Ocean rogue waves: A mystery unveiled?

Numerical simulations of prototypical rogue waves in the ocean. Top left: Normal sea state. Top right: Rogue hole. Bottom left: Rogue wave. Bottom right: Rogue wave group, also known as "three sisters". Ocean nonlinearities are only required to explain why rogue holes are even more rare than positive rogue waves. Whether rogue waves appear isolated or in a group depends on the spectral width of the sea state.

In a collaborative effort, the group of Günter Steinmeyer at the Max-Born-Institut in Berlin together with colleagues from the Leibniz-University in Hannover and the Technical University in Dortmund now report a new approach to shed more light on the rogue wave mystery. To this end, they suggest a new metric for the complexity of the wave motion, namely, the so-called phase space dimension. This metric measures the effective number of waves that interfere at one given location on the ocean surface. More importantly, they also propose a way to measure the dimension, and this measurement could readily be implemented on ships, possibly providing an early warning of rogue waves.

In fact, it seems that the capability of the ocean to form rogue waves is variable. The study suggests that the ocean surface movement is fairly simply structured throughout most of the time. Even in heavy storms, mostly conditions prevail that do not enable rogue wave formation. However, the complexity of the wave patterns may suddenly increase when crossing seas are generated, resulting in rogue-wave prone situations. Using the suggested dimensional analysis, it is exactly these rogue-wave prone situations that can be detected. Nevertheless, the individual rogue wave event remains unforeseeable. Moreover the study suggests that the ocean dynamics are ruled by linear yet still very complex dynamics.

The study therefore opens a new perspective for a better understanding of ocean rogue waves. Much research went into ocean nonlinearities, but it appears that the latter play a minor role for rogue wave formation. In contrast, winds have found very little attention in the rogue wave discussion so far. As winds are ultimately the drivers behind ocean wave formation in general, it therefore seems perfectly possible to identify rogue-wave prone situations from meteorological analysis, identifying situations that may give rise to crossing seas early on. The appearance of an individual rogue wave may remain a mystery, but at least, we may soon be able to predict the "rogueness" of ocean weather hours or days in advance.