Tuesday, March 11, 2008

Ocean Science 101: How our surf is made (Part 3: Characteristics of swells)


The swell stage of a wind wave life-cycle occurs both in the generation phase and the swell phase (duh Adam), which is why the surf gets big when a storm or heavy winds move right over your beach. Usually though the swell phase occurs when the energy of a storm moves away from the area it was created.

As the swell energy moves away from the storm center it becomes more organized and structured. The energy generally moves along the path that was established by the strongest areas of fetch, but because storms are circular in nature swell move out in all directions and is very similar to dropping a rock into a pond…a better description though would be a rock thrown into a pond, where the biggest waves move out along the direction the rock was traveling when it hit the water (stupid rock analogy).

We measure swell energy in a rather unique way…it is actually a time based measurement called swell-period. This refers to the interval between the passage of two successive swell crests past a fixed location in the ocean and is measured in seconds. Swells with periods of 16-seconds or more are generally referred to as “long-period swells”. Swells in the 12- to 15-second range are considered “medium-period swells”. Swells that are 11-seconds and below are called “short-period swells”

Generally the longer the swell-period the more energy it has and the faster it moves. So as swell first starts to leave the storm area all of the swell energy will be piled up and moving more or less together but as it starts to travel across the longer distances the swell energy starts to separate. The long-period swells move faster than the shorter ones so eventually they out-distance them.

The longer the distance between the storm and the beach the more separated out the swell periods will get. For example a swell generated several thousand miles away in the South Pacific will arrive with very distinct differences in swell period. The swell will hit the buoys with the long-period energy first and will follow the progression down through the shorter periods.

A closer storm, like the ones that form just off the coast of California, will blast us with the full spectrum of swell periods almost at the same time…you may get a little jump in long-period energy first but then the peak of the energy will come through in a large lump…it may jump from a 18-second swell straight to a 12-second swell and peak with a bunch of energy in-between.

More Random and complicated stuff about swells

Swells are complex beasts and they live in a weird state of physics, which makes them even more complicated.

To even start the discussion you need to know (and hopefully understand) a thing known as the Coriolis Effect.

Wikipedia describes (because I am getting a headache) the Coriolis Effect as “an apparent deflection of moving objects from a straight path when they are viewed from a rotating frame of reference.

Freely moving objects on the surface of the Earth experience a Coriolis force, and appear to veer to the right in the northern hemisphere, and to the left in the southern. Movements of air in the atmosphere and water in the ocean are notable examples of this behavior: rather than flowing directly from areas of high pressure to low pressure, as they would on a non-rotating planet, winds and currents tend to flow to the right (left) of this direction north (south) of the equator. This effect is responsible for the rotation of large cyclones”

On the surface this seems straight forward enough but with swell it takes on a slightly different twist. Coriolis only affects objects with mass. A swell is almost entirely energy, which has no mass, so it is NOT affected by Coriolis. (I really hate physics sometimes).

The tricky part is that air and water molecules have mass, so you have to take Coriolis into account when you are dealing with currents and storms, (basically all the things that help to create a swell), but the second a swell leaves a storm area as energy in the water it is no longer affected by Coriolis.

I am sure a lot of you are swearing at me right now for even bringing this up but it actually plays a very important role in tracking swells. Because the swell is not affected by Coriolis it travels in a straight line. You can basically draw a line from the storm’s fetch straight to your surf break…if no land gets in the way then you will see waves from that swell.

Important Safety Tip: Oh yeah that straight line, from storm to break, has to be on a globe…not a flat map…when you draw it on a flat map you have to take into account the curvature of the earth, which means that the straight line has to curve along what is called a “great circle line” or is sometimes referred to as a “Rum Line” in ship navigation.

Swell decay

From time to time you will hear a term called swell-decay…this is another one of those tricky concepts. Remember a couple of paragraphs ago when I said swells are energy and have no mass? Well it is not entirely true…if a swell was pure energy it wouldn’t have any friction and all swell periods would move at the same speed and be able to travel across the entirety of the ocean. In reality there is a little bit of mass in a swell, or at least friction.

This occurs as the swell moves through the water…the energy actually extends downward as well as along the surface. The longer the swell period the deeper the energy goes. In this “column” of swell (for lack of a better word) there is some circular motion of water molecules. It is this slight motion of actual mass that allows some of the swell energy to bleed off as friction. In short period swells there is a lot of this circular motion (mostly because the swell’s waves are so close together). It is this energy reduction that is usually referred to as swell-decay.

What happens is that swells will lose energy as they travel longer distances…the longer they go the more they lose. Short-period swells lose the most energy the fastest…so a swell with a period under 10-seconds can only go a couple hundred miles before it has lost so much energy that it can’t create waves any longer.

A long-period swell decays much slower and will only lose a small amount of energy over long distances. It is possible for a 18-20 second swell to travel thousands of miles and only lose 2/3rds of its energy before it hits the beach. So if a swell started off with 10’ of long-period energy it would still have almost 3’ of energy when it arrived at the beach.

This is one of the reasons why Southern California never gets short-period energy from the South Pacific…all of it decays away before it reaches us.

Part 1: Overview – Types of Waves
Part 2: Wave Creation and Swell Generation
Part 4: Breaking waves

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