ABSTRACT
A 338-year oxygen isotope record from teak tree-ring cellulose collected from Mae Hong Son province in northwestern Thailand was presented. The tree-ring series preserves the isotopic signal of the regional wet season rainfall and relative humidity. Tree-ring δ18O correlates strongly with regional rainfall from May to October, showing coherent variations over large areas in Southeast Asia. We reconstructed the summer monsoon season (May to October) rainfall based on a linear regression model that explained 35.2% of the actual rainfall variance. Additionally, we found that in the 19th century, there was a remarkable drought during many years that corresponded to regional historic drought events. The signals of the June to September Indian summer monsoon (ISM) for the period between 1948 and 2009 were clearly found. Spatial correlations and spectral analyses revealed a strong impact of the El Niño-Southern Oscillation (ENSO) on tree-ring δ18O. However, ENSO influenced the tree-ring δ18O more strongly in the 1870-1906, 1907-1943, and 1944-1980 periods than in the 1981-2015 period, which corresponded to periods of weaker and stronger ISM intensity.
ABSTRACT
Thailand is a predominantly agricultural country. An understanding of the dominant driver of decadal-scale changes in Thailand monsoon (TM) rainfall trends is particularly important in terms of agro-meteorological information and monsoon predictions. In this study, a 194-year tree ring chronology of teak trees in northwestern Thailand was developed. Correlations between the tree ring width index (i.e., the Susa index) and climate variables confirmed that this index can be used as a proxy for rainfall in the early monsoon season from May to July. Similar variations with other regional tree ring chronologies confirmed the reliability of the climate signals embedded in the tree ring widths. The possible relationship between the Susa index-based TM rainfall and the Pacific Decadal Oscillation (PDO) was examined. Spectral analysis showed statistically significant PDO periodicities of between 22 and 24 years. The spatial correlations detected across the key regions of the PDO revealed associations with the north Pacific sea surface temperature during recent decades. The long-term relationships between the Susa index and the PDO were nonstationary at the decadal timescale. Positive correlations were found for AD 1824-1875 and AD 1900-1955, whereas negative relationships prevailed for AD 1876-1899 and 1956-2017. The El Niño Southern Oscillation-related anomalous TM was indeed stronger during both the warm and cold phases of the PDO. The PDO is therefore identified as a driving factor of decadal climate variability. This study leads the way to understanding the changes in the TM-PDO relationship over time and demonstrates the utility of teak tree ring width as a potential proxy for PDO teleconnection.
