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 vegetation to persistent droughts in both present-day and future climates. We propose a detailed evaluation of the ability of the ISBA_CC (Interaction Soil–Biosphere–Atmosphere Carbon 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 performed in the Amazon. We also explore the model sensitivity 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, ISBA_CC 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 ISBA_CC to drought. While one of the proposed WSF formulations improves the response of most ISBA_CC fluxes, except respiration, 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 responses to drought under a warmer climate and higher CO₂ concentration is clearly needed