[By Callum Shakespeare]
Last week was perhaps the first time you heard of “internal waves” – the phenomenon believed to have caused the tragic sinking of the Indonesian submarine KRI Nanggala the previous week, resulting in the death of the 53 crew members.
So you may be surprised to learn that you have undoubtedly already encountered internal waves. They exist all around us in the atmosphere and the ocean, although they are generally invisible. If you’ve ever been on a turbulent plane, you’ve felt the effects.
Satellite image of internal waves in the atmosphere and ocean off the northwest coast of Australia. In the atmosphere, we see waves as lines of clouds. In the ocean, waves appear in the reflections of the sun’s rays on the surface of the sea. NASA
Internal waves are generated when a strong wind passes over a steep hill. The air is lifted up and over the hill against the force of gravity, then accelerates on the other side as gravity takes over. This up and down movement triggers an oscillation downwind of the coast. Oscillating motion is an internal wave.
You can visualize this more easily by imagining a bouncing ball rolling on a step on otherwise flat ground. If you roll it fast enough, the ball takes off at the crest of the step and accelerates downward under gravity. When the ball hits the ground, it begins to bounce with a bounce length (or wavelength) that depends on how fast you rolled it.
Internal waves are generated by rapid flow on a steep slope, much like a ball bounces when it is rolled at high speed on a step (illustration provided by the author)
Not surprisingly, internal atmospheric waves are most often found in mountainous regions. If you’ve ever looked at the sky and seen long parallel bands of cloud, especially near mountains, you’ve probably seen an internal wave spreading through the atmosphere. The waves propagate upwards at the same time as they are carried downwind of the mountain by the air flow.
Waves can reach all the way into the stratosphere, which begins about six miles above the ground, before changes in atmospheric structure force the waves to break. Just as waves break on the beach when the water gets shallower, internal waves break up in the atmosphere when the properties of the air (such as flow velocity or density) change rapidly with height. . Such changes are common in the lower stratosphere (six to nine miles), where airliners fly.
And just like the waves on the beach, this break creates a tremendous amount of chaotic motion – or turbulence – creating an unpleasant shaking motion for any aircraft (and their passengers) that are nearby!
So what about internal waves in the ocean? Just like in the atmosphere, they are generated by strong fluxes (in this case, ocean currents) over steep hills. But in this case, the hills are on the seabed.
The steeper the hills and the stronger the currents, the bigger the resulting waves. The seas around Indonesia have a perfect combination of these ingredients: a network of deep basins connected by narrow, shallow channels, through which strong tidal currents flow.
These currents are so strong that they generate a particularly extreme type of internal wave called an âinternal solitary wave,â which concentrates all of the wave energy in one up and down motion, rather than many individual oscillations. These waves can be hundreds of feet high, several miles long, and travel at speeds of five knots.
Solitary waves are greatest at depths of around 160 to 650 feet, where there is a pronounced temperature gradient between the warm surface layer and the cool interior of the ocean – the same depths at which the sub- sailors generally operate. If a submarine sitting at this kind of depth were suddenly hit by one of these waves, it would be carried down (or up, depending on its position in relation to the wave) at a speed of about 30 feet per minute for 10 minutes.
Without swift action to counter wave movement, a submarine could quickly be transported below its maximum operational depth, resulting in hull failure and sinking. An archived US Navy report reveals that submarine commanders were aware of the risks of internal waves as early as 1966.
In addition to the danger they represent for submarines, internal waves also play an important role in ocean circulation. They carry large amounts of energy, helping to maintain ocean currents, mixing heat and carbon dioxide across the oceans, and thus influencing our global climate.
So the next time you are rocked by the turbulence on an airplane, or looking at strange bands of cloud in the sky, think about the internal waves propagating all around you.
Callum Shakespeare is Senior Lecturer in Climate and Fluid Physics at Australian National University.
This article appears courtesy of The Conversation and appears in its original form here.
The opinions expressed here are those of the author and not necessarily those of The Maritime Executive.