INDIVIDUAL AND COUPLED EFFECTS OF THE ENSO AND PDO ON THE INCIDENCE OF AUTUMNAL DRY AND WET PERIODS IN IRAN

Abstract

The El Niño-Southern Oscillation (ENSO) is strongly connected to the inter-annual to intra-seasonal variations of Sea Surface Temperature (SST) over the Pacific Ocean equators. On the other hand, the Decadal Pacific Oscillation (PDO) is related to quasi-decadal fluctuations of the Pacific SSTs in the north-eastern parts of the ocean. The present study has made an effort to analyse the individual and the coupled effects of the ENSO and PDO on autumn precipitation in Iran for the period 1951-2005. Total precipitation data was collected for 41 rain-gauge stations spread in various parts of the country. Moreover, concurrent SST anomalies over the Niño 3.4 region were used as the ENSO indicator. Years related to the rank 1 to 18and 37 to 55 (18 years each) were categorized as the ENSO negative (El Niño) and positive (La Niña) phases, respectively. After obtaining the PDO data, similar procedure was also used to detect 18 years of the high or low phase of the PDO (PPDO or NPDO, respectively). The events that El Niño or La Niña years were coincided with the positive or negative phase of the PDO were then investigated. The results indicated that, precipitation has significantly suppressed or enhanced during the La Niña or El Niño event, respectively. Although the above or below normal precipitation was generally coincided with the PDO positive or negative phase, the effects of this Oscillation on precipitation variability were generally found to be significant for south-east rather than south-west of the country. While precipitation variability in south-west parts of the country was more sensitive to ENSO, the PDO was more influential on precipitation variability in southeast districts. The more dry or wet event was recognized as the periods that La Niña or El Niño is, respectively, coincided with the negative or positive phase of the PDO (La-LPDO and El-HPDO