A-year Continuous Observations of Near-Surface Atmospheric Water Vapor Stable Isotopes at Matara, Sri Lanka

Abstract. Atmospheric water vapor stable isotopes are crucial for understanding hydrological cycle processes under climate change. This study presents 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 depleted δ¹⁸O from -20.4 ‰ to -9.1 ‰. During the northeast monsoon dominated period, the northern Bay of Bengal, the Indian subcontinent, and Southeast Asia were primary moisture sources, displayed enriched 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, outgoing longwave radiation (OLR) is a significant index used to gauge the intensity of convective activity. Lower OLR values, indicative of stronger and deeper convection, were associated with more depleted δ¹⁸O in air masses. These findings help to improve the understanding of influences of the monsoon and local meteorological condition on water vapor isotopes in tropical region and provide new dataset on enhancing water vapor isotopic modeling or atmospheric processes projection in coastal regions.