Résumé : The isotopic composition of water vapour in the North Atlantic free troposphere is investigated with Infrared Atmospheric Sounding Interferometer (IASI) measurements of the D/H ratio (δD) above the ocean. We show that in the vicinity of West Africa, the seasonality of δD is particularly strong (130 ‰), which is related with the influence of the Saharan heat low (SHL) during summertime. The SHL indeed largely influences the dynamic in that region by producing deep turbulent mixing layers, yielding a specific water vapour isotopic footprint. The influence of the SHL on the isotopic budget is analysed on various time and space scales and is shown to be large, highlighting the importance of the SHL dynamics on the moistening and the HDO enrichment of the free troposphere over the North Atlantic. The potential influence of the SHL is also investigated on the inter-annual scale as we also report important variations in δD above the Canary archipelago region. We interpret the variability in the enrichment, using backward trajectory analyses, in terms of the ratio of air masses coming from the North Atlantic and air masses coming from the African continent. Finally, the interest of IASI high sampling capabilities is further illustrated by presenting spatial distributions of δD and humidity above the North Atlantic from which we show that the different sources and dehydration pathways controlling the humidity can be disentangled thanks to the added value of δD observations. More generally, our results demonstrate the utility of δD observations obtained from the IASI sounder to gain insight into the hydrological cycle processes in the West African region.