A synoptic study on seasonal patterns of wet and dry spells in midwest of Iran

Document Type : Research Paper


1 Assistant professor, Faculty of Geography, University of Tehran, Tehran, Iran

2 MSc. Graduate in Physical Geography, Climatology

3 Scientific staff, Faculty of Geography, University of Tehran, Tehran, 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 charts
on Sea Level Pressure (SLP), 850 and 500 Hpa levels. The results of synoptic analysis show that dry spells in MWI
are mostly corresponded to Azores High (AZH) intensifying and its central movement to east and northeast of normal
position. Furthermore the Siberian High (SH) ridge is extended from northeast to Iran. The linkage between SH and
AZH ridges through Iran, Mesopotamia (Iraq) and Arabic Peninsula lead to drought occurrence in the Middle East
especially 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 northward
of Arabia South High (ASH) to Iran and Mesopotamia. This event is marked by a strong ridge in 500 Hpa charts over
study area. The Circumpolar Vortex (CPV) is contracted to polar area during dry spells and expanded equatorward in
wet spells. In Summarized SH extends during wet spells but dose not reach to Iran. In addition Mediterranean East
Trough (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 the
north of Caspian Sea and over Ural Mountain (UM) causes different climatic conditions in Iran.