Congo Basin drying associated with poleward shifts of the African thermal lows
Trends in Congo Basin rainfall in six observational datasets are examined on monthly and annual time scales to motivate an investigation of changes in the atmospheric hydrodynamics. Annual-mean Congo Basin rainfall declines over the 1979–2017 time period in all datasets, with strongest agreement and statistical significance from March through August. The trends are less pronounced over the 1998–2017 time period, especially for boreal spring, with although the boreal summer season continues to dry. Decadal-scale differences in the atmospheric hydrodynamics are examined in three reanalyses to improve our physical understanding of the precipitation trends and add confidence. During much of the spring and fall, changes in the atmospheric circulation reflect regional processes and feedbacks. During the warm season in each hemisphere, Congo Basin precipitation is supported when the circulation about continental thermal lows converges with cross-equatorial flow from the winter hemisphere. The drying trend during these seasons is associated with poleward shifts of the continental thermal lows, which weakens this convergence. Rainfall anomalies are not directly related to local surface warming, and they do not involve modifications of moisture transport from the Atlantic or Indian Oceans. For boreal summer, the drying is related to amplified warming over the Sahara. In austral summer, a southward shift of the thermal low is part of a large-scale, zonal mean pattern shifting the subtropical highs poleward.
KeywordsCentral Equatorial Africa Congo Basin rainforest Tropical forest change African precipitation trends Angola low Tropical expansion
This work was funded by NSF Award #1356386. The Texas Advanced Computing Center (TACC) at the University of Texas at Austin provided the high performance computing and database resources. We also gratefully acknowledge the Global Modeling and Assimilation Office (GMAO) and the GES DISC for the dissemination of MERRA2, the Climate Prediction Division of the Japan Meteorological Agency for the dissemination of JRA-55, and ECMWF for the dissemination of ERAI.
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