15 Dec 2005; Mat-Nat Faculty (UITO), room U1=A228. Thursday 10:15-12:00

Karsten Trulsen (Department of Mechanics, Institute of mathematics, University of Oslo)

"Freak ocean waves".

Recent occurrences of freak waves, notably the Draupner "New Year Wave", have inspired study of nonlinear and statistical properties of the sea surface.

We have developed a weakly nonlinear model (a modified nonlinear Schrödinger equation) suitable for numerical Monte-Carlo simulation of the sea surface, useful for the prediction of spectral and statistical properties of the sea surface as well as wave kinematics. We demonstrate how the omega^{-4} power law for the frequency spectrum is produced by nonlinear transfer from small to large frequencies. We predict that the Tayfun distributions are good nonlinear corrections to the Gaussian/Rayleigh distributions for surface elevation and crest height even when the bandwidth is not small. Recently the so-called Benjamin-Feir index has been proposed as an indicator for the likelihood of freak wave occurrences; we argue that this index is more useful for idealized laboratory conditions than for realistic sea environments.