Exploring Summer Compound Heatwaves in North Africa: Insights from Reanalysis and RCMs

Climate hazards triggered by extreme events have recently gained growing attention due to their potential socioeconomic and environmental risks (Luo et al., 2022).  Heatwaves can be categorized into: those occurring only during the day, only at night, and those encompassing both day and night. Although prior studies have extensively explored the independent heatwaves, the compound day-night events and related atmospheric conditions remain underexplored, in particular over North Africa. 

This study focuses on analyzing summer compound heatwaves (CHW) in North Africa as represented by ERA5 and the CORDEX-CORE ensemble of regional climate models (RCMs) data in the historical period (1979–2005). According to reanalysis data, CHWs predominantly occur over Morocco, except in the Atlas mountains, as well as in eastern Algeria and Egypt, with an annual average of approximately one event per year lasting 3 to 4 days. The CORDEX-CORE multimodel mean shows CHWs across most of the region, with the exception of the Atlas mountains. RCMs multimodel gives a relatively higher annual duration ranging from 4 to 7 days and a frequency, averaging 1.5 events per year.

Our composite diagnostic analyses of near-surface variables suggest that compound heatwaves are associated with increased solar radiation and clear sky during daytime, combined with an increase in the downward longwave radiation along with the specific humidity during the night. In fact, the increased water vapor during the night enhances the absorption of outgoing longwave radiation and increases the re-emission of longwave radiation back to the surface (Luo et al.,2022; Wu et al., 2023), contributing to nighttime surface warming. The composite of geopotential height at mid-troposphere (500 hPa) and upper-troposphere (200 hPa) show pronounced positive anomalies, associated with extreme daytime and nighttime temperatures during compound heatwaves especially on coastal areas. 

The wider spatial distribution of events identified in the multimodel ensemble compared to the reanalysis can be attributed to the overestimation of downward solar radiation in the RCMs across the entire region, in contrast to ERA5 where positive solar radiation anomalies are more localized in coastal areas. A similar pattern is also observed in the specific humidity.

 

References: 

Luo, M., Lau, N.-C. & Liu, Z. Different mechanisms for daytime, nighttime, and compound heatwaves in Southern China. Weather Clim. Extremes 36, 100449 (2022).

Wu, S., Luo, M., Zhao, R. et al. Local mechanisms for global daytime, nighttime, and compound heatwaves. npj Clim Atmos Sci 6, 36 (2023). https://doi.org/10.1038/s41612-023-00365-8