Delayed onset of the South American Monsoon during the Last Glacial Maximum

Regional climate model simulations support geological evidence that the Amazon basin was drier at the time of the Last Glacial Maximum (LGM). The model grid spacing of 60 km reveals the regional dependence of precipitation patterns with, for example, wet LGM conditions in some parts of the high Andes. The primary cause of drying throughout the Amazon Basin is a 2-3 month delay in the onset of the summer monsoon, in association with cool sea surface temperatures in the tropical Atlantic. Once the monsoon is finally triggered, LGM rainfall rates are similar to those of the present day. A shorter, but equally intense, South American monsoon during the LGM is consistent with geological proxy data that indicate that the Amazon rainforest persisted through the glacial period but with modified species populations. Evidence from the charcoal record of more fires and higher atmospheric dust loading is also consistent with the simulations of an extended dry season in South America during the LGM.