The synoptic Patterns of wintertime drought and wet period in Southwestern of Iran

Drought is one of the natural hazards that occur in most parts of the world but its manifestation in the arid and semiarid regions of the world is more. First purpose of this study is the identification of synoptic patterns affecting in drought occurrence and second is its comparison with wet periods. For this purpose, daily and monthly precipitation data over a period of 20 years (2008-1989), were gathered and analyzed using statistical methods such as Z score for 20 meteorological stations in southwestern of Iran. Preliminary results showed that the frequency of drought events in the study area is more than wet occurrences. Trend of droughts, particularly in recent years is increasing, although January and December show a maximum frequency of occurrence of drought, but the phenomenon is declining in both. However, in March and February with a lower frequency of occurrence of drought, the drought is increasing. From synoptic view point the annual shift of sub-tropical high pressure belt, particularly meridional extension the STHP cell in southeast of Saudi Arabia plays an important role in the winter precipitation variation. So, often dry periods with the movement toward the West, taking place on peninsula and the increasing in heights, and wet periods with the movement toward the East, taking place on sea and the decreasing in geopotential heights. Also the location of the Mediterranean trough axis plays an important role in precipitation variations in southwest of Iran. Whatever the trough be closer to the east part of Mediterranean Sea and its axis forms a NW-SE orientation, rainfall in study area will be increased but with increase the trough distance from east part of Sea and with NE-SW orientation, it is usually associated with periods of reduced precipitation and drought events.