Assessing the Contribution of Nonoceanic Factors to the 21st Century Severe Droughts in Tropical South America

Since the beginning of the century tropical South America (SA) has experienced several extreme droughts including the record breaking droughts of 2005 and 2010 in the Amazon, the drought of 2012 in Northeastern Brazil (Nordeste), and the unprecedented 2016 drought that influenced the entire tropical SA including Nordeste, North, and South Amazonia with severe consequence in terrestrial water storage and ecosystems. Droughts in tropical SA are strongly connected to the tropical Pacific and Atlantic sea surface temperature (SST), yet SST-based statistical models under-predicted both the severity and extent of the 2016 precipitation anomalies, suggesting that potential non-oceanic factors may have played a significant role. This study assesses the contribution of some of these factors to recent droughts in the tropical SA, including natural vegetation feedback and the GHG-induced warming, using a regional dynamic vegetation-climate model. Based on results from the regional climate model with different vegetation treatments, vegetation-climate interactions has a small but noticeable impact on intensifying the agricultural and hydrological aspects of the recent droughts, yet its impact on the meteorological aspect (i.e., precipitation deficits) is rather small. Further experiments using the regional climate model driven with observed and detrended SST and lateral boundary conditions revealed that anthropogenic warming in the past several decades may have significantly amplified the severity of the precipitation deficits during the 21^st century droughts. In addition, agricultural land use changes may also enhance the severity of the droughts. Results from this study suggested that the tropical SA will experience more extreme droughts in the future.