The climate of Earth is shaped to a significant degree by the occurrence of intense storms over three regions: the Amazon and Congo basins and the Pacific Ocean. However, little is known about natural oscillations in the amounts of rainfall over the Amazon and Congo basins. Here, we present new satellite observations on tropical rainfall distribution and historical river flow observations to document a natural see-saw oscillation across the Atlantic Ocean: floods over the Amazon basin tend to coincide with droughts over the Congo basin and vice versa. This phenomenon is most significant during the southern hemisphere summer, and was observed most clearly during the decades of 1945-1955, 1960s, and 1970s. The mechanism responsible for this see-saw phenomenon is based on the Gill model of tropical circulations since rising motions associated with floods over either of the two basins is likely to force subsidence and droughts over the other basin.