Since ancient times, we have come up with various explanations for what causes the sea to behave as it does. The churning of the ocean waves has been attributed to mythological beings, such as Poseidon/Neptune, whose wrath sailors have forever been at the mercy of. Anyone who has gazed upon the ocean has noticed that it is not entirely still. The water appears to be moving in undulations. When we are at the beach, we can see the waves cresting and crashing down onto the sand. The power of waves is significant, but what causes them? While we still have much to learn about the nature of water and the ocean, our scientific study of it tells us that there are several causes, but all involve the movement of energy.
What are waves?
According to the U.S. National Oceanic and Atmospheric Administration (NOAA) (n.d.), waves are created when energy moves through the water. The water subsequently moves in a circular motion, although it does not travel very far. Mainly, the only thing waves are doing is transmitting energy across the sea (para. 2). Thus, we now know that waves are energy moving through the water, and while the waves themselves are not traveling very far, the energy they are transmitting can travel through an entire ocean basin (NOAA, 2021a, para. 1). This is not to say that seawater is static because it does move due to currents, but that is for another post.
OK, but the water is clearly moving some distance when we see waves crashing ashore. What’s the deal? Yes, that’s true. The orbital motion of the wave is disturbed when it encounters the seafloor. Furthermore, it is not only the surface of the water but also a column of water beneath it that is moving in an orbital motion (down to roughly half of the wave’s wavelength). As the wave approaches the shore, the bottom part of it slows down and compresses which causes the top part to crest. Once this crest reaches a tipping point, it comes crashing down as surf (NOAA, n.d., para. 3).
Anatomy of a Wave & Terminology
Note: While there are more terms related to waves in physics and mathematics, in this case, we are only concerned with those as related to fluids.
There are a number of terms related to how waves are described (as seen in the above image):
- Wavelength: The distance between wave crests in the direction of propagation.
- Wave height: The vertical distance from trough to crest.
- Wave frequency: The number of wave crests passing a single point each second.
- Wave period: The time required for a wave crest to move from one point to another in the direction of propagation.
- Fetch: The distance over water in which wind blows unobstructed. (The longer the fetch and the greater the wind speed, the more energy is transferred to the water.)
What kinds of energy cause waves?
Now that we know what waves are and some of the terminology behind them, we will now turn to the kinds of energy that cause waves. There are several types of waves as defined by the energy source that creates them.
- Wind-driven/Surface waves: A wave formed by wind moving over the surface of the water. These are common all over the world and the kind you see at the beach.
- Storm surge: A long wave caused by high winds and a continuous low-pressure area. Common when severe storms move inland.
- Tsunami: A very long wave caused when submarine earthquakes quickly displace a large amount of water. (Note: Not every undersea earthquake causes a tsunami.)
- Tides/Tidal waves: Waves caused by the gravitational pull of the sun and the moon on the Earth (NOAA, 2021a, para. 2 – 4). (Note: The terms tsunami and tidal waves are occasionally used interchangeably, but in reality, they are two different things.)
Storm surges and tsunami generally do not crash down on the shore like surface waves. Instead, they come ashore as massive rises in sea level and can travel for some distances inland (NOAA, n.d., paras. 4 – 5). Having personally survived the Great East Japan Disaster on March 11, 2011, I can attest to the fact that tsunami DO NOT behave like in the movies! You cannot surf it, nor would you want to. A tsunami is a huge mass of water that comes ashore very rapidly and destroys virtually everything in its path.
What are rogue waves?
AKA freak waves or killer waves
Sailors and mariners have long referenced singularly massive waves, as distinct from the norm, that appears out of nowhere and inundate their vessels. Such waves are commonly known as rogue waves, freak waves, or killer waves. Only recently has science lent some credence to the existence and causes of such phenomena. Rogue waves are unexpectedly large waves, generally greater than twice the size of the surrounding waves. They can come from directions other than the prevailing wind and waves, and they are sometimes said to look like walls of water with steep sides and unusually deep troughs (NOAA, 2021b, para. 1- 3).
Given the rarity of such waves, their causes are still under investigation, however, scientists theorize a couple of causes. One cause is constructive interference which occurs when two swells, traveling at different speeds and directions, pass through one another. The resulting energy in these waves can coincide and reinforce one another which creates large waves. Should these swells be traveling in the same direction, the resulting large wave can last for several minutes before subsiding. Another cause is the focusing of wave energy. This is where waves (such as in a storm) develop in a current against the normal flow of water. The result is a shortening of the wave frequency where the waves eventually join together and form large rogue waves (NOAA, 2021b, paras. 4 – 6). Based on these known causes, it becomes understandable how rogue waves can appear suddenly and without warning. Furthermore, it is also logical that it would be very difficult to predict exactly when, where, and under what conditions they would occur.
Our current understanding of hydrography has moved beyond the ancient mythological tales and tells us that waves are caused by energy moving through the medium of water. Waves vary in size, but they all represent the effects of wind, earthquakes, and gravitational forces exerting power on water that is only seen in the water itself. Our understanding of the ocean and how it works is constantly evolving. Indeed, we have only just scratched the surface.
National Oceanic and Atmospheric Administration. (n.d.). What causes ocean waves?. https://oceanexplorer.noaa.gov/facts/waves.html.
National Oceanic and Atmospheric Administration. (26 February 2021a). Why does the ocean have waves? https://oceanservice.noaa.gov/facts/wavesinocean.html.
National Oceanic and Atmospheric Administration. (26 February 2021b). What is a rogue wave? https://oceanservice.noaa.gov/facts/roguewaves.html.