What are the causes of waves in the ocean? Energy analysis and wave types
Ocean waves are a ubiquitous part of coastal landscapes and beach vacations. But have you ever stopped to wonder where a wave comes from, how far it travels, or why it forms in the first place?
A wave is formed whenever energy passes through a body of water, causing the water to move in a circular motion. While a number of events, including hurricanes, full moons, and earthquakes, can transfer kinetic or motion-generated energy over water, it is the wind that is most often to blame. The type of wave that is created depends on the above event that triggers the wave action.
Anatomy of a wave
When the wind blows over a smooth water surface, two things happen: Friction is created when the air rubs against the water, and this frictional force begins to stretch the water surface. As the wind blows continuously, the surface of the water turns into choppy waves, then turns into white hats, then begins to stretch upward, forming a ridge, the highest point of a wave.
While the uppermost part of a wave is known as the crest, its bottom is called the trough. The vertical distance between the crest and the trough tells you how high the wave is.
The height of a wave depends on the wind speed, its duration (how long it blows) and its fetch (how far it blows in one direction). Slow wind speeds create small waves. Likewise, if the winds blow for only a short time, or if they blow on a short fetch, smaller waves will occur. For a great wave to form, these three factors must be important. For example, a constant 33 mph (30 knots) wind blowing for 24 hours over a 340 mile (547 km) fetch causes average wave heights of 11 feet (3.3 m), according to NOAA and the book Oceanography and seamanship.
As to the height of a wave capable of growing, NOAA notes that although “rogue” waves over 65 feet (19.8 m) can occur under extreme storm conditions, such wave heights are extremely rare. During Hurricane Sandy, several ocean buoys measured individual wave heights of over 45 feet (13.7 m).
The waves make curls
Have you ever swam in an ocean wave? You probably felt like it was lifting you up and down in a rocking motion, but that’s not entirely true. Waves actually cause water-borne objects to move in a circular movement, so in reality it lifted you up and forward as it approached, then down and back as it passed.
The speed at which a wave moves depends on the depth of the water in which it moves and its wavelength (the distance between two successive waves). Longer waves generally move faster in the water.
At the same time, all of this is happening above the waterline, a turbulent water column is also moving just below. However, when a wave from the deep ocean approaches the shore and this shadow wave meets the shallower seabed, its movement is interrupted. It slows down, compresses and forces the crest of the wave higher in the air. This causes the wave to imbalance and the wave crashes into what is called a “breaking wave”. As for wave energy that started out as wind power, it dissipates in waves.
Types of waves
Wind-driven surface waves are the most common types of waves, but they’re not the only types of waves you’ll find at sea.
When the moon, rather than the wind, pulls across the surface of the ocean, tidal waves form. Yes, the moon’s gravity is actually pulling on the surface of our planet. (This gravitational pull affects both land and water, but it’s the more malleable water that has the most impact.)
The type of tidal wave that forms depends on which side of the earth you are on. When your area is directly facing the moon, you will experience rising water levels creeping inward from the beach (high tide) due to the swelling of the oceans towards the moon. But when your area is furthest from the moon, the sea level will drop and move away from the shore (low tide) as they are essentially pulled towards the center of the earth.
Only two high tides and two low tides occur daily on Earth (one high tide and one low tide by the sides of the Earth).
While tsunamis are sometimes referred to as a tidal wave, it is not the same thing. Although they act like tidal waves in that they move up the shore and inland, they are largely triggered by underwater earthquakes. On average, two tsunamis occur each year in the Pacific Ocean, which is the most seismically active ocean basin in the world.
When the winds of a hurricane blow across the surface of the sea, gradually pushing the water away in front of it, it creates a series of long waves called storm surges. As the storm nears the coast, water has “pooled” into a dome several hundred miles wide and tens of feet high. This ocean swell then moves towards the shore, inundating the coast and eroding the beaches.