Heat waves in the West African Sahel: nature, drivers and predictabilty

The Sahel in West Africa is a vast semi-arid region stretching from the Atlantic coast to the Red Sea and marks a transition between the Guinean forests and the Sahara desert, home to several millions of people. It is a climatologically hot region with a peak of heat observed in boreal spring before the summer monsoon. However, heatwave research in the region is still in its infancy despite that, as in most regions of the globe, high temperatures have escalated over recent decades as a result of climate change. This thesis builds knowledge on the characteristics, physical understanding and predictability of heatwaves in the Sahel at the weather and intraseasonal scales. These timescales have received little attention from previous work despite being important for operational risk management. The analyses are built on two main datasets including the fifth generation of the European reanalyses (ERA5) and the hindcast of the ECMWF ensemble extended-range forecastings system (ENS-ext). Sahelian intraseasonal heatwaves are overall short-lived with mean duration between three and five days. Record-breaking events can nonetheless last up to two weeks in some locations. Heatwaves in the Sahel are also characterised by low frequencies of occurrence with typically one to two events a year across most of the region. On the other hand, they are very severe in intensity given that the mean state of the atmosphere is already hot. The eastern and central parts of the Sahel are the most affected by heatwaves in terms of duration, frequency and intensity whereas the proximity to the Atlantic Ocean attenuates extreme heat events in western Sahel. The physical quantification of heat can be done in considering temperature only or adding other environmental variables to build multivariate thermal indices. The upstream choice of thermal index for heatwave study is important in the Sahel. Indeed, heatwaves sampled using different indices show only moderate synchronicity between them. Besides, the Sahel is characterised by a low concomitance between daytime and nighttime heatwave events, which is a relieving factor when it comes to the impact on health. Regarding the thermodynamic processes, heat advection and greenhouse effect of moisture are found to be the main underlying causes. Both these processes are made possible by a significant perturbation of the low-level flow, favouring a transport of hotter or more humid air masses towards Sahelian locations. At the large-scale, this circulation anomaly is often associated with convective anomalies in the Guinean region of West Africa, where convection is at its peak during the spring season. Consequently, the link between heatwaves in the Sahel and tropical modes of variability including the Madden Julian Oscillation (MJO) and the equatorial Rossby (ER) and Kelvin waves (EK), which are important drivers of the Guinean convection, was investigated. The examination reveals that the probability and, to a lesser extent, intensity of Sahelian heatwaves are significantly modulated by the passage of tropical modes over the West African domain. Depending on their convective phases, i.e. either enhanced or suppressed, they can increase or decrease heatwave probability in the region. The modulation is sensitive to the diurnal period and geographical location with nighttime heatwaves more impacted by the modes over eastern Sahel and daytime heatwaves more affected over western Sahel. Among the investigated modes, the MJO was found to have the greatest impact on heatwaves owing to its larger spatial extension and longer periodicity. The skill of Sahelian heatwave prediction by numerical models was assessed using ENS-ext which is regularly considered as one of the best forecasting systems at the intraseasonal scales. Significant skill was found in the first two to three weeks of the forecast. Longer predictability can be achieved using a “flexible” evaluation i.e. tolerating errors within a given temporal window, especially at the longest lead-times, where even approximate indication of heatwave occurrence is valuable for targeted anticipatory actions. With increasing lead-times, heatwaves are found to be more predictable at night than at day. Interestingly, ENS-ext is able, on one hand, to relatively well predict the local activity of tropical modes over West Africa and, on the other hand, to simulate their observed impact on Sahelian heatwaves. Furthermore, heatwave prediction skill is higher when tropical modes are active, implying that they are good sources of heatwave predictability. As a result, in the future, more precision and longer predictability can be obtained for heatwave prediction in the Sahel in view of the continual improvement in the representation of convection and tropical modes by models. Given its findings, this thesis has many implications for heatwave risk management in the Sahel across a range of sectors including, but not limited to, health, labour productivity, water and energy