A regional climate model (RegCM3) has been employed to investigate the role of Zagros Mountain in the formation and maintenance of summertime subtropical anticyclone over Iran. The monthly mean fields of relative vorticity and vertical velocity were compared within all levels of the troposphere, with and without Zagros Mountain simulations. In the mid-troposphere, the relative vorticity values without the Zagros Mountain simulation indicate an extensive decrease of both anticyclonic circulation and maximum negative vorticity over Zagros Mountain and the surrounding area. In the upper troposphere, there was also a decrease of anticyclonic circulation, but to a lesser degree. In the lower troposphere, positive vorticity below the 500 hPa level decreased without the Zagros Mountain simulation. These findings show that the omission of Zagros Mountain within the simulation decreased the negative vorticity values and weakened the anticyclonic circulation within the middle and upper troposphere. Cyclonic circulation changed to anticyclonic circulation in the lower troposphere. Further investigations of vertical velocity fields also indicated that the dominant ascending air flow over the western Iran plateau was substituted with a descending flow without the Zagros simulation. The results clarified the important role of Zagros Mountain in the formation and maintenance of the summertime subtropical anticyclone over Iran.

Keywords