Ocean waves represent the movement of energy through water, not the water itself traveling across vast distances. Water particles largely move in a circular motion, returning close to their original position as the disturbance progresses. Various forces constantly interact with the ocean’s surface and depths, giving rise to the diverse range of waves observed globally.
Wind as the Primary Wave Generator
Wind blowing across the water’s surface generates the most common ocean waves. As wind moves over the ocean, it transfers energy to the water, initiating small ripples that grow into larger, more organized waves with familiar crests and troughs.
The size and power of wind-generated waves depend on three factors: wind speed, duration, and fetch. Wind speed refers to how fast the wind blows; stronger winds produce larger waves. Duration indicates how long the wind has been blowing; sustained wind transfers more energy, leading to greater wave development. Fetch is the uninterrupted distance over open water that the wind blows in a consistent direction, providing a larger area for energy accumulation and wave growth. When all three factors are significant, such as during a storm over a wide expanse of ocean, large waves can form.
Gravitational Pull and Tides
Gravitational forces from celestial bodies, primarily the Moon and to a lesser extent the Sun, create tides. These forces pull on Earth’s oceans, causing the water to bulge outwards on the side closest to the Moon and also on the opposite side of Earth. As Earth rotates, different regions pass through these bulges, resulting in the regular rise and fall of sea levels known as high and low tides.
Tides are very long waves, with wavelengths spanning thousands of kilometers across ocean basins. Unlike wind-generated waves that have periods of seconds to minutes and affect only the ocean’s surface, tides have much longer periods, ranging from hours to a day, and involve the movement of the entire water column. The Moon’s gravitational influence is about twice as strong as the Sun’s in generating tides due to its closer proximity to Earth. When the Sun, Moon, and Earth align, their combined gravitational pull creates more extreme high and low tides, known as spring tides.
Underwater Disturbances: Tsunamis
Tsunamis are powerful ocean waves caused by sudden, large-scale water displacements, typically triggered by geological events beneath or near the ocean floor. Underwater earthquakes, particularly in subduction zones, are the most common cause, as they can suddenly uplift or subside the seafloor. Other triggers include massive submarine landslides and volcanic eruptions that displace large water volumes.
Tsunamis differ from wind-generated waves in their characteristics and behavior. They are shallow-water waves, meaning their wavelength is much greater than ocean depth, affecting the entire water column. In the deep ocean, tsunamis can travel at speeds comparable to a jet plane, over 800 kilometers per hour (500 mph), yet they are barely noticeable at sea due to their very long wavelengths and low height. As a tsunami approaches shallower coastal waters, it slows down, but its height increases, transforming into a towering wall of water or a rapidly rising flood that can inundate vast areas.
Other Contributions to Ocean Waves
While wind, gravity, and seismic events are primary drivers, other localized phenomena also contribute to ocean wave formation. Vessels moving through water generate ship wakes, created by water displacement. These wakes can be significant near coastlines or in busy waterways.
Changes in atmospheric pressure, especially during severe weather like hurricanes, contribute to storm surges. These long waves result from strong winds pushing water towards the shore and lower atmospheric pressure raising sea levels, intensifying closer to land. Internal waves can form below the ocean’s surface along boundaries between water layers of different densities. Generated by tides interacting with seafloor topography or strong ocean currents, these waves carry energy and momentum through the ocean’s interior, influencing mixing and circulation.