ABSTRACT Precipitation drives the terrestrial water cycle; determining its sources and variability is critical for advancing knowledge in atmospheric circulation, hydrology and climate impacts in urban montane environments. The present study analysed event‐based rainfall isotopes to assess how moisture sources, influenced by synoptic‐scale weather systems and local meteorological conditions, control precipitation during the wet season in Mexico City. We found that the isotope signature of rainfall exhibited substantial event to‐event variation (absolute range: 21.3‰ for δ 18 O and 171.7‰ for δ 2 H), reflecting the combined effects of both local and remote atmospheric processes along moisture transport pathways. Trajectory analysis indicated that most air masses travelled over inland areas or followed combined land–ocean pathways. The former was associated with air parcels originating over the Gulf of Mexico, the Pacific Ocean and continental regions, whilst the latter were mainly linked to air parcels travelling over the Caribbean Sea. Collectively, these air masses produced isotopically depleted rainfall with high deuterium‐excess values compared to those that moved predominantly over oceanic regions. The moisture attribution analysis showed that 16% of the precipitation during the studied period was mainly sourced from recycled moisture over land surface regions within the mixing layer, with the remaining fraction attributed to unidentified moisture uptake locations, highlighting the importance of local recycled moisture relative to externally advected sources. Consistently, high deuterium‐excess values in rainfall were often observed throughout the wet season (> 10‰; ~80% of the rain events sampled), providing further support for the re‐evaporated terrestrial moisture source contribution. Most rainfall events and accumulated precipitation at the study site were associated with air masses originating over continental sources and typically modulated by trough weather systems, with nearly 30% of the rainfall attributed to recycled moisture within the mixing layer. In contrast, air parcels transported from the Gulf of Mexico and the Caribbean Sea – often influenced by troughs and tropical waves – were dominated by advected moisture sources (> 90%) from unidentified spatial locations. Under these conditions, rainfall exhibited the greatest variability in isotopic composition and deuterium‐excess. Although only a few rain events associated with frontal cold systems were sampled, these events exhibited the highest levels of recycled moisture. These findings provide new insights into rainfall origin, moisture transport and sources for the Mexico City Metropolitan Area, and foundation for future climate and hydrology studies in the region.
Muñoz‐Villers et al. (2026) studied this question.