Abstract
To investigate stable isotopic variability of precipitation in Singapore, we continuously analysed the -value of individual rain events from November 2014 to August 2017 using an online system composed of a diffusion sampler coupled to Cavity Ring-Down Spectrometer. Over this period, the average value (O-18(Avg)), the lowest value (O-18(Low)), and the initial value (O-18(Init)) varied significantly, ranging from -0.45 to -15.54 parts per thousand, -0.9 to -17.65 parts per thousand, and 0 to -13.13 parts per thousand, respectively. All 3 values share similar variability, and events with low O-18(Low) and O-18(Avg) values have low O-18(Init) value. Individual events have limited intraevent variability in -value () with the majority having a below 4 parts per thousand. Correlation of O-18(Low) and O-18(Avg) with O-18(Init) is much higher than that with , suggesting that convective activities prior to events have more control over -value than on-site convective activities. The d-excess of events also varies considerably in response to the seasonal variation in moisture sources. A 2-month running mean analysis of O-18 reveals clear seasonal and interannual variability. Seasonal variability is associated with the meridional movement of the Intertropical Convergence Zone and evolution of the Asian monsoon. El Nino-Southern Oscillation is a likely driver of interannual variability. During 2015-2016, the strongest El Nino year in recorded history, the majority of events have a O-18 value higher than the weighted average O-18 of daily precipitation. O-18 shows a positive correlation with outgoing longwave radiation in the western Pacific and the Asian monsoon region, and also with Oceanic Nino Index. During El Nino, the convection centre shifts eastward to the central/eastern Pacific, weakening convective activities in Southeast Asia. Our study shows that precipitation -value contains information about El Nino-Southern Oscillation and the Intertropical Convergence Zone, which has a significant implication for the interpretation of water isotope data and understanding of hydrological processes in tropical regions.