عنوان مقاله [English]
Results of the analysis shows that, when La Nina is weak in autumn and accrued after changing strong El Nino to the very brief neutral phase of ENSO, drought occurs in the fall season in Iran. In order to get this, 4 cases, is identified with these conditions, and rainfall anomalies were calculated in these cases. The result show that in this condition, precipitation reduces in the center, east, southeast, west and also northwest of Iran and a very dry fall dominates in these parts. In all analyzed years, most of Iran experiences a negative abnormality more than 60%. Also Synoptical studies for selected cases was conducted. It showed that, fall droughts caused by weak and short La Nina dominance after a severe El Nino are controlled by six major pressure patterns. These major are: Arabian and North African subtropical high pressure, Siberian high pressure, Azores high pressure, Mediterranean low pressure and Iceland low. When ridges of Arabian high pressure develop towards the north, prevent eastward movement of Sudanese low pressure. Sometimes the ridges of this pressure center develop to higher altitudes and prevent entrance of Mediterranean current from west of Iran, so Mediterranean current pass in higher altitudes. The development of African high pressure towards south and east of the Mediterranean Sea and development of the Azores high pressure ridges towards north and east, and covering its ridges on the center and west of the Mediterranean Sea, causes weakness or lack of formation of the Mediterranean trough. The most common pattern among effecting patterns on drought of studding years is increase of central pressure of Iceland low pressure center in the north of Europe, which its reinforcement or weakness, effects Mediterranean low pressure. Weakness of Iceland low pressure center causes weakness or lack of formation of Mediterranean trough. So the existence of Siberian high pressure in the north, which causes development of dry and cold air on Iran and also the existence of Arabian, north African and Azores high pressures, and of course weakening of the Iceland low center, that's respectively prevent incoming of Sudanese low pressure and formation of Mediterranean trough, beside, lack of sufficient moisture feeding on Iran, have all caused severe low precipitation in the fall season in analyzed years. As a result, in the occurrence of a weak La Nina after a severe El Nino, should expect dry autumns in Iran, especially in the center, east, southeast, west and also northwest regions. And this result can be considered as a seasonal forecast.