a synoptic study on seasonal patterns of wet and dry spells in midwest of iran

the effective patterns and mechanisms of synoptic systems during the wet and dry spells in midwest of iran(mwi) analyzed and discussed on seasonal scales from 1974 to 2003. synoptic analysis is based on synoptic chartson sea level pressure (slp), 850 and 500 hpa levels. the results of synoptic analysis show that dry spells in mwiare mostly corresponded to azores high (azh) intensifying and its central movement to east and northeast of normalposition. furthermore the siberian high (sh) ridge is extended from northeast to iran. the linkage between sh andazh ridges through iran, mesopotamia (iraq) and arabic peninsula lead to drought occurrence in the middle eastespecially iran. the sudan low (sl) and mediterranean trough (mt) have a limited extend during drought periods(dry spells). the most important occurrence index of droughts in upper charts is reinforcing and extending northwardof arabia south high (ash) to iran and mesopotamia. this event is marked by a strong ridge in 500 hpa charts overstudy area. the circumpolar vortex (cpv) is contracted to polar area during dry spells and expanded equatorward inwet spells. in summarized sh extends during wet spells but dose not reach to iran. in addition mediterranean easttrough (met) reinforced and extended from 25°e to 40°e and its meridional expands reaches to the red sea (rs)and sudan low. the other result is that during wet and dry spells, the orientation of blocking highs ridges in thenorth of caspian sea and over ural mountain (um) causes different climatic conditions in iran.