Ocean currents trigger rogue waves

Inhomogeneous media can change the nonlinear properties of waves propagating on them. In the ocean, this phenomenon can be observed when waves travel on a surface current. In the case of negative horizontal velocity gradients (i.e. an accelerating opposing current or a decelerating following current), waves shorten and heighten, enhancing wave steepness. As a result, a nonlinear mechanism known as modulational instability develops, leading to the formation of large-amplitude waves (the so-called rogue waves), even if they would otherwise be unexpected. Laboratory experiments and numerical simulations with a current-modified version of the Euler equations are presented to assess the role of an opposing current in changing the statistical properties of unidirectional random wave fields. Results demonstrate in a consistent and robust manner that an opposing current induces a sharp and rapid transition from weakly to strongly non-Gaussian properties with a consequent increase of the probability of occurrence of rogue waves. Agreement with numerical simulations confirms that this transformation can be attributed to quasi-resonant nonlinear interactions triggered by the background current.