Investigation of relationship between southern oscillation index (SOI) and reconstructed precipitation of central Zagros

Iran’s precipitation mainly affects by air masses that arrive to it from adjacent area directly or affects as teleconnection pattern due to remoteness from water sources. Due to irregular arrival of the air masses to Iran and far distance to teleconnection centers، occurrence of precipitation variations is one of the prominent characteristics of Iran’s climate. Also due to short period of instrumental data in the meteorological stations، there is no possibility for understanding of the climatic variations during the last centuries. Reconstruction of climatic variations based on current evidences، shows a more realistic understanding of the climatic variations of the region. Trees are the useful climatic evidences that experience different climatic situation of temperature and precipitation during their life period and the effects of these climatic variations is reflected in their tissues. In temperate climates، trees add a ring annually to their previous rings. Therefore، by studying of the tree-ring widths، useful climatic information can be obtained over the last centuries of the region. In the previous studies، relationship between the Southern Oscillation Index (SOI) and Iran’s precipitation only has been studied in a relatively short period due to the lack of instrumental data. In this study، we present a longer period of relationship between monthly precipitations of the region with Southern Oscillation Index (SOI) by reconstruction of precipitation based on Oak tree rings in the central Zagros region over the last centuries (1705-2010). In the present paper، three sites of Oak species with a short distance to the meteorological stations were selected in Lorestan (Shineh)، Kermanshah (Faryadras) and Ilam (Dalab) provinces. In Shineh and Faryadras sites Quercus infectoria species and in Dalab site Quercus Persica species used for sampling. Two increment cores at different sides of each tree were extracted by a Suunto increment borer at breast high. In each site 10 to 14 trees were sampled. After air drying، the surface of cores was prepared by a razor blade. The ring widths were measured with a LINTAB5 measuring system with a resolution of 0. 01 mm، and all cores were cross-dated by visual and statistical tests (sign-test and t-test) using the software package TSAP-Win. Raw ring widths were standardized by ARSTAN program to remove non climatic signals. Three site chronologies were obtained from the three sites. To increase the length of the chronology and due to the same trends and high correlations between the three site chronologies، all growth curves were combined and the regional chronology was calculated. The values of mean sensitivity (MS)، signal to noise ratio (SNR) and first auto correlation (AC1) were calculated for the regional chronology. Mean monthly precipitation of Khorramabad (1951-2010)، Kermanshah (1951-2010) and Ilam (1987-2010) meteorological stations were used to calibrate the tree-ring/climate relationship. Due to the high correlation (p<0. 01) between the regional chronology and mean monthly precipitation of the region، October-May، precipitation of the region was reconstructed using a linear regression model. For investigation of relationship between the reconstructed wet years and the El Nino events، Historical El Nino events from 18th century to present were obtained from NOAA database. The longest value of SOI (1876-2010) was obtained from Bureau meteorological station. Finally the relationship between October-May monthly values of SOI and the reconstructed October-May precipitation of central Zagros was investigated in the common period (1877-2010). Similar growth patterns of trees in the three sites indicate that they influence by the same growth factors. Relationship and correlations between monthly precipitation and the regional chronology showed that precipitation has positive effect on tree growths in the region. The positive correlation with precipitation during the pre-growing season show the fact that precipitation during these months provides moisture availability in semi-arid regions and favors formation of early wood cells، which account for the majority of the total ring-width (Liu et al.، 2011). Compression between the reconstructed precipitation and the actual precipitation of the central Zagros region shows some years with inconsistency between observed and reconstructed data. Most of the reconstructed wet years have been occurred coincidence with El Nino events. There is a negative relationship between the reconstructed October-May precipitation of central Zagros and Southern Oscillation Index (SOI). It should be noted that negative values of SOI indicate El Niño events and during the El Niño phases precipitation in most parts of Iran is above average. According to this، Southern Oscillation Index (SOI) during October to May has positive effect on precipitation in central Zagros. The highest correlation (p<0. 01) between SOI and the reconstructed precipitation was found on November and December which is supported by the results of Azizi (2000)، Yarahmadi and Azizi (2007) and Khoshakhlagh (1998). Also there is a significant correlation (p<0. 05) between SOI and the reconstructed precipitation in April and May. It may be due to the far distance between the study region and the El Nino origin. Therefore the study region during April-May influence by El Nino with a lag time. The length of the regional chronology is 305 years (1705-2010). Total and monthly precipitations from October to May have positive effect on tree growths in the region. The results showed that dry conditions occurred in 1730s، 1760s-1790s، 1810s، 1840s-1850s، 1870s-1880s، 1900s، 1920s، 1940s، 1960s and 1980s. Comparison of wet years and historical El Nino events showed that 77 of reconstructed wet years have occurred with El Nino events simultaneously. Investigation of relationship between the Southern Oscillation Index (SOI) and the reconstructed October-May precipitation of the central Zagros region showed that SOI has positive effect on precipitation in the region. The highest negative correlation between the Southern Oscillation Index (SOI) and the reconstructed precipitation was found in December. The highest correlation (p<0. 01) between SOI and the reconstructed precipitation of the region was found in November، December and January. There was a significant correlation (p<0. 05) between SOI and the reconstructed precipitation in April and May as well.