Coupling between Vegetation, Fire, Cloud, Precipitation and Large-Scale Atmospheric Circulation in Amplifying Rainfall Variability and Drought over Tropical Continents

Professor Inez Fung is a pioneer and world leader in use of an integrated earth system approach to address outstanding climate and ecosystem science questions. Her work encompasses carbon, water, aerosols and the interconnection among them. Inspired by her research, we present our research results to show that the coupling between vegetation, biomass burning, cloud and precipitation can significantly amplify rainfall variability, especially droughts, over humid tropical continents, including the Amazon and Congo. These regions have experienced unusually frequent extreme droughts with strong delay of the wet season onsets and lengthening of the biomass burning season in the past few decades, followed by recent reversal of the trends of biomass burning and wet season onset over the Amazon. This rainfall variability is much larger than that which can be explained by oceanic variability, and that can be represented by global climate models. We will present observational evidence to suggest that biomass burning aerosols tend to reduce shallow and deep convection in a drier and more stable environment, and conversely in a wetter and less stable environment, as the balance of thermodynamic and cloud microphysical effects of aerosols change with the ambient environment. In doing so, biomass burning aerosols can amplify the rainfall anomalies, through a chain of feedbacks involving the land surface, shallow and deep convection, atmospheric diabatic heating and the large-scale circulation responses. This amplification effect is especially important for the dry anomalies during the dry to wet season transition, contributing to the delay of wet season onset and so amplifying rainfall variability. These results highlight the importance of biosphere variability and its interaction with the atmosphere to improve our predictive understanding of rainfall variability in a changing climate.