Rainwater isotopic composition variability during extremely rainy years in the São Francisco River headwaters, southeastern Brazil

The stable isotope composition of oxygen (δ¹⁸O) and hydrogen (δ²H) in precipitation is essential for tracing hydrometeorological processes and understanding their impacts on surface and groundwater. This study investigates the seasonal isotopic variability of precipitation in the headwaters of the São Francisco River, southeastern Brazil, highlighting the influence of local and regional meteorological parameters and large-scale atmospheric phenomena such as the South Atlantic Convergence Zone (SACZ). Stable isotope data of oxygen and hydrogen were obtained from 33 monthly precipitation samples, together with meteorological records (precipitation, temperature, relative humidity). Additional information was derived from ERA‑5 satellite products (total column water vapor and vertical velocity) and backward air‑mass trajectories using the HYSPLIT model, to assess the role of moisture availability and convection in isotopic variability. Comparative analysis between dry and wet seasons revealed seasonal patterns and highlighted the influence of the South Atlantic Convergence Zone (SACZ), which intensified isotopic depletion during prolonged rainfall events. Isotopic seasonality is explained by distinct moisture transport patterns between wet and dry seasons, reflecting the dynamics of large-scale atmospheric circulation.