دانشکده کشاورزی، مرکز پژوهشهای علوم جوّی اقیانوسی دانشگاه شیراز، شیراز، ایران
عنوان مقاله [English]
The El Niño-Southern Oscillation (ENSO) is strongly connected to the inter-annual to inter-seasonal variations of Sea Surface Temperature (SST) over the Pacific Ocean equators. On the other hand, the Decadal Pacific Oscillation (PDO) is related to near decadal fluctuations of the Pacific SSTs in the northeastern parts of the ocean. The influence of these oscillations on the global climate is generally more obvious when the ENSO or PDO is in its extreme condition. For such circumstances, the SST anomalies over a per-defined Ocean waters are highly positive or negative (positive or negative phase, respectively).
The present study has made an effort to analyze the individual and the coupled effects of the ENSO or PDO on the occurrence of the autumnal dry or wet periods in southern parts of Iran for the period 1951-2005. Regional maps of precipitation and vector wind were also produced to extend the outcome of the present study for the Middle Easte region. Total precipitation during the October-December period was collected for 30 rain-gauge 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 were also extracted from the webpage of the National Oceanic and Atmospheric Administration (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 as for the opposite phases of the PDO were examined. Furthermore, the autumnal precipitation 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 vector wind anomalies were also produced using the http://www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl webpage. These maps were generated for the opposite phases of ENSO, PDO and for the La-LPDO and El-LPDO periods. The GrADS software was 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 for understanding the impact of the considered tele-connections on the atmospheric circulation and rain-bearing airflows.
The results indicated that, for the southwestern parts of the country, precipitation has been significantly suppressed or enhanced during the La Niña or El Niño event, respectively. This suppression or enhancement, however, was not significant for the southeastern districts. Although the above or below normal precipitation in the southern parts of Iran was generally coincided with the PDO positive or negative phase, the effects of this oscillation on the precipitation variability were found to be significant for southeastern rather than southwestern parts of the country. In other words, while the precipitation variability in the southwestern parts of the country is more sensitive to the ENSO status, the PDO is more influential on the precipitation characteristics in southeastern 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-HiPDO, respectively). For the La-LPDO events, the precipitation deficiency was estimated as 68% to 100% for the southwestern and 37% to 67% for the southeastern districts. On the other hand, the occurrence of the El-HiPDO has enhanced the autumnal precipitation by about 50% to 90% in the southwest and 30% to 50% in the southeast parts of Iran.
According to the given results, ocurrence of the El Niño event intensifies the westerly or southwesterly airflows which carry the Red Sea, Mediterranean Sea and Persian Gulf's moistures 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 southwesterly circulation over northwestern parts of the Indian Ocean also pushes substantial amounts of water vapor to the Arabian Peninsula and southern parts of Iran for such spells. The characteristics of atmospheric circulation during the PDO positive phase are mostly similar to that of the El Nino, though the latter is more vigorous than the former. The most/least strengths of these moisture-laden circulations are associated with the El-HiPDO/La-LPDO epochs.