Regional atmospheric dynamics govern interannual and seasonal stable isotope composition in southeastern Brazil

Abstract

Climatic factors controlling seasonal and interannual changes of stable isotope composition in precipitation remains understudied across the tropics. Daily isotopic measurements of rainfall (2014–2017, Rio Claro, Southeast of Brazil) were used to identify the climatic factors governing the isotopic variability. Overall, precipitation presented large variations, ranging between −18.36‰ and +4.89‰ for δ18O, −136‰ to + 43.4‰ for δ2H, and −11.8‰ to +26.6‰ for d-excess. The Local Meteoric Water Line (δ2H = 8.05 ± 0.1·δ18O + 13.08 ± 0.5) indicated a strong influence of moisture recirculation along the transport of air masses. During the summer (wet season), high moisture content prevailed in the atmosphere. The latter was associated with recycled moisture from Amazonia region due to the influence of the South Atlantic Convergence Zone and Frontal Systems, resulting in depleted precipitation (δ18O ≤ −6‰). During the winter (dry season), the combination of Cold Fronts and minimal moisture transport from the Amazonia region, resulted in more enriched rainfall (δ18O ≥ −1‰). Atmospheric recirculation processes along the moisture pathway were also reflected on the seasonal d-excess variation. The strong correlation of δ18O and relative humidity along HYSPLIT trajectories confirmed the influence of regional atmospheric dynamics over isotopic composition of precipitation. Our findings provide new understanding of rainfall generation within the Guarani aquifer recharge area.