Refraction of a Gaussian seaway

Refraction of a Longuet‐Higgins Gaussian sea by random ocean currents creates persistent local variations (in the form of lumps or streaks) in average energy and wave action distributions. These variations explicitly survive averaging over wavelength and wave propagation direction. The lumps and streaks in average local action mean that the uniform sampling assumed in the venerable Longuet‐Higgins theory does not apply. Proper handling of the nonuniform sampling results in greatly increased probability of freak wave formation. The present theory represents a synthesis of Longuet‐Higgins Gaussian seas and the refraction model of White and Fornberg, which used a non‐Gaussian nonstatistical plane wave incident seaway. Using the linearized equations for deep ocean waves, we obtain quantitative predictions for the increased probability of freak wave formation when the refractive effects are taken into account. The wave height distribution depends primarily on the “freak index,” γ, which measures the strength of refraction relative to the angular spread of the incoming sea. Dramatic effects are obtained in the tail of this distribution even for the modest values of the freak index that are expected to occur commonly in nature. Extensive comparisons are made between the analytical description and numerical simulations.