Study on the role of annual movements of Arabian subtropical high pressure in the late start of precipitation in southern and southwestern Iran

摘要

According to Iran geographical location and the general circulation of the atmosphere (or atmospheric circulation), rainfall starts in many parts early in autumn and ends in mid or late spring based on the latitude. Due to the dominating synoptic conditions during some years, rainfall starts earlier than the normal conditions, and during some years, rainfall starts later than the normal conditions. Such synoptic conditions showed that the start of rainfall, especially in the south and southwest of Iran, is closely related to the location and displacement of Arabian subtropical high pressure in autumn. A study conducted on rainfall beginning day of 30 stations in a statistical period in 36 regions in south and southwest of Iran showed that during 12years of rainfall began later than normal conditions. Arabian subtropical high pressure was extracted 1000, 850, and 700hPa at four sea levels since September 15 to the beginning day of rainfall. This study showed that during those years when rainfall began later than normal, there were three general states. During some years, the high pressure system has been out of the southwest of Iran later than normal conditions. In a number of years, the high pressure system has left the area, but has moved westward, or the southern border of the Persian Gulf. The governing synoptic conditions of these three patterns have made it impossible for any rainfall system entering the region. In these patterns, Sudan low pressure system has been blocked as the most effective rainfall system in the southern latitude on Sudan and Ethiopia, and it is not possible to expand on the region or move to another route. In the three patterns, Arabian subtropical high pressure generally has an orbital expansion, and in this way, Mediterranean trough has no potential to expand southward. As a result, instability cannot be transferred to the region.