On long-crested ocean rogue waves originating from localized amplitude and frequency modulations

Rogue waves, which form on the ocean’s surface, can cause significant damage to marine installations and pose a serious threat to ship safety. Understanding the physical mechanisms behind extreme wave focusing is crucial for predicting their formation and mitigating their impact. Two intensively discussed wave amplification frameworks are the linear and nonlinear focusing mechanisms. These are also known as superposition principle and modulation instability, respectively. We report an experimental study investigating the formation mechanisms in a unidirectional representative JONSWAP-type sea state and show that the nonlinear focusing principle can be sub-categorized into either a localized amplitude or a so far less-studied phase-related frequency modulation, or both being at play. The frequency modulation-type mechanism occurs at a lower probability, as suggested from the distribution of more than 200 recorded extreme events, however, it cannot be underrated or disregarded.