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 δ 18 O 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 19 th 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 δ 18 O. However, ENSO influenced the tree-ring δ 18 O 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.

Citation: Cai, B., N. Pumijumnong, M. Tan, C. Muangsong, X. Kong, X. Jiang, and S. Nan (2010), Effects of intraseasonal variation of summer monsoon rainfall on stable isotope and growth rate of a stalagmite from northwestern Thailand, J. Geophys.

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Thailand has been regarded as a transition zone between the Indian and western North Pacific monsoons which produce most of the precipitation in this area and sustain large-scale agricultural activities. Their different moisture sources and associated transport trajectories could result in distinguishable seasonal variations in rainfall oxygen isotopes (δ18O). Knowledge about seasonal variability in Thailand monsoon could facilitate agricultural water management and improve our understanding of the Asian monsoon system. In this study, a detailed examination of the intra-seasonal variability of tree-ring cellulose δ18O in teak trees from Northwestern Thailand was performed. The results show clear intra-seasonal variability in cellulose δ18O, with heavier values in the early rainy season and much lighter values in the later rainy season. Climate correlation analysis indicates that there is a significant and positive correlation between sub-seasonal variation in cellulose δ18O and rainfall δ18O. A weak and negative correlation with relative humidity (RH) has also been identified. However, there is no relation between intra-seasonal variation in cellulose and that of the amount of rainfall. These results imply that intra-seasonal variability of cellulose δ18O in teak trees from Northwestern Thailand may inherit such variability in rainfall δ18O and might be slightly shaded by RH. The spatial correlations with large-scale precipitation in the Merged Analysis (CMAP) and Outgoing Longwave Radiation (OLR) data sets indicated seasonal changes in moisture origin and may potentially be used to explore long-term monsoon climate variability on a seasonal scale in subtropical Southeast Asia.

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Teak ring-width is one of the promising paleoclimate proxies in the tropical region. Tree-ring chronology spanning from 1840 to 2016 (177 years) was derived from seventy-six trees from Phrae Province, northern Thailand. A total of 141 core samples were cross-dated, a standardized master was constructed, and the tree residual master chronology was developed by ARSTAN program. The tree-ring chronology has a significant positive correlation with the monthly rainfall and relative humidity during the monsoon season (May - June). In addition, the growth of tree-ring width also significantly inversely correlated with Niño 3, Niño 3.4, and Niño 4 indices during the second half of the dry season (January - March). We reconstructed summer monsoon season (May - June) rainfall based on a linear regression model which explained 21.95% of the actual rainfall variance. The trend of the reconstructed rainfall record shows a decrease of 0.6 mm per decade and substantially showed four wet periods and five dry periods. These results suggest that this teak chronology has a good potential to be a high-resolution proxy for reconstructing the past local climate in northern Thailand.

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