Infragravity wave generation by nonlinear interaction between wind waves and wind gusts

Infragravity (IG) waves are of outmost importance to various nearshore processes. Due to their long periods and lengths, they influence the water elevation (and hence the location of the breaker line), and cause harbor and basin resonances. Their primary generation mechanisms are related to shoaling and breaking of nearshore wind waves. A common assumption is that the origin of deep water IG waves is the reflection from near or far coastlines. Through the analysis of propagation times of IG wave packets along ray paths, ECMWF reanalysis data, and our Acoustic Doppler Current Profiler field measurements in the East Mediterranean, we found evidence of deep water IG wave generation by off-shore 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 IG energy near Aogashima, Japan during winter. A new model of deep water IG wave generation is developed, based on nonlinear interactions of wind wave triads with submesoscale wind oscillations. The observed under-prediction of IG waves is resolved using a new gustiness-based model. The formulation of the new source term fits stochastic wave forecasting models. It can explain observations such as dual-peaked and multi-directional incident IG spectra and is found to be significant (and sometimes even dominant) during strong storms in the open ocean.