One Drier, One Wetter: A Tale of Two Rainforests

Relatively limited observations over the Amazon and Congo Basin rainforests indicate that annual mean rainfall has increased in the Amazon and decreased in the Congo basin on decadal time scales. This change in precipitation is reproduced in two second-generation reanalysis, ERA-Interim and JRA-55. The precipitation differences are diagnosed in the reanalyses to relate the change to fields that are constrained by observations, namely, atmospheric winds, specific humidity, sea surface temperatures, and geopotential height. An evaluation of the atmospheric column moisture budget is used to understand the physical processes of the potential change in rainforest precipitation because the precipitation anomalies are reflected in the anomalous vertically-integrated moisture convergence. Therefore, confidence that these opposite-signed changes in precipitation are occurring in the two rainforested regions is bolstered not only by the agreement between observations and reanalyses, but by their relationship to more reliably-observed fields and the development of a physical understanding of the change. Decadal differences are calculated to compare the 1979-1988 time period with 2007-2016 period, and include an analysis of both annual mean and seasonal trends.

Over the Amazon, vertically-integrated moisture convergence is primarily associated with zonal wind convergence, with regional support from meridional wind convergence. Moisture advection plays a minor role. The largest decadal difference also emerges in the zonal wind convergence field. Warming over the Amazon basin is stronger than over the tropical Atlantic, increasing the land/sea contrast and associated circulation systems.

Over the Congo Basin, vertically-integrated moisture convergence is primarily associated with meridional wind convergence. In austral summer, for example, the Northern Hemisphere trade winds converge with southerly flow in the northwestern quadrant of the Angola low. Relatively strong warming of the Indian Ocean coupled with subdued warming over much of southern Africa are associated with a weakening of the Angola low, which weakens meridional convergence and precipitation over the Congo Basin.