Stable isotope characteristics of precipitation in Malaysia: establishment of local meteoric water line

This paper discussed the use of stable isotope compositions of precipitation (δ²H and δ¹⁸O) to understand its characteristics and the parameters controlling meteorological conditions in Malaysia. The meteoric water line for Malaysia, or MMWL established in this study are δ²H = (7.43 ± 0.05)δ¹⁸O + (7.33 ± 0.11) (n = 595, r² = 0.97) for individual values, and δ²H = (7.07 ± 0.20)δ¹⁸O + (4.90 ± 0.75) (n = 51, r² = 0.96) for annual precipitation weighted means. No major variations in local meteoric water lines established between the stations, except for one (i.e., Cameron Highlands), which was found to be significantly different due to its high deuterium excess, or d value, attributed to moisture recycling (re–evapotranspiration) from the surrounding dense rainforest area. The precipitation moisture came from the two monsoon systems; the Northeast monsoon (NEM) and Southwest monsoon (SWM). They differ only slightly in their isotope signatures, where the NEM (winter monsoon) is 0.38‰ more negative than the SWM (summer monsoon), and the rainfall amount and temperature have a relatively weak influence on the stable isotopes in precipitation. Rainfall δ¹⁸O is poorly correlated with the monthly rainfall amount and temperature, with very low r² values (–0.4‰/100 mm; 0.09 and 0.7‰/°C; 0.06, respectively). The seasonal variability of isotopic compositions of precipitation in this region is controlled by the insubstantial but conspicuous inverse correlation amount effect. The seasonal patterns of δ²H and δ¹⁸O are evident, with higher values observed during the NEM and lower values observed during SWM. However, the temporal variations of rainfall δ²H and δ¹⁸O during NEM and SWM are almost indistinguishable due to local climatic conditions, prevailing atmospheric circulations, and sources of precipitation. The apparent sign of El Niño and La Niña events (2015 and 2017, respectively) of the El Niño–Southern Oscillation (ENSO) cycle were successfully identified from this study, indicated by the heavier δ¹⁸O from the peaks in the annual precipitation amount versus δ¹⁸O annual precipitation weighted mean plot, while La Niña corresponds to the lighter values of the rainfall isotopes.