Extreme dry seasons in the Amazon: the role of vegetation processes and atmospheric circulation

The interactions between vegetation activity and climate have been widely recognized as important drivers of the dynamics of the Earth system. This is particularly relevant in the context of global change, where both ecosystems and climate system are changing. In recent years, several studies have focused on the relationship between vegetation activity and hydro-climatic processes in the Amazon basin, with particular interest in the relationship between vegetation changes and regional atmospheric circulation. Recent studies suggest a lengthening of the dry season and a more frequent occurrence of extreme droughts in this region during the past decades. In particular, previous modeling studies suggest that changes of vegetation cover could partially explain these longer dry seasons in the Amazon. On the other hand, observational analyses indicate that such lengthening is also related to an enhanced moisture transport toward equatorial America, favoring convection over the equatorial region and subsidence over the Amazon. In this study, we aim to understand the role of both vegetation processes and regional atmospheric circulation on the more frequent occurrence of extreme dry seasons in the largest tropical forest of the world. We use long-term observations, derived from climate reanalysis data in the central and southern portions of the Amazon basin, to characterize the dynamics of the transition between seasons and contrast it with key indicators of vegetation dynamics derived from remotely-sensed observations of vegetation activity. On the other hand, to understand the role of atmospheric circulation, we use the Dynamic Recycling Model (DRM), a 2D semi-Lagrangian model that estimates the transport of water vapor originated as evaporation from different sources and evaluate its changes during longer dry seasons in the Amazon. Our results indicate that long and short dry seasons in the Amazon exhibit different energy and water fluxes, mediated by vegetation activity. Furthermore, our modeling approach indicates that both regional circulation associated with ocean-atmosphere interactions and local processes related to vegetation cover are important to determine moisture advections from oceanic sources toward northern South America and the Caribbean. However, local reductions of atmospheric moisture supply to the Amazon forest are mainly related to reductions of precipitation recycling rather than to regional circulation changes. These findings highlight the importance of considering the role of large/regional-scale circulation as well as local-scale processes on water vapor transport over the Intra-American region and the Amazon and their influence on extreme dry seasons over this forest.