Development of frequency-dependent ocean wave directional distributions

•We present experimental and numerical investigation of ocean wave directional spread.•A frequency dependent directional spread develops from no frequency dependence.•Generation of frequency dependent directional spread in wave basin is not necessary.•Many of the directional spectra from the Ekofisk field have a bimodal character.•The bimodality is consistent for all four methods including the Burg maximum entropy.

The paper discusses the development of a frequency dependent directional spread from an initial condition of frequency-independence. The study applies basin directional measurements from the Maritime Research Institute Netherlands (MARIN), simulated data from a nonlinear wave equation, and field measurements from the Ekofisk field. The basin experiments and numerical simulations are initialized with a JONSWAP spectrum with frequency-independent directional distributions. In both cases we observe the development of a strong frequency-dependence of the directional spread. The numerical simulations suggest that static nonlinear contributions to the surface elevation partially explain the behavior below the spectral peak in accordance with [1]. There are also dynamic nonlinear contributions on both sides of the spectral peak.