HYDROLOGICAL PROCESSES Hydrol. Process. 19, 3413–3423 (2005) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5978 Interannual variation of stable isotopes in precipitation at Bangkok in response to El ˜ Nino Southern Oscillation Kimpei Ichiyanagi 1 * and Manabu D. Yamanaka 2 1 Institute of Observational Research for Global Change (IORGC), Japan Agency for Marine–Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan 2 IORGC/Graduate School of Science and Technology, Kobe University, Kobe, Japan Abstract: Evidence for a close relationship between the interannual variation of stable isotopes in precipitation and the El ˜ Nino Southern Oscillation (ENSO) is presented for Bangkok, Thailand. Anomalies of sea surface temperature in the Ni˜ no-3 region of the equatorial Pacific (ENSO index) and υ 18 O in precipitation in Bangkok were positively correlated for May and October. The composite mean precipitation was much greater in the isotopic low phase than in the high phase for both May and October. This suggests that the amount of precipitation is the main factor determining the observed variation of stable isotopes in precipitation in Bangkok. Composite analyses of 850 hPa temperature, evaporation, outgoing longwave radiation (OLR), and precipitation showed that the variation in the amount of precipitation in Bangkok is a response to the ENSO–Asian summer monsoon coupling in May, and a direct response to ENSO in October. The composite mean d-excess values in both the low and high phases in October and in the low phase in May were about 10‰, and were less than 7‰ during the high phase in May. A large difference in the evaporation field between the low and high phases of May exists over the Indian Ocean, suggesting that evaporation was not in equilibrium during the high phase in May. Future studies will consider the precipitation amount effect based on daily or event-based sampling. Copyright  2005 John Wiley & Sons, Ltd. KEY WORDS stable isotopes; precipitation; Asian monsoon; ENSO; water vapour origin INTRODUCTION Stable water isotopes (υ 18 O, υD) are hydrologically conserved and can contribute to a better understanding of the global hydrological cycle. Dansgaard (1964) reported the global distribution of stable isotopes in precipitation, and discussed the two effects that influence the isotopic composition of precipitation: temperature and precipitation amount. The temperature effect is used as a palaeo thermometer to reconstruct past climate, mainly in polar regions. The precipitation amount effect is a linear relationship between the amount of precipitation and the isotopic composition of precipitation observed from the tropical to mid-latitude regions. Stable water isotopes are useful tracers for climatic events, such as El ˜ Nino Southern Oscillation (ENSO) and monsoons. Ichiyanagi et al. (2002) showed how ENSO teleconnection patterns in the South Pacific affect Antarctic climate and found a strong negative correlation between υ 18 O in Antarctic precipitation and ENSO. Aragu´ as-Aragu´ as et al. (1998) described spatial and temporal variability in the stable isotopes in precipitation over the Asia–Pacific region and remarked on the obvious precipitation amount effect caused by the Asian summer monsoon. In addition, some studies have used atmospheric general circulation models to simulate the isotopic signal affected by precipitation amount, temperature, moisture source, and atmospheric circulation in tropical regions (e.g. Cole et al., 1999; Jouzel et al., 2000). ENSO signals in the water isotopes of precipitation were seen over * Correspondence to: Kimpei Ichiyanagi, Institute of Observational Research for Global Change, Japan Agency for Marine–Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan. E-mail: kimpei@jamstec.go.jp Received 4 August 2003 Copyright  2005 John Wiley & Sons, Ltd. Accepted 27 March 2004