Hobie Surf: Surf Science 101: Genesis of Waves

Welcome to a new feature on our blog, think of it as your Sunday morning read. A chance to get up, sip your coffee and learn something new, or enrich your knowledge on something you already know a lot about. Gary Larson, will take you through the science behind the things we all enjoy, but aren’t really sure exactly why/how they happen in his course  Surf Science 101. Enjoy!!

Waves

Ladies and gentleman summer is upon us and that means south-swells are aplenty. You’re excited for the 18-20 second, New Zealand ground-swell from about 200° but not exactly sure why? Well, feel free to saddle-up for the inaugural weekly installment of Surf Science 101, where I will attempt to enlighten you on some of the perplexing phenomenon we, as surfers, often encounter in the ocean. This week I will be exploring the formation of ocean waves, the variables involved and how they can travel for thousands of miles before breaking along the coastline.

Contrary to the explanations I have heard, from middle America folk to the well seasoned Southern California surfer, waves that we surf on a daily basis are not caused by the moon, tides or the large freighters criss-crossing the ocean. The only variable that is absolutely essential for wave propagation is wind. That’s it. If the planet Earth ditched its moon, ceased all tidal fluctuations and sank every boat in all seven-seas, but wind still blew over the surface of the ocean, we would still have waves.

That being said, there are three important factors that govern wave heights; wind speed, wind duration and fetch. Fetch is understood as the distance over water that wind blows in approximately the same direction. If there is an increase or decrease in any of these three variables the wave height will either increase or decrease, respectively. For example, if a 40-knot wind blew for 24 hours over a 100 mile fetch the wave heights would be larger than if the wind speed was 30-knots and the fetch was 50 miles.

Wave Propagation Chart. Wave heights increase when the wind speed and/or fetch length increases.


It is the friction between the movement of air (wind) over the surface of the water, in this case the ocean, that is the genesis of wave production. The first waves that are created through the air-ocean interaction are called wind waves. These waves start off relatively small with very short wavelengths, or distance between successive wave crests (or troughs), and are found in the fetch. If the wind speed remains constant or increases and the fetch length increases, the wind can transfer energy to the ocean surface more efficiently as the roughness of the ocean surface increases. I like to think of it as if the wind waves act as sails and wind blows against the backs of the waves increasing their speed, energy and amplitude. Now let’s look at an example of the formation of a storm, the wind generating force, and how waves propagate from the storm center.

The synoptic sea-level pressure map above shows a large storm in the south Pacific, during July 7th, 1998, defined by an area of extreme low-pressure centered at about 60°S latitude, 140°W longitude, or about half-way between New Zealand and southern Chile. You can approximate the storm center by looking at the contour lines and noticing where the decreasing sea-level pressure (SLP) centers into, going from 1015mb all the way down to 980mb, where the last contour line is available on this map. Because this storm is located in the southern hemisphere the Coriolis Effect will cause the storm to rotate in a clockwise direction meaning that the wind will be blowing from the south on west side of the storm and from the north on the east-side. The thing to notice on this map is the position of the high pressure area, to the west of the low, basically over New Zealand. Air always moves from high pressure areas to low pressure areas. The large the difference in pressure between these two systems and their proximity to each other ensures that there will be high wind speeds around the low.

Now, by looking at the synoptic wind speed map of the same area during the same day we can see that the fastest wind speeds are to the west of the center of the storm (turquoise color) along the boundary of the high pressure system. The winds in this area reached constant speeds of about 35-knots with a fetch of what looks to be about 1500 nautical miles.

Looking at a swell synoptic map showing the wave height and direction of travel. Notice how the largest waves (the purple blob) are in the area just slightly west of the storm center at about 160°W Longitude. This is also the area with the highest wind speeds and, if we refer back to the first image of the SLP contours, it is where the contour lines between high pressure and low pressure were close together signaling a rapid change in air pressure in the region.

When the wind waves move out of the area of wind influence, they become free moving waves or what we all know as swell. Swell deriving, and moving away from a common generating force, such as the southern hemisphere storm we looked at, will gather into groups of waves with approximately the same period and speed.

I will discuss swell characteristics in a later post where I will delve into swell period, speed, and attempt to answer how during many swell events, waves can be over-head at one beach and practically flat at a beach only a few miles away.

Gary Larson

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