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Through the analysis of propagation times of infragravity wave packets along ray paths, reanalysis data, and our field measurements in the East Mediterranean, we find evidence of deep water infragravity wave generation by offshore storms. We confirmed the results also using deep water pressure cell measurements in the Pacific. The known nearshore generation mechanism showed large discrepancies with the observed infragravity energy near Aogashima, Japan, during winter. A new model of deep water infragravity wave generation is developed, based on nonlinear interactions of wind wave triads with submesoscale wind oscillations. The observed underprediction of infragravity waves is resolved using this new gustiness-based model. The new source term is found to be of importance during strong storms in the open ocean and underlines the importance of accounting for submesoscale wind oscillations in wind wave models.

Plain Language Summary Ocean infragravity waves are surface waves with periods, lengths, and speeds much greater than the ones of wind waves and lower than those of tides. The primary generation mechanisms are related to shoaling and breaking of nearshore wind waves. A common assumption is that the origin of deep water infragravity waves is the reflection from near or far coastlines. By analyzing pressure cell measurements and wind and wave model data, we found evidence that deep water generation mechanisms also play a significant role. We present a new process connecting seemingly unrelated phenomena of the comparably slow wind gusts and very fast infragravity waves through nonlinear resonances. Its implementation compensated for underestimations of modeled infragravity waves in deep waters of the Pacific. Besides additional energy, the newly found mechanisms introduce also new frequencies and directional properties which may play a significant role in numerous situations. Infragravity waves are the main cause of harbor resonances. They are also related to various other fields such as sediment transport, altimetry measurements, wave-ice sheet interactions, and Earth's hum.