Synoptic drivers of humid heatwaves in West Africa

Humid heatwaves (HHWs) can cause heat stress in humans by reducing the ability to sweat in higher humidity. West Africa is an area of high interest due to its rapid population growth, high vulnerability, and latitudinal variation in HHW drivers. There is little understanding in how HHWs are triggered at synoptic scales. Using reanalysis and satellite-derived rainfall, we find different drivers in the three key regions of the Guinean coast, Sahel and Sahara. HHWs are associated with elevated near-surface specific humidity in all three regions. Near-surface temperature is also elevated in the Guinean Coast and Sahel regions while the Sahara region experiences a decrease during events. The rise in both temperature and humidity can be explained by the combination of increased near-surface downward shortwave radiation and trapping of moisture in the lower troposphere. The main moisture source is rainfall two days prior. After rainfall, clearer skies brought by dry mid-tropospheric northeasterly winds drive increased shortwave radiation, providing energy for surface evaporation and increasing temperature. In the Sahara region, the background air temperature is already very high, so there is enough energy for surface evaporation despite the mitigating impact of rain on temperature, indicated by the increase of surface latent heat flux to the atmosphere by as much as 118%. African Easterly Waves (AEWs) are a key driver of rainfall in Sahel and Sahara regions, and, therefore, are also a source of HHW predictability. The probability of a HHW increases during an AEW passage by as much as 24% in Western Sahara, which contains major population centres of over 1 million people. We also find most HHW events occur south of the intertropical discontinuity, which moves north and south with the onset and cessation of the African monsoon. While the majority of HHW events occur during the African Monsoon season in the Sahara, most events occur immediately before and after the start of the monsoon season further south. In addition, we analyse vertical profiles of cloud from CloudSat and CALIPSO, and show a clear anomaly from climatology during HHWs, with reduced cloud in the moister Guinean coast and Sahel to the south and increased cloud associated with rain in the more arid region of the Sahara. Understanding of the drivers and predictability of HHWs is important for risk management and adaptation measures such as the development of early warning systems.