Relationship between Amazon and high Andes rainfall

Abstract

A regional climate model is used to study the connections between Amazon and high Andes rainfall. The modeling effort focuses on understanding the rainfall response during the Last Glacial Maximum (LGM) (∼21,000 years ago), as paleoproxy evidence generally suggests that the high Andes were wet during this period, while conditions over the Amazon were drier, although there is more uncertainty about the latter. Analysis of the yearlong present‐day and full LGM integrations reveals that the delayed onset of Amazon convection in the LGM during austral spring is associated with an enhancement of rainfall over the high Andes. In the absence of convection over the Amazon in the austral spring, zonal geopotential height gradients over the Amazon strengthen, resulting in anomalous easterly flow between the Amazon and the high Andes. Instead of the low‐level flow paralleling the Andes (e.g., northwesterly), the flow impinges more directly onto the eastern slopes of the Andes during the LGM, where it rises and transports moisture from the Amazon up into the mountains, enhancing convection over the Andes. This circulation mechanism explains how the moisture and rainfall over the high Andes of Peru and Bolivia can increase (such as the case for the LGM) despite large‐scale drying over South America. Present‐day observations are then explored for 1984, 2002, and 2003 to identify whether mechanisms identified in the paleoclimate modeling are observable in the present‐day climate. Comparison of these particular springs reveals a response analogous to the LGM model response, suggesting that this mechanism that connects tropical Atlantic sea surface temperatures/Amazon convection and high Andes rainfall is operating in the present climate.