The ocean is a dynamic environment where water movements generally follow predictable patterns, yet certain events produce waves that defy normal expectations. These extreme water phenomena, often called “big waves,” are a diverse category of powerful movements generated by entirely different physical forces. Understanding these events requires precise terminology, as the power of a wave can come from wind, seismic activity, or rapid changes in atmospheric pressure. The names given to these waves—from rogue waves to tsunamis—reflect the distinct mechanisms that drive them to exceptional heights or destructive potential.
Rogue Waves The Ocean’s Spontaneous Peaks
A rogue wave, also known historically as a freak wave or monster wave, is defined in oceanography as any wave whose height from crest to trough is more than twice the height of the surrounding significant wave height. The significant wave height is calculated as the average height of the largest one-third of waves in a given sea state. Rogue waves are notorious for appearing suddenly, often in areas where the sea is not particularly rough, and they typically last for only a brief period before dissipating.
The primary mechanism believed to cause these spontaneous peaks is constructive interference, where multiple smaller wave crests traveling at different speeds or directions momentarily align and combine their energy. When the crests of several wave trains converge exactly in phase, their amplitudes are added together to form a single, massive peak. Strong currents or nonlinear effects in wave dynamics can enhance this process; for instance, storm-driven waves moving against a powerful current can compress the waves, causing them to shorten and steepen.
The existence of these enormous waves was confirmed when the Draupner wave struck the Draupner oil platform in the North Sea on January 1, 1995. This wave measured a maximum height of 25.6 meters (84 feet), exceeding the predictions of all known wave models at the time. Unlike tsunamis, which are long-wavelength phenomena, rogue waves are localized events that look like a steep, towering wall of water often preceded by an unusually deep trough, sometimes referred to as a “hole in the ocean.”
Displacement Waves Tsunami and Artificial Origins
Displacement waves are generated when a large amount of water is suddenly and vertically moved, sending energy radiating outward. The most common example is the tsunami, typically caused by a major underwater earthquake that vertically moves the seafloor. This tectonic shift pushes the entire water column, initiating a series of waves with energy that spans the full depth of the ocean.
Secondary causes of tsunamis include large submarine landslides, volcanic eruptions, or the impact of a meteorite.
In the deep ocean, a tsunami is characterized by an extremely long wavelength, often spanning hundreds of kilometers, and a low amplitude, usually less than one meter high. Due to this geometry, the wave is barely noticeable to ships at sea, yet it travels at speeds comparable to a jet aircraft, exceeding 800 kilometers per hour in deep water. The speed of a tsunami is determined by water depth, causing it to behave as a shallow-water wave even in the deepest parts of the ocean.
When the tsunami approaches the coastline and moves into shallower water, it undergoes a process called shoaling. The wave’s leading edge drags along the rising seafloor, causing its speed to slow significantly to approximately 30 to 50 kilometers per hour. To conserve the wave’s energy, the wavelength shortens dramatically, and the wave’s amplitude is forced upward. This conversion of kinetic energy into potential energy causes the tsunami to build into a towering surge or bore that can reach heights of tens of meters when it inundates the shore.
The concept of generating a displacement wave artificially was explored during World War II with the “Tsunami Bomb,” code-named Project Seal. Tests concluded that a single explosion was insufficient to create a large tsunami. Modern analysis confirms that even the largest underwater nuclear detonation cannot replicate a seismic tsunami because the explosion’s energy is localized. It primarily creates steam and compressional waves, not the sustained, large-scale vertical seafloor movement necessary to displace the entire water column across an ocean basin.
Meteorological Waves and Coastal Effects
Not all powerful water movements involve breaking crests or seismic origins; many are driven purely by atmospheric conditions. A storm surge is a phenomenon where strong winds and low atmospheric pressure combine to create an abnormal rise of water above the predicted astronomical tide. The low pressure at the center of a hurricane or cyclone lifts the water surface, while high-velocity winds push a dome of water toward the shore. This is not a traditional breaking wave, but a sudden, massive elevation in sea level that causes widespread coastal flooding.
Another form of meteorological water movement is the seiche, a standing wave that occurs in enclosed or semi-enclosed bodies of water, such as bays, harbors, or large lakes. Seiches are initiated when a rapid shift in barometric pressure or strong winds pushes water toward one side of the basin. The water then oscillates back and forth across the body of water, similar to water sloshing in a bathtub. This rhythmic oscillation can continue for hours or even days, with the largest vertical movement occurring at the ends of the water body.
Seiches are sometimes called a form of meteotsunami when they are caused by fast-moving weather systems that generate a wave amplified as it enters a coastal feature. These meteorological events highlight that a “big wave” can manifest not just as a towering crest, but as a dangerous, prolonged rise in water level driven entirely by the atmosphere.