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January 14, 2024

What is the difference between a tsunami and a rogue wave?

A large wave
Summary
This is a comparison of two severe wave phenomena

Tsunamis could be defined as a series of very long waves, often due to earthquakes or volcanic eruptions. Rogue waves are waves that are at least two times the significant wave height of the surrounding waves, often due to the interaction between different wave trains (groups of waves with crests and troughs) travelling in opposing directions or the interaction between swells and currents, flowing in opposing directions. They are similar in that both could generate catastrophic consequences.

You probably remember the devastating tsunami in the Indian Ocean on Boxing Day in 2006, which was a consequence of a major earthquake with an epicenter near Indonesia. Southeast Asia observed waves with a magnitude of up to 30m near the northwestern end of Sumatra, and the tsunami even propagated throughout the oceans, where the tsunami clearly was recorded by tide gauges on the South American east coast.

More recently, on the 1st of January 2024, another tsunami was occurring in Japan. Although not as powerful as the one in 2004, but still devastating for the Japanese society. In fact, Japan is the country that suffers the most tsunamis in the world because of their position over four different tectonic plates.

But what is a tsunami and how is it formed? A tsunami is a series of very long waves, most often originally generated by an earthquake on converging tectonic plate boundaries but could also be caused by for example volcanic eruptions under the sea, or landslides.
Both the speed and the growth of a tsunami is highly dependent on the ocean depth. In contrast to wind generated waves, tsunamis move through from the bottom of the ocean to the water surface. And out in the deep oceans, tsunamis will normally mainly pass beneath the water surface and will only in rare cases reach more than 1m higher peaks than the ocean swell itself. Which is why mariners at deep oceans usually do not notice tsunamis.

In deep oceans, tsunamis could come with speeds of more than 400 knots (roughly 200 m/s), with a distance of hundreds of kilometers between the wave crests.
Despite these high numbers, the growth of the tsunami is not significantly noticeable in deep oceans, but they start to build up higher as the waves travel inland, which is due to gradually decreasing water depth. As the tsunami approaches shallower water and land, it also loosens in speed (perhaps slowing down to just a few hundredths of their speed in the deep ocean).

As said above, tsunamis near the coast could reach impressive heights of up to 30m or even higher. But most of them are just one-tenth as high. And as they come onshore, they develop like rapidly rising floods, which could inundate coastal areas several kilometers inland.

Rogue waves, monster waves, or freak waves as they are also called, could statistically be defined as waves that are at least twice as high as the surrounding significant wave height. These waves are a rather rare phenomenon and are very hard to study and predict as they seem to come out of nowhere and their direction of propagation often deviates from the prevailing wind and wave direction. They can essentially come from any direction, surprising any human or vessel, or offshore wind turbine if you like. Rogue waves can cause severe damages to even the largest cargo ships.

The causes of the formation of rogue waves are somewhat uncertain till this day. One theory of their formation is that when several swells, with crests and troughs, of different speed and direction collide, a rogue wave could form. If the respective swells hit each other under the most favorable conditions, they can create a sudden tremendous transfer of energy as their crests could coincide and reinforce each other. Each individual swell energy thus combines into one entity of wave energy, resulting in a rogue wave.

Rogue waves are more likely to form during storm events (low-pressure events), in very confused seas where you see a lot of swells of different directions. The formation areas for mid-latitude lows are in general where warm ocean currents meet cold ocean currents.
An example of this is off the southeast coast of South Africa, where the warm Agulhas Current is located - please see figure 1.
As the Agulhas Current flows southwest, it will at times collide with southwestern swells moving northeast. The wavelength will then become shorter, and the swells will thus become steeper as a result of this interaction with opposing directions. The waves may thus dynamically merge and form rogue waves. In fact, this region off South Africa is one of the most common regions on the globe for rogue waves to form which is thus due to the fast-running Agulhas Current interacting with swells in connection with developing lows.

A map of the Agulhas Current
The Agulhas Current is located along the continental shelf southeast of South Africa - flowing from approx. Durban down to off Port Elizabeth. Image from Windy.com.

Maybe the most famous rogue wave is the Draupner wave, that hit the Draupner platform in the North Sea on New Year’s Day in 1995. This wave was nearly 26m high, compared to the 12m high waves in the vicinity. Another famous rogue wave event occurred more recently, in November 2020 off Ucluelet on Vancouver Island. Records of this rogue wave showed that the wave “only” measured about 17.6m but with surrounding waves of about 6m makes this rogue wave proportionally more extreme (nearly three times as high as the waves in the vicinity).

So how does a rogue wave differ from a tsunami?
One distinction is their cause of formation. Whilst tsunamis are due to more geological features like earthquakes or volcanic eruptions undersea, rogue waves are due to more oceanographic and meteorological features. They also differ from each other in the way that tsunamis are barely evident above deep oceans and only become significantly evident when approaching shoreline. On the other hand, rogue waves can attain their great heights regardless of whether it is deep water or closer to the shoreline.

Tsunamis and rogue waves are both types of large oceanic waves, but they differ significantly in their origins, characteristics, and causes. Here are the key distinctions between the two:

Formation and cause
- Tsunami: Tsunamis are primarily caused by underwater disturbances, such as earthquakes, volcanic eruptions, or underwater landslides. The vertical displacement of the ocean floor displaces a large volume of water, creating a series of powerful waves.
- Rogue Wave: Rogue waves, on the other hand, are often associated with the convergence of multiple smaller waves. They can be triggered by strong winds, ocean currents, or the interaction of different wave systems.

Wave characteristics
- Tsunami: Tsunamis are characterized by long wavelengths and travel at high speeds across entire ocean basins. They have relatively low amplitudes in deep water but can grow in height as they approach shallower coastal areas.
- Rogue Wave: Rogue waves are known for their sudden and extreme increases in height compared to surrounding waves. They often appear unexpectedly and can be significantly taller than the average waves in the area.

Frequency
- Tsunami: Tsunamis are relatively infrequent but can have devastating consequences on land when they occur.
- Rogue Wave: Rogue waves are considered rare, but they can appear more frequently than tsunamis. They are often unpredictable and challenging to anticipate.

Spatial distribution
- Tsunami: Tsunamis can travel across entire ocean basins, affecting coastlines that are thousands of kilometers away from the source.
- Rogue Wave: Rogue waves are usually localized events, occurring in specific areas of the ocean rather than spreading across large distances.

Duration
- Tsunami: Tsunamis can last for several hours, with a series of waves arriving at intervals.
- Rogue Wave: Rogue waves are typically short-lived events, with a sudden surge in wave height that may last only a few minutes. Understanding these differences is crucial for coastal communities and maritime activities to develop effective warning systems and safety measures.

Regardless of their differentiation, tsunamis and rogue waves are both giant waves that could cause catastrophic consequences! 

If you would like to learn about the difference between swell and wind waves you are very welcome to read this article.