ENSO-driven variability of rainfall isotopes and moisture sources across monsoon seasons in southern Thailand

Study region Southern Thailand comprises three climatologically distinct zones: the Petchaburi–Prachuap Khiri Khan coast, the southwestern coast, and the southeastern coast, all strongly influenced by monsoon circulation and coastal–orographic interactions. Study focus This study investigates the spatial, seasonal, and interannual variability of rainfall stable isotopes (δ¹⁸O, δ²H, and d -excess) across southern Thailand to identify the dominant atmospheric and climatic controls on precipitation isotopic composition. Daily rainfall isotope observations collected from 2019 to 2022 across three coastal zones are analyzed in conjunction with monsoon classification, ENSO phase identification, and backward air-mass trajectory modeling (HYSPLIT) to resolve moisture transport pathways and circulation influences. New hydrological insights for the region Rainfall isotopes exhibit strong spatial gradients linked to coastal proximity and orography, and clear seasonal contrasts associated with monsoon dynamics. Enriched δ¹ ⁸O values dominate during the pre-monsoonal southeast flow, while pronounced depletion occurs under intense southwest monsoon rainfall; the northeast monsoon shows intermediate signatures. ENSO variability systematically alters Local Meteoric Water Lines and moisture source contributions, with trajectory analysis identifying the Indian Ocean as the dominant source and episodic Pacific and continental inputs during specific circulation regimes.