Temporal variation of stable isotopes in precipitation at Bangkok, Thailand

Some studies discussed spatial and temporal variability of the stable isotope composition of precipitation in the Asia Pacific region. However, the relationship between isotope signature of precipitation and climate is not well understood, because the long-term isotopic data in precipitation is limited. The purpose of this study is to understand the temporal variation of stable isotopes in precipitation at Bangkok and precipitation mechanism in Monsoon Asia region in response with El Nino/Southern Oscillation. The monthly averaged stable isotopes (Oxygen-18, Deuterium) in precipitation and meteorological data for the period of 1968 to 1995 at Bangkok (13.73 deg. N, 100.5 deg. E), which are provided by the Global Network for Isotopes in Precipitation. The vertically integrated precipitable water and moisture flux datasets in NCEP/NCAR global atmospheric reanalysis with 2.5 deg. x 2.5 deg. resolution are used to determine the circulation anomalies for the period of 1979 to 1995. To consider the influence of El Nino/Southern Oscillation to precipitation in Bangkok, correlations between oxygen-18 of precipitation and SST in Nino-3 region (that is ENSO index) are considered for each month. The positive correlations with statistically 95% significant level are found only May and October, which is onset and offset period of Asian Monsoon in Thailand. The correlation coefficient is 0.39 for May and 0.62 for October. The isotopic low-phase years in May are 1981, 86, 88 and 91, and isotopic high-phase years are 1979, 80, 83, 87, 92, 94 and 95. Also, the isotopic low-phase year in October are 1988, 90 and 95, and isotopic high-phase years are 1979, 80, 82, 91 and 94. The anomaly of moisture flux is westward (eastward) in the Indian Ocean and eastward in the Pacific Ocean, respectively. Anomaly of moisture flux is shown convergence (divergence) pattern centered in the Indochina Peninsula and westward (eastward) in the Equatorial Pacific in low (high) phase. These PW and moisture flux anomalies suggest the close connection to the ENSO