Large‐Scale and Local Atmospheric Controls on Rainfall Isotopic Variability in an Urban Area of the Central Amazon

ABSTRACT

Stable water isotopes ( ¹⁸ O and ² H) serve as sensitive tracers of atmospheric processes. In the Amazon Basin, one of the planet's major convective centres, research on the isotopic composition of rainfall declined after the 1990s, especially across central Brazil. The limited temporal resolution of earlier datasets, combined with the absence of detailed synoptic observations, has left significant gaps in understanding how isotopic variations reflect the atmospheric dynamics that control moisture transport and rainfall origins. This study addresses this gap by examining the atmospheric drivers of daily rainfall isotopic variability in recent precipitation (2023–2025) from Manaus, a major urban centre in the central Amazon. The main goal was to investigate these atmospheric controls through an integrated analysis of local meteorological observations, reanalysis products and air mass back trajectories, and to compare recent isotopic variations with historical records (1965–1990) to assess potential changes over time. Results indicate a distinct recent isotopic pattern, linked to the drier conditions prevailing in 2023–2024. This change was marked by a decrease in deuterium excess (from 13.4‰ to 11.3‰), consistently higher ¹⁸ O values, and a 1‐month advance in the isotopic values typically associated with the dry season in 2024. The analysis of daily isotopic variations identified two distinct control regimes: a large‐scale regime during the wet season (governed by the Intertropical Convergence Zone, lower isotope values) and the dry season (governed by the South Atlantic Subtropical High, high isotope values), and a thermodynamically driven local regime that dominates season transitions, promoting more isotope variability. These findings highlight the utility of stable isotopes as powerful tracers for both quantifying the influence of interannual atmospheric anomalies and disentangling the interplay between large‐scale and local controls. Capturing these regimes at high temporal resolution is crucial for assessing how urbanisation and land‐use change affect moisture budgets across the Amazon.