Southern African Mountains Affect Rainfall and Cloud Cover in the Tropics

It is well known that mountain ranges play a key role in general atmospheric circulation. The mountains in southern Africa, despite being relatively small, have important climate impacts. Studies have found that the southern African mountains increase the stratocumulus cloud deck off the Namibian coast in atmosphere only Global Climate Models (GCMs) however the role of these mountains have not been studied previously using a fully coupled GCM. We take advantage of a Geophysical Fluid Dynamics Laboratory (GFDL) coupled model to examine the influence of southern African mountains on the Namibian stratocumulus deck, the South Atlantic ocean-to-atmosphere energy transport, and the Intertropical Convergence Zone (ITCZ). We also run experiments with an atmosphere only model for comparison. For both models, a control simulation with realistic topography is compared to a simulation where the mountains in southern Africa are removed. As in previous studies, the removal of mountains results in thinning of the Namibian stratocumulus deck. In the coupled model, the increased sea surface temperature in the southern Atlantic due to the reduction of low clouds forces the Atlantic ITCZ to shift southward toward the warmer hemisphere. However, changes in the ocean circulation cool the South Atlantic atmosphere, lessening the ITCZ shift and changing the structure of precipitation. These results show the importance of mountains on shaping Atlantic rainfall and highlight the role of dynamical ocean processes in atmospheric dynamics.