Understanding the Congo Droughts: The Role of Soil Moisture-Precipitation Feedbacks

The Congo rainforest, the driest and second largest rainforest on Earth, has experienced a widespread and long-term drying trend, with more intense and frequent droughts over the past four decades. Extended droughts have significantly warmed and dried the land surface, reduced forest productivity, lengthened the dry season, and increased the risk of forest fires and biomass burning in the region. These changes, if continued, could alter the composition and structure of the Congolese rainforest, impact biodiversity and carbon storage, and have long-term environmental, societal, and economic implications. A surprising fact about the Congo drying trend is that the reductions in precipitation (P) have been accompanied by increases in thunderstorm activity over the region. The increase in thunderstorms is counterintuitive since most of the rainfall in the Congo comes from thunderstorms, thus one would expect thunderstorms and P to increase or decrease in tandem. The mechanisms driving this counterintuitive relationship, termed the “Congo P paradox”, remain unknown. Research conducted here follows the working hypothesis that the ultimate cause of the Congo drying trend is changes in surface temperature over the Indian and Pacific Oceans, but the intensity and duration of droughts is increased by local feedbacks between soil moisture (SM) and P. Furthermore, the opposing trends in P and thunderstorm activity occur because the increases in the most intense thunderstorms, also called Mesoscale Convective Systems (MCSs), are accompanied by reductions in weak to moderate thunderstorms. The increases in P associated with increases in MCSs are overcompensated by decreases in precipitation due to reductions in weaker thunderstorms, resulting in opposing trends for P and thunderstorm activity. The research also explores the idea that SM-P feedbacks account for the Congo P paradox due to their effects on both P and thunderstorms. The key issue here is that SM and P interact with each other in multiple ways and the mechanism through which SM and P interact to prolong drought and intensify MCSs are subtly different. The work involves analysis of observational data from satellites and other sources as well as high-resolution simulations from the Weather Research and Forecasting (WRF) model. The project supports a postdoctoral fellow, a graduate student, and an undergraduate, and various forms of outreach are conducted including engagement with researchers based in the Congo. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.