6.3 Internal waves and tsunamis

Internal Waves

Formation of internal ocean waves

Internal waves form below the ocean surface, at boundaries where water layers of different densities meet. Unlike surface waves driven by wind, internal waves are generated by disturbances at these density interfaces deep within the water column.

Several mechanisms trigger their formation:

• Tidal interaction with topography: When tidal currents flow over underwater features like seamounts, ridges, or continental shelf edges, the disruption generates oscillations at density boundaries. This is one of the most significant sources of internal waves globally.
• Wind-driven energy transfer: Strong surface winds can push energy downward into the water column, disturbing the layered structure below.
• Seasonal thermocline changes: As the thermocline deepens or shallows with the seasons, the shifting density structure creates conditions favorable for internal wave generation.

Density stratification in wave generation

Internal waves depend on the ocean being stratified, meaning it's organized into layers of different density. These density differences arise from variations in temperature and salinity.

• The thermocline (temperature boundary), halocline (salinity boundary), and pycnocline (overall density boundary) are the key interfaces where internal waves propagate.
• Stronger stratification produces more pronounced internal waves. A sharp pycnocline provides a well-defined boundary for waves to travel along, while weak stratification dampens them.
• Buoyancy is the restoring force for internal waves. When a parcel of water gets displaced from its equilibrium depth, buoyancy pushes it back, and the resulting overshoot creates oscillation. This is analogous to how gravity restores surface waves.
• Pycnocline strength directly controls wave amplitude and frequency: a stronger density contrast supports larger, more energetic waves.

Characteristics of internal waves

Internal waves behave quite differently from the surface waves you can see from shore:

• They travel along density boundaries (pycnoclines) in the ocean interior, not at the air-sea interface.
• They move both horizontally and vertically, often propagating at an angle to the surface.
• They're much slower than surface waves, with typical speeds of about 2–3 m/s.
• Wavelengths range from meters to hundreds of kilometers, and periods range from minutes to hours. Some internal waves have periods that match tidal cycles, which is why they're sometimes called internal tides.
• Their amplitudes can be surprisingly large. Internal waves with vertical displacements of tens of meters are common, and some exceed 100 m.

Energy and momentum transfer

Internal waves play a major role in ocean mixing and circulation, even though they're invisible from the surface.

• They transport energy and momentum through the deep ocean. When internal waves break (similar to surface waves breaking on a beach, but happening at depth), they mix water across density layers.
• This deep mixing is critical for redistributing nutrients from the deep ocean toward the surface, fueling biological productivity.
• Internal waves influence large-scale ocean circulation patterns and contribute to vertical heat transport.
• Marine ecosystems feel their effects directly. Internal waves can push nutrient-rich deep water up onto continental shelves and alter plankton distribution, creating patchy concentrations that attract fish and other organisms.

Observation and measurement techniques

Detecting internal waves is challenging because they occur below the surface, but several methods have proven effective:

• Satellite altimetry: Internal waves create subtle surface signatures (small changes in sea surface roughness) that satellites can detect.
• Acoustic Doppler Current Profilers (ADCPs): These instruments measure water velocity at multiple depths throughout the water column, revealing the oscillating currents associated with internal waves.
• Thermistor chains: Strings of temperature sensors deployed at various depths record the temperature fluctuations caused by internal waves passing through.
• Seismic oceanography: Low-frequency sound waves originally developed for geological surveys can image internal wave structures by detecting density contrasts in the water column.

Tsunamis

Tsunami causes and mechanisms

A tsunami forms when a large volume of ocean water is suddenly displaced, generating a series of waves that radiate outward from the source. The key word is sudden: gradual changes don't produce tsunamis.

The most common causes:

• Earthquakes: The primary trigger. Specifically, earthquakes at subduction zones that cause vertical displacement of the seafloor are most dangerous. A magnitude 7.0+ earthquake with significant uplift or subsidence of the ocean floor can displace enough water to generate a major tsunami.
• Submarine landslides: Large underwater slope failures displace water rapidly. These are sometimes triggered by earthquakes but can also occur independently. Landslide-generated tsunamis tend to be devastating locally but dissipate faster over distance.
• Volcanic events: Eruptions, caldera collapses, or volcanic flank collapses can displace enormous volumes of water. The 2022 Hunga Tonga eruption generated a tsunami detected across the Pacific.
• Meteorite impacts: Extremely rare, but a large ocean impact would displace enough water to create catastrophic waves.

Characteristics of tsunamis

Tsunamis are fundamentally different from wind-generated waves. Their defining feature is their enormous wavelength.

• Wavelengths commonly exceed 100 km. Because ocean depth (average ~4 km) is much less than the wavelength, tsunamis behave as shallow-water waves even in the open ocean.
• Speed is governed by the shallow-water wave equation: $c = \sqrt{gh}$, where $g$ is gravitational acceleration (9.8 m/s²) and $h$ is water depth. In 4,000 m of water, this gives roughly 200 m/s, or about 720 km/h, comparable to a jet aircraft.
• In the deep ocean, a tsunami might be only 0.5–1 m tall and nearly undetectable by ships. But as it enters shallow water, it slows down and the wave energy compresses. This process, called wave shoaling, dramatically amplifies wave height.
• Tsunamis lose very little energy as they cross ocean basins because of their long wavelength. A tsunami generated in Chile can cause destruction in Japan hours later.

Coastal impacts and hazards

The destructive power of a tsunami comes not from wave height alone but from the sheer volume and momentum of water moving onshore.

• Inundation can extend kilometers inland in low-lying coastal areas. The 2011 Tōhoku tsunami in Japan traveled up to 10 km inland in some locations.
• The water carries entrained debris (vehicles, building materials, trees), which greatly increases its destructive force.
• Multiple waves typically arrive over a span of hours, and the first wave is not always the largest. People who return to the coast after the first wave recedes are at serious risk.
• Rapid water level changes generate powerful currents that can destroy harbor infrastructure, snap mooring lines, and scour foundations.
• Secondary hazards include sewage and chemical contamination, fires from ruptured fuel lines, and collapse of weakened structures.

Tsunami detection and mitigation

Modern tsunami warning systems combine multiple data sources to detect events and issue alerts as quickly as possible.

1. Seismic monitoring provides the first indication. Networks detect the earthquake within minutes and estimate its magnitude and location.
2. DART buoys (Deep-ocean Assessment and Reporting of Tsunamis) sit on the seafloor and measure pressure changes caused by a passing tsunami wave overhead. These confirm whether an earthquake actually generated a tsunami.
3. Tide gauges and coastal sea-level stations track water level changes near shore, providing real-time confirmation of wave arrival.
4. Warning centers (such as the Pacific Tsunami Warning Center) integrate all this data, run numerical models to predict wave arrival times and heights, and issue alerts through sirens, emergency broadcasts, and mobile notifications.

Beyond detection, mitigation strategies include:

• Hazard mapping to identify vulnerable zones and guide land-use planning
• Building codes and zoning regulations that restrict construction in high-risk areas or require tsunami-resistant design
• Public education on natural warning signs (strong earthquake shaking, unusual ocean withdrawal) and evacuation routes

Challenges in tsunami preparedness

Even with modern technology, significant challenges remain:

• Near-field events give very little warning time. Communities close to the earthquake source may have only 10–30 minutes before waves arrive, leaving almost no time for formal warnings.
• False alarm fatigue: Issuing warnings that don't result in a tsunami erodes public trust. But being too cautious risks missing a real event. Warning centers constantly balance these tradeoffs.
• Public awareness gaps: Many coastal residents and tourists don't know the natural warning signs of a tsunami or their local evacuation routes.
• Infrastructure resilience: Building tsunami-resistant structures is expensive, and many vulnerable communities lack the resources.
• International coordination: Tsunamis cross borders. Effective warning systems require cooperation between nations, standardized communication protocols, and shared monitoring networks.