The Synoptic Analysis of Widespread Dry Monthly Periods in the Southern Coasts of Caspian Sea

The main feature of the Earth system is climate changes altering climatic elements such as precipitation and temperature¡ in short and long-term periods. Drought is one of the climate changes affecting on several arid¡ semi-arid and even wet regions of the world with different intensities. The main goal of this study is investigation on the synoptic effective mechanisms in order to identify the monthly dry periods in the southern coasts of Caspian Sea as well as presenting its patterns. First of all¡ in order to recognize the patterns affecting on the drought of this area¡ 30-year statistical data (1977-2006) extracted and then¡ the wet and dry months were identified using the standard score index (Z). Afterwards¡ the synoptic maps were provided using the Grads software based on geopotential data and the pressures of the sea levels¡ 850¡ 700¡ 500 hpa. Moreover¡ these maps were used to extract the relative pattern. Statistical processing of precipitation and temperature data illustrates that in most of the wet months¡ standard precipitation index (z) and temperature Z are positive and negative¡ respectively. Due to high rainfall¡ temperature is lower than normal and its reverse is observed in dry months. It means that the low rainfall coincides with the negative precipitation and positive temperature indexes. Synoptic maps show that Siberian high pressure and blocking system over Eastern Europe (located on the Ural Mountains) have the most influence on the southern coasts of Caspian Sea¡ Therefore¡ by means of creation of the blocking systems and their spatial displacement are originated the wet and dry periods in the area.