We present here a chronology of tree ring cellulose δ18O for the period 1588-2002 based on individual measurements of seven Fokienia hodginsii trees growing in northern Laos. Response function analysis of meteorological data revealed that this oxygen isotope chronology has a significant negative correlation with monsoon season precipitation, the water level of the Mekong River, and the Palmer drought severity index (PDSI). Our reconstructed monsoon season PDSI, which accounts for 41.5% of PDSI variance, showed that wetter phases occurred during the periods AD 1660-1695 and AD 1705-1790, that the main drier periods were AD 1630-1660, AD 1900-1940, and AD 1954-2002, and that there has been a trend of decreasing moisture during the monsoon season over the last 200years. A reduction in monsoon activity can also be seen in various tree ring oxygen isotope records from the Himalaya, Tibet Plateau, and Southeast Asia. Rising sea surface temperatures over the tropical Pacific and Indian Ocean could be responsible for this reduction in the Asian summer monsoon. By combining proxies sensitive to the El Niño-Southern Oscillation (ENSO) in northern Laos and Vietnam, we were able to reconstruct the annual multivariate ENSO index (MEI) and local ENSO event history, and so improve our understanding of long-term variations in ENSO and its influences on Southeast Asia. •The long δ18O chronology built just by tree ring oxygen isotopes was robust.•ENSO event history during the last 400years was reconstructed in Southeast Asia.•Reduction of Asian monsoon activity during the last 200years was found.
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