Predicting the response of the Amazon rainforest to persistent drought conditions under current and future climates: a major challenge for global land surface models

While a majority of global climate models project drier and longer dry seasons over the Amazon under higher CO2 levels, large uncertainties surround the response of veg- etation to persistent droughts in both present-day and future climates. We propose a detailed evaluation of the ability of the ISBACC (Interaction Soil-Biosphere-Atmosphere Car- bon Cycle) land surface model to capture drought effects on both water and carbon budgets, comparing fluxes and stocks at two recent throughfall exclusion (TFE) experiments per- formed in the Amazon. We also explore the model sensi- tivity to different water stress functions (WSFs) and to an idealized increase in CO2 concentration and/or temperature. In spite of a reasonable soil moisture simulation, ISBACC struggles to correctly simulate the vegetation response to TFE whose amplitude and timing is highly sensitive to the WSF. Under higher CO2 concentrations, the increased water- use efficiency (WUE) mitigates the sensitivity of ISBACC to drought. While one of the proposed WSF formulations im- proves the response of most ISBACC fluxes, except respira- tion, a parameterization of drought-induced tree mortality is missing for an accurate estimate of the vegetation response. Also, a better mechanistic understanding of the forest re- sponses to drought under a warmer climate and higher CO2 concentration is clearly needed.