Heat waves monitoring over West African cities: uncertainties, characterization and recent trends
Authors: Ngoungue Langue Cedric Gacial , Lavaysse Christophe , Vrac Mathieu , Flamant Cyrille 
Published: July 2022

Abstract. Heat waves can be one of the most dangerous climatic hazards affecting the planet; having dramatic impacts on the health of humans and natural ecosystems as well as on anthropogenic activities, infrastructures and economy. Based on climatic conditions in West Africa, the urban centers of the region appear to be vulnerable to heat waves. In this study, we assess the potential uncertainties encountered in the process of heat waves monitoring and analyse their recent trend in West Africa cities. This is investigated using two state-of-the-art reanalysis products namely ERA5 and MERRA for the period 1993–2020. Three types of uncertainties are discussed. The first type of uncertainty is related to the reanalyses themselves, with MERRA showing a cold bias with respect to ERA5 over the Sahel and Guinean regions except over some countries (Guinea Bissau, Sierra Leone, Liberia). Furthermore, large discrepancies are found in the representation of extreme values in the reanalyses over the southern Sahel and the Guinea coast. The second type of uncertainty is related to the sensitivity of heat waves frequency to the threshold values used to monitor them. Heat waves detected using the lowest threshold value are very persistent and last for several days; while the duration of heat waves related to high threshold values is shorter. The choice of indicators and the methodology used to define heat waves constitutes the third type of uncertainty. Three sorts of heat waves have been analysed, namely those occurring during daytime, nighttime and both daytime and nighttime concomitantly. Four indicators have been used to analyse heat waves based on 2-m temperature, humidity, 10-m wind or a combination of these. Nighttime and daytime heat waves are in the same range of occurrence while concomitant day- and nighttime events are extremely rare because they are more restrictive. The climatological state of heat wave occurrence shows large differences between the indicators. We found that humidity plays an important role in nighttime events; concomitant events associated with wet-bulb temperature are more frequent and located over the north of Sahel. Most of the events detected in the regions 75 % have a duration around 3–6 days. The most dangerous events with a duration of at least 10 days contributed up to 12 % of the total number of events.For all indicators, the interannual variability of heat waves in the west Africa region evidences 4 years with a significantly higher frequency of events (1998, 2010, 2016 and 2019) possibly due to higher sea surface temperatures in the Equatorial Atlantic corresponding to El Nino events. All indicators also highlight that the cities in the Gulf of Guinea region experienced more heat waves than those lying along the Atlantic coastline and those located in continental Sahel during the last decade. The heat wave events occurring in the Guinean region show short duration and weak intensity, while in the coastal and continental regions, events are persistent with strong intensity. We find a significant increase in the frequency, duration and intensity of heat waves in cities during the last decade (2012–2020) compared to the previous two decades. This is thought to be a consequence of climate change acting on extreme events.