Effect of changes in precipitation amounts and moisture sources on inter- and intra-annual stable oxygen isotope ratios (δ18O) of teak trees from northern Thailand

Abstract

Thailand is situated in a transition zone of overlap between the Indian monsoon (IM) and the western North Pacific (WNP) monsoon. The region serves as an important area to study the influences of different moisture sources. This study presents the first stable oxygen isotope analysis on seven teak (Tectona grandis Linn.) trees collected from Phrae Province in northern Thailand at different temporal resolutions. Isotopic analyses of teak tree rings at both annual and sub-annual timescales reflected the variations in their source water oxygen isotope ratios (δ18O, i.e., rainfall) to different degrees (Annual scale: r = 0.31, p < 0.05, Monthly scale: r = 0.63, p < 0.01). A 146-year-long annually resolved tree-ring cellulose δ18O series, spanning between AD 1871 and 2016, exhibited moderate negative relationships with both local (r = −0.58, p < 0.01) and regional rainfall amounts, representing the climatology of rainfall totals during the entire summer monsoon period from May to October. A spatial correlation analysis between monthly resolved tree-ring cellulose δ18O with large-scale Climate Prediction Center (CPC), Outgoing Longwave Radiation (OLR), and Merged Analysis of Precipitation (CMAP) datasets, as well as an air mass backward trajectory analysis, indicated that the intraseasonal tree-ring cellulose δ18O values reflected changes in moisture originating from the Indian and Pacific Oceans, rather than the amount of local precipitation. These results imply that variations in northern Thai teak cellulose δ18O values are modulated not only by the local rainfall amount but also by large-scale convection, which varies between different seasons and over time. Advance knowledge of the regional monsoons in the transition area can therefore lead to a broad and complete understanding of the entire Asian monsoon circulation.