Water vapour is the primary greenhouse gas, itself causing around 60% of the effect. Nevertheless, the cloud processes linked to it are one of the primary sources of uncertainty in climate projections, particularly for tropical and subtropical regions such as the Sahel.
As a result, it is crucial to understand the transport, the sources and the reserves of atmospheric humidity. Researchers from the HSM unit at the IRD and their French and Nigerian partners from the LSCE and the IRI(1) have developed a new measurement technique to further this understanding.
This innovative method records the isotopic composition(2) of water vapour in situ. It consists of a laser technique, based on the absorption spectra of the different water molecules that make up atmospheric vapour.
The stable water isotopes are a well-known geochemical tracer for reconstituting historic climate variations, using archives such as ice cores. But their capacity to teach us about current variability has not been established. Using a new technique enabling the observation of these isotopes in atmospheric vapour, the researchers have demonstrated that they are far more sensitive to physical atmospheric processes than to basic meteorological variables such as humidity or pressure. They thus offer a means of precisely studying elements and phenomena as diverse as the origin of masses of air, cloud formation or the different types of humidity transport.
This technique has been used by the research team at the IRI campus in Niamey, with financial support from the IRD. Under the leadership of this Nigerian institute, isotopic measurements have been taking place over the Sahel continually since 2010.
Water from the Mediterranean
Climatologists have analysed these measurements over the course of a full year, including the monsoon period in 2010 and the dry season that followed. Unexpectedly, the isotopic date showed a very strong variability in the availability of atmospheric water over the course of the year. And this was true even during the dry season, where it was understood that the absence of rain meant that there was no activity over the desert. The study also shows that this water vapour is due to regular intrusions of humid air from North Africa, with an eruption during January and February – the height of the dry season. This result suggests that the Sahel climate depends on very slight changes in atmospheric circulation at a regional level, particularly in the Mediterranean. In addition, this humidity, already present when the monsoon arrives in the Sahel several weeks later, has an important role to play in triggering rainfall. The research is also shedding new light on the variability of the tropical atmosphere outside of the monsoon period, scarcely documented until now.
Deconstructing the squall line
The temporal resolution of the data acquired (a few seconds) has enabled the specialists to deconstruct the genesis and the propagation of typical squall lines for monsoon thunderstorms. They have closely studied hydration processes (such as the evaporation of water droplets) and the inverse dehydration processes (such as the dry air caused by descending vertical air currents) that directly control the availability of water in the atmosphere. Water isotopes have varying characteristics depending on the process that created them, and researchers have demonstrated that both of these processes are strongly present in the Sahel, capable of generating gust fronts that can lead to thunderstorms.
The newly-obtained data will allow an improvement in the development projections for the precipitation regime in the Sahel, currently based on models in which the strong variability in water levels observed has not been fully represented**.
Routinely used by the IRI, this technique is being made available to other African laboratories for the whole continent. It can also be extended to other tropical regions where the precipitation development could reduce the availability of water resources, such as the Andes. Projections there are even more uncertain than in the Sahel. The complexity of the tropical atmospheric water cycle is coupled with another major problem: relief, also poorly represented in climate models
(1) This research forms part of the Analyse Multidisciplinaire de la Mousson Africaine (AMMA), and has been carried out by researchers from the Montpelier HydroSciences unit (IRD-CNRS-UM1-UM2) and the Climate and Environmental Sciences laboratory (CEA-CNRS-UVSQ) and their partners from the Institut des RadioIsotopes (Université Abdou Moumouni).
(2) Hydrogen and oxygen have several stable isotopes which differ in their number of neutrons. These result in different forms of H2O molecules, varying in weight. The isotopic composition is the relative quantity of these different molecules in water vapour.
(3) The Sahel monsoon is formed by flows of humidity originating in the south-southwest which appear at the start of the summer.