Water vapour isotopes over West Africa as observed from space: which processes control tropospheric H ₂ O ∕ HDO pair distributions?

Abstract. The West African Monsoon (WAM) is crucial for rainfall in West Africa, impacting socio-economic conditions. Its complexity arises from interactions between large-scale circulation, convective dynamics, and microphysical processes, making it challenging to disentangle individual contributions to the hydrological cycle. Recent advances in retrieving the isotopic composition of tropospheric water vapour from space promote the paired analysis of H2O and HDO to study atmospheric moisture pathways and processes. Using data from the satellite instruments IASI (Infrared Atmospheric Sounding Interferometer), AIRS (Atmospheric Infrared Sounder) and TROPOMI (Tropospheric Monitoring Instrument), along with the IMERG (Integrated Multi-Satellite Retrievals for GPM) precipitation product, we analyse the variability of H2O and HDO (given as δD) over West Africa at convective and seasonal scales. Key findings include the following: (1) monsoon convection over the Sahel induces an anti-correlation between H2O and δD in the mid-troposphere. This is due to dry intrusions from the Saharan upper troposphere into Sahelian squall lines, fostering rain evaporation and mid-tropospheric δD depletion. (2) Over the Guinea coast, convective precipitation is associated with moist and enriched signals, with surface evaporation from the tropical Atlantic reducing rain evaporation and δD depletion. (3) During the Sahelian monsoon peak, an anti-correlation between precipitation and δD forms year to year, indicating the amount effect in tropospheric water vapour. (4) In the Sahelian winter, when precipitation is minimal, {H2O, δD} signals point to mixing of dry air masses of different origins. This study is the first to apply comprehensive isotopic datasets from IASI, TROPOMI and AIRS to the WAM, demonstrating the utility of satellite-based {H2O, δD} pairs in detecting impacts of microphysical and dynamical processes on water vapour isotopic composition.