Wave breaking is a widely observed yet complex phenomenon that plays a crucial role in oceanic processes. It significantly affects air-sea interactions, including the transfer of mass, momentum, and energy. Previous research largely focused on two-dimensional (2D) waves, which travel in a single direction, even though ocean waves are inherently three-dimensional (3D). This article presents experimental results that explore the effects of three-dimensionality on wave breaking, aiming to deepen understanding and refine the existing methods used to model wave breaking and energy dissipation.
Wave Breaking in 3D vs. 2D
Most oceanographic studies on wave breaking rely on assumptions of two-dimensionality, where waves propagate along a single axis. This approach simplifies the modeling of waves and their interactions but does not adequately reflect the complexities of natural oceanic conditions. The authors of this article seek to address this gap by experimenting with 3D waves that spread directionally, revealing how the breaking process changes when wave energy spreads across multiple directions.
Experimental Setup
The researchers conducted their experiments in a state-of-the-art circular wave tank at the University of Edinburgh. This unique facility allowed them to create waves traveling in all directions without the constraints typically present in traditional experimental setups. By using focused wave groups with different directional spreading conditions, the researchers could assess wave breaking across a wide range of scenarios, including both unidirectional (2D-like) and highly directionally spread (3D) wave fields.
Key Findings and Wave-Breaking Regimes
The experiments demonstrated that directional spreading fundamentally alters the mechanisms of wave breaking. The researchers identified three distinct regimes of wave breaking based on the degree of directional spreading:
Travelling-Wave Breaking (Type I): This mode resembles 2D wave breaking, where the wave crest spills or plunges as the wave becomes unstable.
Standing-Wave Breaking (Type II): As directional spreading increases, the wave breaking shifts from a horizontal overturning motion to a vertical jetting motion, characteristic of standing waves. This type of breaking is marked by the formation of vertical jets that shoot upwards from the wave crest.
Travelling-Standing-Wave Breaking (Type III): In this regime, a fast-moving ridge forms along the wave crest, with breaking occurring at the top of this ridge. This breaking mechanism differs from both travelling-wave and standing-wave breaking, introducing new dynamics that have implications for energy dissipation and air-sea exchange processes.
Implications for Energy Dissipation and Air-Sea Exchange
The authors discovered that 3D waves could reach a much higher steepness before breaking compared to their 2D counterparts. In some cases, the waves were observed to be up to 80% steeper than at the breaking onset, a phenomenon not seen in 2D waves. This finding challenges the validity of existing models for energy dissipation and offshore structure design, which are often based on the assumption that wave steepness is limited by breaking onset in 2D waves.
In terms of air-sea exchanges, the experiments suggest that directional spreading influences the amount of energy dissipated during wave breaking. The vertical jetting associated with standing-wave breaking, for example, creates a different mechanism for air entrainment, which may affect processes such as bubble formation and gas exchange at the ocean surface.
Post-Breaking-Onset Behavior
The study also explored the behavior of waves after breaking onset, which revealed that 3D waves are not necessarily limited by the breaking process. Unlike in 2D waves, where breaking typically restricts further growth in steepness, the researchers found that in 3D waves, the crests could continue to rise after breaking, especially in conditions with high directional spreading. This result has significant implications for understanding the formation of extreme waves, such as rogue waves, and for the design of offshore structures that must withstand such events.
Parameterization for Wave Forecasting
One of the key outcomes of this research is the development of new parameterizations for predicting wave-breaking onset in 3D waves. The researchers introduced two single-parameter measures of directional spreading (denoted as Ω0 and Ω1), which capture the degree to which waves are standing or travelling. These parameters could potentially improve the accuracy of phase-averaged wave models, such as those used in wave forecasting and offshore engineering, by providing a more realistic representation of the conditions under which wave breaking occurs.
Future Directions and Applications
The findings from this study open new avenues for research in wave modeling and the design of coastal and offshore infrastructure. Future work could focus on refining the energy-dissipation terms in wave models to account for the influence of 3D wave dynamics. Additionally, the results highlight the need for further investigation into how directional spreading affects other oceanographic processes, such as the generation of sea spray, bubble-mediated gas exchange, and the formation of extreme waves.
This article provides groundbreaking insights into the phenomenon of wave breaking, challenging long-standing assumptions about the limitations imposed by breaking onset. By shifting the focus from 2D to 3D wave dynamics, the authors reveal new mechanisms of wave breaking that significantly affect energy dissipation, air-sea exchange processes, and the formation of extreme waves. These findings have important implications for improving wave forecasting models and designing offshore structures to withstand the more extreme conditions present in highly directionally spread seas.
More information: Mark McAllister, Three-dimensional wave breaking, Nature (2024). DOI: 10.1038/s41586-024-07886-z. www.nature.com/articles/s41586-024-07886-z
English (United States) ·