Internal waves are a type of wave that occurs within a fluid, particularly in oceans, lakes, or the atmosphere, where different layers exist due to variations in density. Unlike the familiar waves we see on the surface of the water, internal waves propagate below the surface, along the interfaces between these layers of different densities. These density differences can be caused by changes in temperature (thermocline), salinity (halocline), or both (pycnocline).
Think of it like jelly layered in a dish; if you tap the side, waves travel along the boundaries between the different colored layers within the jelly, not just on top. In the ocean, warmer, less dense water sits on top of colder, denser water, creating these layers where internal waves can form.
How Internal Waves Are Generated
The formation of internal waves requires a disturbance to the stable layering of the water body. They are generated when the interface between the water layers is disturbed. This disturbance can come from various sources.
Primary Causes of Formation
One common way internal waves form is through the interaction of moving water with underwater topography. For example, when tidal flow pushes the layered water body over a shallow underwater obstacle, like a sill or a ridge. As the tidal current flows over the raised seabed, it lifts and displaces the denser water from below and pushes down the less dense water from above at the crest of the obstacle. This disturbance sets off waves that travel along the density interface.
Other potential mechanisms include:
- Wind stress: Strong winds can mix the upper layers or create pressure variations that disturb the interfaces below.
- Underwater landslides or seismic activity: These events can cause significant displacement of water layers.
- Flow interactions: Currents flowing past islands, coastlines, or other underwater features can shed eddies and generate waves.
Here's a simplified breakdown of the process:
- Layered Water: Water exists in layers of different densities (e.g., warm on top, cold below).
- Disturbance: Something disrupts the boundary (interface) between these layers.
- Wave Generation: The displaced water tries to return to its stable position due to gravity (like a stretched spring), causing oscillations that propagate as waves along the interface.
These waves can vary greatly in size, from small ripples to massive waves hundreds of meters tall (measured vertically), traveling vast distances through the ocean's interior.
