Source Regions of Infragravity Waves Recorded at the Bottom of the Equatorial Atlantic Ocean Using OBS of the PI-LAB Experiment.

Infragravity waves are oceanic surface waves with typical frequencies between 0.004 and 0.04 Hz, and wavelength of the order of tens of kilometers. They are generated from the interaction of the higher frequency wind waves and swell at the coastlines. Over the last decade, there is a growing interest in them as they are related among others with sediment transport and nearshore hydrodynamics, seiches in harbors and coastal regions, coastal barrier breaching, and Earth's hum. They could also contaminate high-resolution satellite radar altimetry measurements of sea level. In this work we study one-year long recordings from the Ocean Bottom Seismometers (OBS) deployed during the Passive Imaging of the Lithosphere Asthenosphere Boundary (PI-LAB) experiment, which consists from three linear arrays and 37 broadband seismometers equipped with differential pressure gauges. The pressure records from each station were down-sampled to 1Hz and amplitude normalized to reduce the effects of earthquakes using the data's envelope. All possible station pairs were cross correlated for each one of the 349 days. Afterwards, a stack of the daily data for each station pair was created producing a noise correlation function for periods between 60 - 200 s. Beamforming indicates greater energy at 150 s period for the summer months (June-July) than the other months, with a bimodal distribution of back azimuths with average back azimuths either from the west or east. The most energetic peaks tended to arrive from the east. We backprojected the wave energy onto a global grid and find infragravity waves arriving from all possible directions but the dominant sources of coherent energy propagation are located along West coast of Africa, near Nigeria and Cameroon, originating from the northern part of the Gulf of Guinea and to a lesser extent the East coast of South America near Brazil. This asymmetric distribution of sources may be related to geometry of the West African coast line, which may be optimal to leak infragravity waves into the open ocean.