Propagation of the overtide M 4 through the deep Atlantic Ocean

The nonlinear overtide M 4 is generated in shallow waters and is generally no more than a few mm amplitude in the deep ocean. Such small, periodic waves can now be mapped empirically by analyzing the long (15‐year) time series of Topex/Poseidon and Jason‐1 satellite altimetry. In a few locations of the deep Atlantic, M 4 amplitudes reach nearly 10 mm, confirming recent modeling efforts by Lyard and colleagues. These relatively large amplitudes evidently represent a complex interference pattern of free waves radiating from a few shallow sources, especially the Patagonian Shelf. They form approximately a standing‐wave resonance in the Gulf of Guinea.