Phase‐Coherent Amplification of Ocean Swells Over Submarine Canyons

Abstract

Ocean swells can be a significant coastal hazard, potentially causing coastal disasters with costly damage to infrastructure and tragic loss of lives. We have observed this in the coastal region of Toyama Bay in the Sea of Japan, which has been devastated by severe winter swell events (the so‐called YoriMawari‐nami; YM wave). An extreme swell event was recorded in February 2008 in which the wave height at a site in the bay reached 9+ m, which corresponds to 16 times the local climatological average. Substantial efforts by the Japanese coastal engineering community have been expended to reproduce YM wave events with phase‐averaged wave models but, to our knowledge, with no success. During an archetype YM event, we found that the Noto peninsula filtered the incoming wavefield such that in the bay incident waves were quasi‐monochromatic. The swell then refracted over a submarine canyon resulting in a bimodal directional distribution of wave energy. This initiated conditions favorable for coherent interference, violating the assumptions of the phase‐averaged approach. Coherent interference is the key mechanism for understanding the formation of the large amplitudes and for matching observed high space‐time variability. Our results demonstrate the significance of the coherent interference for wave statistics over coastal bathymetry; only by accounting for phase‐resolving phenomena were we able to adequately reproduce the observations.Waves, which originate in strong storms in the north of the Sea of Japan, have recently devastated the coastal region of Toyama Bay. These type of events have been known for centuries, and the locals refer to them as YoriMawari‐nami. Engineers have struggled to understand the origin of YoriMawari‐nami events, though important for protecting our coasts, and they have been unable to reproduce them with typical models (phase‐averaged wave models). We used data to characterize these events, which are always long, old waves (swells) with unusually large wave heights compared to the past wave climate. We use wave models and ray‐tracing techniques to show how a swell arrives in the bay and bends over submarine canyons. The submarine canyon, like a prism separating light into different colors, separates out swells, and wave phases (the exact timing of an individual wave cycle) can no longer be ignored. In fact, the wave phases become crucial for understanding the enhancement of wave height. We were able to reproduce the observations with a phase‐resolving model, giving hope for predicting these events in the future. The coasts of Toyama Bay in Japan have been devastated by severe swell events called YoriMawari‐nami We characterize these events with data, ray tracing, and phase‐averaged and phase‐resolving models Phase‐coherent amplification emerged as the key mechanism for matching observations