Most of the world's water is 3 billion years old, and it moves at different rates through the hyrdrosphere. Water vapour circulates for days or even just a few hours while deep ocean water can be in circulation for thousands of years. From a geographical viewpoint, there are four main oceans, the Atlantic Ocean, the Indian Ocean, the Pacific Ocean and the Arctic Ocean. Lesser bodies of water are the Antarctic and the Southern Oceans.
In The Secret Life of Waves on BBC4 television, David Malone delves into the mysteries of the movement of waves across our oceans and seas, how they are formed and how they behave.
Ocean Waves: a Transference of Energy, not of Matter
Dr. Richard Porter of Bristol University explains that waves are simply the transport of energy travelling along water, not the transport of water itself. There is no displacement of matter or of molecules.The energy is caused by wind and it cannot be lost or destroyed. As waves run into shallow water, the water beneath cannot support the warmer water near the surface, so that the surface waves are compelled to move faster, overtaking the bottom layer and breaking as they reach for the shoreline.
Some of the energy returns to the sea, some dissipates through friction along the shifting sands, while some may be converted into sound - the wonderful roaring of a breaking wall of white surf we are all familiar with.
Dr. Porter's metaphor for the popping of billlions and billions of water bubbles as the waves break is "The Song of the Ocean."
Examples: a Dripping Tap and Newton's Cradle
Using a dripping tap as an example, Dr. Porter explains that the plop we hear is not a drop of water as it is landing. What you hear is a pop as air escapes from the bubble. This is because bubbles consist of pockets of gas, surrounded by water. The roar of the sea is actually the pulsating and breaking of all these bubbles of water and the escaping air. The individual sounds depend on the size of the bubble. Big bubbles make a louder, deeper sound than small ones. If the bubble is very small, you won't hear it since the vibration would be insignificant.
Professor Michael McIntyre of Cambridge University gives an example to describe the transport of energy in the ocean. The executive toy, Newton’s Cradle, consists of five balls suspended from a frame that appear to swing to and fro, but only the balls at each end actually move. The middle three remain static but allow the energy to pass through them.
Different Kinds of Ocean Waves
The height of a wave is due to the relationship between the wind energy directed into the waves and its duration.
- Capillary waves: These are ripples. It is the nature of surface tension to try to flatten waves, so that you see a regular undulation of water. Every "monster" forty-foot high wave, begins as a ripple. If you throw a cork into a rippling pond, it will bob up and down in one place.
- Gravity waves: When the ripples increase to a height of 2cm or more, the force of gravity becomes dominant, and pushes the wave down so that it moves forward.
- Swells: These occur with the combination of wind velocity, wind area (known as “fetch”) and duration. Waves accelerate and merge and with this enormous accumulation of energy, they can travel at the same height for long distances.
- Rogue waves: These are also known as killer waves, or freak waves and are formed from swells, moving at different speeds and coming from various directions. Rogue waves are larger than surrounding waves, have steep sides and deep troughs. They are also extremely unpredictable in behaviour.
Waves don’t just travel along the surface of the water. There are waves moving in the depths of the ocean, maybe in a different direction from the surface waves. Oceanic Rossby waves run between the warm upper layer of the ocean and the cold, deeper part. A tsunami is not a wind wave but is caused by a submarine earthquake, volcanic activity or subsidence. This can produce a huge, often destructive wave.
Some waves manage to flow in the reverse direction, for example, the Gulf Stream, a warm current that travels sideways in a north-easterly direction from the Gulf of Mexico to N.W. Europe. The Gulf Stream is also known as the North-Atlantic Drift.
Without the Gulf Stream, says Professor McIntyre, Europe would freeze over.
Sources:
"The Secret Life of Waves," presented by David Malone, BBC4, 3 February 2011.
National Weather Service, Jetstream – Online School for Weather
"Oceans" World Atlas, Copyright George Philip Ltd., Multiple Consultants, W.H. Smith, London, 2001.
Janet Cameron - MA. Cert.Ed. is a retired university lecturer and author of twelve books, women's short fiction and a magazine column.
Join the Conversation