Individual and coupled effects of the ENSO and PDO on autumnal dry and wet periods in the southern parts of Iran

The El Niño-Southern Oscillation (ENSO) is strongly connected to the inter-annual tointer-seasonal variations of Sea Surface Temperature (SST) over the Pacific Oceanequators. On the other hand, the Decadal Pacific Oscillation (PDO) is related to neardecadal fluctuations of the Pacific SSTs in the northeastern parts of the ocean. Theinfluence of these oscillations on the global climate is generally more obvious when theENSO or PDO is in its extreme condition. For such circumstances, the SST anomaliesover a per-defined Ocean waters are highly positive or negative (positive or negativephase, respectively).The present study has made an effort to analyze the individual and the coupled effectsof the ENSO or PDO on the occurrence of the autumnal dry or wet periods in southernparts of Iran for the period 1951-2005. Regional maps of precipitation and vector windwere also produced to extend the outcome of the present study for the Middle Easteregion. Total precipitation during the October-December period was collected for 30 raingauge stations spread in various parts of southern Iran. In addition to precipitation,monthly values of the October-December SST anomalies over the Niño 3.4 region werealso extracted from the webpage of the National Oceanic and AtmosphericAdministration (NOAA). These monthly data were then averaged to three monthly(seasonal) records that were used as the ENSO indicator. Years related to the rank 1 to 18 and 37 to 55 (18 years each) were registered as the ENSO negative (El Niño) and positive (La Niña) phases, respectively. A similar procedure was also used to detect 18 years of the negative or positive phase of the PDO. The events that El Niño or La Niña years were coincided with the positive or negative phase of the PDO were then investigated. It was found that out of 18 years of La Nina, for 10 or 7 years, the PDO was in its negative (La- LPDO) or positive phase (La-HPDO), respectively. Similarly, out of 18 years of El Niño, the PDO was in its positive (El-HPDO) or negative (El-LPDO) phase for 8 and 5 years.For each individual station, mean precipitation for the El Niño, La Niña events as well asfor the opposite phases of the PDO were examined. Furthermore, the autumnalprecipitation was also investigated and compared for the La-LPDO, La-HPDO, El-LPDO,El-HPDO events.In addition to the rain-gauge data, the regional maps of precipitation and 850 hPa vectorwind anomalies were also produced using the http://www.esrl.noaa.gov/psd/cgibin/data/composites/printpage.pl webpage. These maps were generated for the oppositephases of ENSO, PDO and for the La-LPDO and El-LPDO periods. The GrADS softwarewas then used to overlay the produced maps of precipitation and the 850-hPa vector wind.The interpretation of these overlaid maps was found to be an efficient approach forunderstanding the impact of the considered tele-connections on the atmosphericcirculation and rain-bearing airflows.The results indicated that, for the southwestern parts of the country, precipitation hasbeen significantly suppressed or enhanced during the La Niña or El Niño event,respectively. This suppression or enhancement, however, was not significant for thesoutheastern districts. Although the above or below normal precipitation in the southernparts of Iran was generally coincided with the PDO positive or negative phase, the effectsof this oscillation on the precipitation variability were found to be significant forsoutheastern rather than southwestern parts of the country. In other words, while theprecipitation variability in the southwestern parts of the country is more sensitive to theENSO status, the PDO is more influential on the precipitation characteristics insoutheastern districts. The more dry or wet event was recognized as the periods that LaNiña or El Niño is, respectively, coincided with the negative or positive phase of the PDO(La-LPDO and El-HiPDO, respectively). For the La-LPDO events, the precipitationdeficiency was estimated as 68% to 100% for the southwestern and 37% to 67% for thesoutheastern districts. On the other hand, the occurrence of the El-HiPDO has enhancedthe autumnal precipitation by about 50% to 90% in the southwest and 30% to 50% in thesoutheast parts of Iran.According to the given results, ocurrence of the El Niño event intensifies the westerlyor southwesterly airflows which carry the Red Sea, Mediterranean Sea and Persian Gulfsmoistures to most parts of the Middle Eastern regions including most parts of Iran, Iraq,eastern coasts of the Mediterranean Sea, the Arabian Peninsula and Afghanistan.Furthermore, intensification of the near-surface easterly, southeasterly or southwesterlycirculation over northwestern parts of the Indian Ocean also pushes substantial amountsof water vapor to the Arabian Peninsula and southern parts of Iran for such spells. Thecharacteristics of atmospheric circulation during the PDO positive phase are mostlysimilar to that of the El Nino, though the latter is more vigorous than the former. Themost/least strengths of these moisture-laden circulations are associated with the El-HiPDO/La-LPDO epochs.