Evolution of the Atlantic Intertropical Convergence Zone, and the South American and African Monsoons Over the Past 95-Myr and Their Impact on the Tropical Rainforests

The widening of the South Atlantic Basin led to the reorganization of regional atmospheric and oceanic circulations. However, the response of the Atlantic Intertropical Convergence Zone (ITCZ), and South American and African monsoons across paleoclimate states, especially under constant paleogeographic and climatic changes, has not been well understood. Here we report on paleoclimate simulations of the Cenomanian (similar to 95 Ma), early Eocene (similar to 55 Ma), and middle Miocene (similar to 14 Ma) using the Community Earth System Model version 1.2 to understand how the migration of the South American and African continents to their modern-day positions, uplift of the Andes and East African Rift Zone, and the decline of atmospheric CO2 changed the Atlantic ITCZ, and the South American and African monsoons and rainforests. Our work demonstrates that the South Atlantic widening developed the Atlantic ITCZ. The South Atlantic widening and Andean orogeny led to a stronger South American monsoon. We find the orogeny of the East African Rift Zone is the primary mechanism that strengthened the East African monsoon, whereas the West African monsoon became weaker through time as West Africa migrated toward the subtropics and CO2 levels fell below 500 ppm. We utilize the Koppen-Geiger Climate Classification as an indicator for maximum rainforest extent. We find that during the Cenomanian and early Eocene, a Pan-African rainforest existed, while the Amazon rainforest was restricted toward the northwestern corner of South America. During the middle Miocene, the Pan-African rainforest was reduced to near its modern-day size, while the Amazon rainforest expanded eastward.