle troposphere subtropical anticyclones over southwest Asia were stronger and experience the considerable north and eastward propagation. Conversely, the cyclonic circulation of the lower troposphere was significantly stronger. The results revealed that the eastward extension of the Iranian subtropical anticyclone at mid-troposphere and the associated increase of anticyclonic circulation over northern India and Pakistan are followed by the westward movement of monsoon depressions and their entering the Arabian Sea. In this case, the monsoon depressions by creating or strengthening the convergence centers over the south and south east of Iran have an important role in increasing the cyclonic circulation and precipitation occurrence. It is also found that Iran low is the main factor of moisture transport and the rainfall occurrence of 1-10 July 1994 in southeastern Iran. The formation of this low pressure in the north of the Persian Gulf, in addition to the increase of positive vorticity over the area, is the key factor in 5-10 July rainfalls of south eastern Iran, by making suitable southern winds over southeastern Iran and transporting the Oman sea moisture in a thin layer to the study region.