Emergence of new heat stress hotspots over the West Africa

We calculate the hourly wet bulb globe temperature (WBGT) values for the last 50 years over Western Africa to assess the emergence of new heatwave hotspots and the interplay between moist and dry heatwaves. In the formulation used, WBGT is derived using the grided data from ERA5 and ERA5-HEAT: 2-m air temperature, relative humidity, 10-m wind speed and mean radiant temperature (a measure of incidence of radiation on a body), and is thus representative of outdoor conditions.

We find that the heat stress estimated through WBGT does not peak over the same geographical regions as the air temperature, suggesting an important role of humidity in intensifying heatwaves over certain regions. While the highest temperatures are reached in the northern Sahel and Saharan regions, the highest heat stress values are found further to the south, in the region bordering Senegal, Mauritania and Mali and in southwest Niger. These are the same regions where the WBGT threshold of 33 °C (conditions dangerous even at resting metabolic rates (MR) < 115 W) have recently been crossed for up to 40 hours per year.

The duration of exposure to WBGT > 30 °C (conditions dangerous at light physical activity, MR < 180 W) has been increasing over almost the entire West Africa, at rates from 30 to 100 hours/decade. Over the Senegal - Mauritania - Mali border and southern Niger, exposure to WBGT > 33 °C has been increasing by 1-4 hours/decade.

Dangerous WBGT thresholds can be crossed at a wide range of temperatures and are often not associated with the highest temperature percentiles. For example, in Niamey, a WBGT of 30 °C has been crossed in the temperature range from ~ 29 to 44 °C, in Thies (Dakar) and Ouagadougou from ~ 28 to 43 °C, and in Abidjan from ~ 28 - 36 °C.  In September 2019 and July 2020, in Niamey we find the first occurrences of air temperatures below 36 °C being associated with very dangerous heat stress values (WBGT > 33 ° C).

We conclude that for much of continental West Africa, and particularly for the Senegal - Mauritania - Mali border region and southern Niger, extreme heat alerts should at a minimum include indicators accounting for temperature and humidity, in order to capture the dangerous moist heatwave conditions occurring at temperatures well below the highest temperature percentiles. More complex indicators that additionally account for wind and radiation are very desirable for estimates of outdoor safety.