Monsoon zonal seesaw: Development of the South Asian monsoon aridified North Africa in the Cenozoic

North Africa experienced a pronounced transition from humid to arid conditions during the early Cenozoic. Meanwhile, the South Asian monsoon underwent substantial intensification, largely driven by the uplift of the Himalayan-Tibetan Plateau. Here, we demonstrate that the two events, which greatly shaped the paleoenvironment and modern regional climate, are not independent responses to paleogeography changes; rather, the former is primarily driven by the latter through the monsoon-desert mechanism, in which South Asian monsoon diabatic heating produces large-scale subsidence and aridity over North Africa. Using multiple ensembles of paleoclimate simulations and a model for the atmospheric response to imposed diabatic heating, we show that the intensification and poleward shift of Asian monsoon rainfall each make comparable contributions to the aridification of North Africa. Our findings identify a persistent dynamical pathway linking the South Asian and North African monsoons and suggest that it played a major role in early Cenozoic North African aridification, while its climatic expression at other times depended on the relative strengths of competing forcings.