The intraseasonal and interannual variability of the West African monsoon greatly
impacts water resource security and flood risks in the intertropical regions of Africa. The stable
isotope composition of precipitation may help to understand and monitor the processes
controlling the monsoon cycle. It is controlled by the different steps of the atmospheric water
cycle, i.e. seawater evaporation, mixing with continental vapor, Rayleigh processes during
rainfall formation, and post-condensation processes such as partial raindrop evaporation.
However, in the West African region where the monsoon duration is often less than 6 months,
the current monthly time step of available data (e.g. the Global Network of Isotope in
Precipitation) provides limited information on intraseasonal variations. In addition to more
classical tracers (δ18O and d-excess), the triple oxygen isotope composition of precipitation,
expressed by 17O-excess, is a novel tracer, whose potential has not yet been fully explored. In
contrast to d-excess, the 17O-excess is not significantly impacted by temperature-dependent
equilibrium fractionation and therefore not sensitive to Rayleigh distillation, but highly
sensitive to evaporation fractionation. In this study, we present the first long-term and high
frequency record of δ18O, d-excess 17O-excess in precipitation from a West African site.
Precipitation was collected at quasi-event scale since April 2018 at the AMMA-Catch
observatory in Benin (http://www.amma-catch.org/), in the framework of the HUMI-17/ ANR
project (https://anr.fr/Projet-ANR-17-CE01-0002). The station (lat. 9°44’ N; long. 1°34’ E) is
located 400 km north of the Atlantic coast, downwind from the main oceanic air mass
trajectories coming from the Gulf of Guinea, identified using the HYSPLIT lagrangian air mass
back trajectory model. The triple oxygen and hydrogen isotope compositions of precipitation
samples were measured at CEREGE using a WS-CRDS Picarro L2140-i. The long-term
reproducibility is 6 per meg for 17O-excess, 0.023‰ for δ18O and 0.17‰ for d-excess
respectively. We assess the magnitude and timing of interannual and intraseasonal 17O-excess
variations, together with d-excess and δ18O, and evaluate how the combination of these
different tracers can inform on the monsoon characteristics, the occurrence of raindrop reevaporation
in relation with local climatic drivers, and the potential influence of hydroclimatic
conditions in the source regions of the Tropical Atlantic.

Isotopic composition precipitation North Benin