One-year continuous observations of near-surface atmospheric water vapor stable isotopes at Matara, Sri Lanka, reveal a strong link to moisture sources and convective intensity

Abstract. Atmospheric water vapor stable isotopes are crucial for understanding hydrological cycle processes under climate change. This study presents the results from a year-long in situ monitoring of atmospheric water vapor stable isotopes (δ18O, δD) at Matara, Sri Lanka, from March 2020 to February 2021 to assess how oceanic sources and moisture transport influence coastal atmospheric moisture isotopic composition. We identified clear seasonal patterns in the isotopic composition, with δ18O, δD, and d-excess showing substantial variation between the southwest and northeast monsoon periods. The primary moisture sources were the Arabian Sea and the Indian Ocean during the southwest monsoon (May to September), characterized by small amplitude fluctuations of δ18O (−20.4 ‰ to −9.1 ‰). During the northeast monsoon, the northern Bay of Bengal, the Indian subcontinent, and Southeast Asia were primary moisture sources, resulting in large-amplitude fluctuations in δ18O (−23.9 ‰ to −7.5 ‰) and higher d-excess values (up to 25 ‰). The study also identified significant influences of sea surface temperature and sea surface relative humidity on the isotopic composition of water vapor. Additionally, we could use outgoing longwave radiation (OLR) to gauge the intensity of convective activity. Observational periods with low OLR, indicative of stronger and deeper convection, were associated with air masses that were more depleted in δ18O than periods with high OLR. These findings facilitate a better understanding of how the monsoon and local meteorological conditions affect water vapor isotope compositions in tropical regions. Furthermore, the new dataset will enable us to improve water vapor isotopic modeling and projections of atmospheric processes in coastal regions.