“…Moreover, when waves propagate over an area of increasing adverse current (positive current gradient), the wavelength is forced to shrink with a concurrent increase of the wavenumber and wave height (see Peregrine, 1976). This results in an enhancement of the wave steepness, which is suspected to make nonlinear processes, such as the modulational instability mechanism and hence extreme waves, more likely (see, for example, Gerber, 1987;Lai et al, 1989;Stocker and Peregrine, 1999;Lavrenov and Porubov, 2006;Tamura et al, 2008). Using numerical simulation of cubic nonlinear dynamics, in this respect, Janssen and Herbers (2009) and Hjelmervik and Trulsen (2009) have tried to quantify the effect of a nonuniform current on the probability of occurrence of extreme waves.…”