High tide is often misunderstood as a cause of big waves. High tide is the maximum elevation of the sea surface at a predictable time, while a wave is a traveling disturbance of energy across that surface. These two phenomena are generated by separate physical forces, though they interact near the shoreline. Waves result from meteorological conditions like wind and storms, while tides are an astronomical event linked to the Moon and the Sun. Wave size and power are determined long before they approach the coast.
The Primary Drivers of Ocean Waves
The energy and size of ocean waves are determined by three factors: wind speed, duration, and fetch. Wind speed dictates how much kinetic energy is transferred from the atmosphere to the water’s surface. Stronger winds apply greater pressure and generate more energetic waves.
A strong wind must blow for a long enough duration to allow waves to build to their full potential. If a gale-force wind blows only briefly, the resulting waves will be smaller than those created by a moderate wind sustained over many hours. Fetch is the uninterrupted distance over open water that the wind blows in a consistent direction.
The combination of high wind speed, long duration, and extensive fetch allows waves to grow into massive, organized systems called swell. Once waves leave their creation area, they are known as deep-water waves, independent of local weather conditions. These swells can travel thousands of miles across ocean basins, carrying energy from distant storms to coastlines.
The Astronomical Forces Behind Tides
Tides are the predictable, periodic rise and fall of global sea levels, governed entirely by gravitational forces. The Moon is the primary force responsible because its relative proximity to Earth gives it the strongest gravitational influence. The Moon’s pull creates a bulge of water on the side of Earth facing it.
A second bulge forms on the opposite side of the planet due to inertia, as the Moon pulls the Earth away from the water. As the Earth rotates, a location passes through these two bulges, resulting in two high tides and two low tides each day. The Sun also exerts a gravitational pull, but it contributes only about 46 percent of the tide-generating force.
The alignment of the Sun and Moon affects tide magnitude. Spring tides occur when the three bodies are aligned, and neap tides result when they form a right angle. These astronomical forces cause only a slow, global change in water level, unlike the rapid energy transfer that creates waves.
How Water Level Changes Affect Wave Break
While tides do not create big waves, the change in water level significantly alters how existing waves interact with the seafloor near the shore. As a deep-water wave approaches the coast and depth decreases, the process of shoaling begins. During shoaling, the lower portion of the wave drags on the seabed, causing the wave to slow down, compress, and increase in height.
The wave eventually breaks when it reaches a water depth approximately 1.3 times its height. The varying depth from high tide to low tide shifts the location and nature of this breaking point. At low tide, the seabed is shallower sooner, causing the wave to break farther from shore and often more abruptly, leading to a steeper, “pitching” wave.
Conversely, at high tide, the increased water depth allows the wave to travel closer to the shore before shoaling causes it to become unstable and break. This deeper water may result in a softer, more gradual break, sometimes causing the wave to “mush out” or lose steepness and power. High tide results in a different wave experience, as the incoming swell interacts with the submerged bathymetry.