Quantifying the ENSO-Related Shifts in the Intensity and Probability of Drought and Wet Periods in Iran

摘要

This paper quantifies the impact of the El Nino-Southern Oscillation (ENSO) on the intensity and occurrence probability of dry and wet periods in Iran during boreal autumn and winter. Three phases (warm, cold, and neutral) were defined based on the Southern Oscillation (SO) status, and precipitation composites were constructed for each phase. The 30th and 70th percentiles of neutral phases were used as the thresholds for distinguishing normal conditions from dry and wet anomalies, respectively. The shifts in the amount and occurrence probability of these thresholds associated with warm and cold ENSO phases were then quantified. It has been found that, compared to the neutral period, warm events substantially reduce (increase) the intensity and occurrence probability of autumnal drought (wet) periods, particularly for southern districts. On the other hand, when a vigorous La Nina prevails, the chance of wet (dry) conditions is low (high) and the probability of severe autumnal drought is intensified.During winters of warm ENSO phases, although most of the country receives precipitation above the drought threshold, in the southeastern and northwestern districts of Iran, the risk of winter drought is high. For these phases, there is little chance that precipitation in these areas is above the wet threshold. A mechanism is proposed to justify the seesaw fluctuation of winter precipitation over the southwestern and southeastern Caspian Sea coasts. It is likely that the interaction between the Siberianhigh and ENSO controls rainfall variability over these coasts. It was found that during cold ENSO phases, winter drought (wet) periods in southern Iran are mostly coincident with wet (dry) conditions over the tropical Bengal Gulf (TBG) region. Such a strong coincidence was not found when rainfall in southern Iran and the Indian Ocean Extension region was compared. For western and northwestern parts of Iran, the probability and intensity of winter drought was found to be low during La Nina events.