Analyzing intensifying thunderstorms over the Congo Basin using the Gálvez-Davison index from 1983–2018

The Congo Basin situated in equatorial Africa is home to the second largest rainforest on the globe and plays an important role in the Earth’s climate system. For instance, the Congo is one of the most convective regions in the world and enhances the tropical large-scale circulation. Although tropical convection is crucial for rainfall and the sustenance of the rainforest, the most intense thunderstorms do not necessarily produce the largest amounts of rainfall over equatorial Africa. Aiming to better understand the decreasing rainfall trend over the Congo, trends in thunderstorm intensity from 1983 to 2018 are analyzed in this paper using the Gálvez-Davison index (GDI), i.e., a thermodynamic index used to measure thunderstorm potential. Consistent with previous studies, thunderstorm activity increased during all seasons over the Congo. The GDI suggests that the increasing trends in thunderstorm activity are attributable to an increase in hydrostatic instability. More specifically, the GDI shows that (1) an increase of cold troughs at 500 hPa, (2) an increase in the temperature gradient between 700 and 950 hPa, and (3) a decrease of the equivalent potential temperature (θ_e) gradient with height collectively act to promote taller, more intense thunderstorms. This study concludes by proposing feedback mechanisms explaining the intensification of thunderstorms. The mechanisms highlight the cooling and moistening of the mid-troposphere, drying and warming at the surface and lower troposphere, and a decrease in vertical stability and convective inhibition. These factors may act to re-enforce the drying trend which has stressed the Congo rainforest over the past 40-years.