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International audience; A good knowledge of surface fluxes and atmospheric low levels is central to improving our understanding of the West African monsoon. This study provides a quantitative analysis of the peculiar seasonal and diurnal cycles of surface thermodynamics and radiative fluxes encountered in Central Sahel. It is based on a multi-year dataset collected in the Malian Gourma over a sandy soil at 1.5°W-15.3°N (a site referred to as Agoufou) with an automated weather station and a sunphotometer (AERONET), complemented by observations from the AMMA field campaign. The seasonal cycle of this Tropical region is characterized by a broad maximum of temperature in May, following the first minimum of the solar zenith angle by a few weeks, when Agoufou lies within the West African Heat-Low, and a late summer maximum of equivalent potential temperature within the core of the monsoon season, around the second yearly maximum of solar zenith angle, as the temperature reaches its Summer minimum. More broadly, subtle balances between surface air temperature and moisture fields are found on a range of scales. For instance, during the monsoon, apart from August, their opposite daytime fluctuations (warming, drying) lead to an almost flat diurnal cycle of the equivalent potential temperature at the surface. This feature stands out in contrast to other more humid continental regions. Here, the strong dynamics associated with the transition from a drier hot Spring to a brief cooler wet tropical Summer climate involves very large transformations of the diurnal cycles. The Summer increase of surface net radiation, Rnet, is also strong; typically 10-day mean Rnet reaches about 5 times its Winter minimum (~30 W.m-2) in August (~150 W.m-2). A major feature revealed by observations is that this increase is mostly driven by modifications of the surface upwelling fluxes shaped by rainfall events and vegetation phenology (surface cooling and darkening), while the direct impact of atmospheric changes on the total incoming radiation is limited to shorter time scales in Summer over this Central Sahelian location. However, observations also reveal astonishing radiative signatures of the monsoon on the surface incoming radiative flux. The incoming longwave flux does not reach its maximum during the monsoon season when the atmosphere is the most cloudy and humid, but earlier, prior to the onset of rainfall, as the dry and warmer atmosphere suddenly becomes moist. This feature points to the significance of the atmospheric cooling during the monsoon season and of the aerosol amounts in Spring. It also reveals that prior to the rainfall onset, the monsoon flow plays a major role on the diurnal cycle of the low-level temperature, due to its radiative properties. Conversely, the incoming solar radiation at the surface increases slightly from late Spring to the core monsoon season even though the atmosphere becomes moister and cloudier; this again involves the high aerosol optical thickness prevailing in late Spring and early Summer against a weaker shortwave forcing by monsoon clouds. The climatological combination of thermodynamic and radiative variations taking place during the monsoon eventually leads to a positive correlation between the equivalent potential temperature and Rnet. This correlation is, in turn, broadly consistent with an overall positive soil moisture rainfall feedback at this scale. Beyond these Sahelian-specific features, and in agreement with some previous studies, strong links are found between the atmospheric humidity and the net longwave flux, LWnet at the surface all year long, even across the much lower humidity ranges encountered in this region. They point to, and locally quantify the major control of water vapour and water-related processes on the surface-atmosphere thermal coupling as measured by LWnet. Namely, they are found to be more tightly coupled (LWnet closer to 0) when the atmosphere is moister and cloudier. Observational results such as presented here provide valuable ground truth for assessing models over a continental area displaying a challenging variety of surface-atmosphere regimes throughout the year, from a desert-like to a rainy tropical-like climate during the core of the monsoon. Indeed, the mechanisms emphasized by these data do not all comply to existing conceptual schemes.

The future change in dry and humid heatwaves is assessed in 10-yr pan-African convective-scale (4.5 km) and parameterized convection (25 km) climate model simulations. Compared to reanalysis, the convective-scale simulation is better able to represent humid heatwaves than the parameterized simulation. Model performance for dry heatwaves is much more similar. Both model configurations simulate large increases in the intensity, duration, and frequency of heatwaves by 2100 under RCP8.5. Present-day conditions that occur on 3–6 heatwave days per year will be normal by 2100, occurring on 150–180 days per year. The future change in dry heatwaves is similar in both climate model configurations, whereas the future change in humid heatwaves is 56% higher in intensity and 20% higher in frequency in the convective-scale model. Dry heatwaves are associated with low rainfall, reduced cloud, increased surface shortwave heating, and increased sensible heat flux. In contrast, humid heatwaves are predominately controlled by increased humidity, rainfall, cloud, longwave heating, and evaporation, with dry-bulb temperature gaining more significance in the most humid regions. Approximately one-third (32%) of humid heatwaves commence on wet days. Moist processes are known to be better represented in convective-scale models. Climate models with parameterized convection, such as those in CMIP, may underestimate the future change in humid heatwaves, which heightens the need for mitigation and adaptation strategies and indicates there may be less time available to implement them to avoid future catastrophic heat stress conditions than previously thought. Significance Statement Temperatures are higher in dry heatwaves, but humid heatwaves can be more dangerous, as the ability to cool by sweating is limited. We found that dry heatwaves are caused by decreased cloud, allowing the sun to heat the surface, whereas humid heatwaves are caused by increased cloud, rainfall, and evaporation from the surface. We found that a state-of-the-art very high-resolution climate model predicts a larger future change in humid heatwaves compared to a more traditional global climate model. Previous estimates of the prevalence of humid heatwaves in the future may therefore be underestimated. If we do not cut emissions of greenhouse gases, present-day African heatwave conditions could be experienced on up to half of all days of the year by 2100.

International audience

International audience; Key words Non photosynthetic vegetation, Sahel, STI, dust interannual variability, MODIS. North Africa is the largest dust source on Earth. However, the drivers of dust emission interannual variability in this region are still debated. Early studies outlined the role of previous-season rainfall and vegetation growth, while some recent studies emphasize the role of wind variability. Here we use a newly developed estimation of dry-season nonphotosynthetic vegetation cover in the Sahel to address this question [1]. This estimation is based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) shortwave infrared bands and covers the 2002-2015 period. Firstly, we showed that the annual vegetation growth anomalies caused by variability of rainfall in June-September (rainy season) translate into anomalies of dry vegetation cover that persist throughout the dry season until May, i.e. until the very end of it. Secondly, we showed that these vegetation anomalies explain 43% (50%) of the year-to-year variance in Sahelian-mean dry-season aerosol optical depth (AOD) as derived from MODIS Deep Blue (or AERONET Sun photometers). Similar explained variance is found with 10 m wind speed and dust uplift potential from the ECMWF. Wind and dry-season vegetation anomalies are not correlated and are further combined in a linear two-variable model. This model is compared to the independent dust concentration data from the Sahelian Dust Transect. The central Sahel proves more important than the western Sahel for dry-season AOD variability, but no relationship to Land Use was found. Dry-season vegetation, for which we now have large-scale observations, is being implemented in models. Figure: (a) Dry-season STI and corresponding dry-vegetation cover in % from MODIS. Letters mark the AERONET stations. The red lines are the 600 mm and 100 mm isohyets. (b) Map of correlation of dry-season STI (October-June) with previous growing-season NDVI (July-September). (c) Map of correlation between dry-season STI and Sahelian AOD.