Stable Isotopes of Daily Precipitation in Singapore (2013/2014): A Reflection of Moisture Source and Atmospheric Circulation

Abstract:

The governing factors of stable isotopic compositions of precipitation in the tropics are highly uncertain and controversial, but the knowledge of such factors is very useful for our understanding of the regional climate. We analyzed d¹⁸O and d²H of daily precipitation samples collected from several meteorological stations in Singapore between November 2013 and the present, and examined their possible control factors. The d¹⁸O value varies from 1.41 to -17.52‰, and the d²H value from 11.92 to -126.70‰. The samples with the lowest d-values were collected during two different periods: the beginning of the 2013/2014 Asian winter monsoon and the end of the intra-monsoon season (May 2014).

Air mass trajectories and atmospheric re-analysis data reveal that some subtropical intro-seasonal systems are likely the major causes of the lowest d-values observed in precipitation; these systems brought moisture with very low d-values to the study area in December 2013 from the North Hemisphere subtropics, and in May 2014 from the South Hemisphere subtropics. During the period without significant intrusion of subtropical weather systems, local moisture from the Indian Ocean and the West Pacific Ocean is the major source for the precipitation in the area, and thus the corresponding d-values of the precipitation tend to be relatively high.

There is no significant correlation existing between d-values of daily precipitation and rain amounts. A weak negative correction, however, is observed between the d-values of daily precipitation and in-situ relative humidity. This suggests some significant evaporative enrichment in raindrops during the condensation in the dry seasons, e.g., between February and March 2014, during which the precipitation samples collected generally have the highest d-values. Further data analysis and studies are needed to investigate the influences of other climate processes on the stable isotopic compositions of daily precipitation in Singapore.