Abstract
Prolonged periods of extreme heat also known as heatwaves are a growing concern in a changing climate. Over the Sahel, a hot and semi-arid region in West Africa, they are still relatively poorly understood and managed. In this research, five multivariate thermal indices derived from the ERA5 database were used to characterize Sahelian heatwaves for statistical analysis and as a sampling basis to investigate their underlying thermodynamic causes. Results show that on average most locations in the Sahel suffer from one or two heatwaves a year lasting 3–5 days but with severe magnitude. The eastern Sahel is more at risk than the west, experiencing more frequent and longer lasting events. Despite similar statistics of intensity, duration and frequency across the heatwave indices, for a given diurnal phase, there is surprisingly low agreement in the timing of events. Furthermore daytime and nighttime heatwaves have little synchronicity. In terms of associated thermodynamic processes, heat advection and the greenhouse effect of moisture are identified as the main causes of Sahelian heatwaves. The processes are nevertheless sensitive to the indices, consequence of the distinctness of their respective samples. Therefore attention should be given to the choice of either index in operational monitoring and forecasting of heatwaves. This will allow to effectively target different exposed socio-economic groups and resultantly enhance the efficiency of early warning systems.
Highlights
Heatwaves, extended periods of extreme heat (Gasparrini and Armstrong 2011) are increasingly recognised as a major hazard to the society, with impacts directly on human health (e.g., Guirguis et al 2013; Arbuthnott and Hajat 2017) and important socio-economic sectors including agriculture (Smoyer-Tomic et al 2003) energy (Añel et al 2017), transport (Palin et al 2013), infrastructure (Chapman et al 2013) and tourism (Perry 2000)
Since this study aims at targeting the effective impacts on comfort, the criteria for the choice are essentially how they simulate the exposure of Sahelian populations to diverse extreme heat-related environmental conditions
The longest lasting daytime heatwaves on record (Fig. S3.a and S4.a) persist for up to two weeks over some locations. (ii) Daytime events are longer lasting than nocturnal events (Figs. 1a vs 2a) persisting on average up to 4.5 days (Fig. 2a) notably over the Central Sahel. (iii) the spatial variability in duration is not pronounced, broadly the Central and East Sahel experiences longer lasting events than the West during both day and night across most indices (Fig. 1a and 2a). (iv) Across the thermal indices, differences are relatively small, over Eastern Sahel, Apparent Temperature (AT) presents the longest events at night (AT-night but has some of the shortest events during the day (AT-day)
Summary
Heatwaves, extended periods of extreme heat (Gasparrini and Armstrong 2011) are increasingly recognised as a major hazard to the society, with impacts directly on human health (e.g., Guirguis et al 2013; Arbuthnott and Hajat 2017) and important socio-economic sectors including agriculture (Smoyer-Tomic et al 2003) energy (Añel et al 2017), transport (Palin et al 2013), infrastructure (Chapman et al 2013) and tourism (Perry 2000). Under a warming climate there is clear evidence that heat extremes have been increasing in recent decades (Hartmann et al 2013) and are likely to continue to do so under all plausible greenhouse gas emission trajectories (Collins et al 2013). Like other regions there are clear upward trends in Sahel temperatures
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