The Motion of the Ocean

3 April 2009 By Chris Tibbs

As captains and crew prepare to deliver their yachts on their own bottom to the Med and other summer cruising grounds, it can be just as important to get a handle on the pattern of waves and swell mid-ocean as it is to gather the latest weather forecasts.

Out at sea, we find a mixture of waves and swell, often coming from very different directions. This can make for a rough passage, ranging from merely uncomfortable to quite dangerous. It also can cause discomfort to guests with delicate stomachs. It is usually the sea state that causes a captain to reduce speed, rather than the windspeed.

Whilst waves and swell are both generated by the wind, "waves” can be defined as “the waves generated by the wind blowing at the time, or recent past, in the area of observation”, while “swell” is defined as, “waves that have moved into the area that are generated by previous winds in other areas.” Swell may have travelled hundreds, or possibly even thousands of miles, and thus will have a long wave length in relationship to its height.

Watching North Atlantic storms and cold fronts is one way to predict a swell, but a quicker and easier solution is to look at wave models online.

One of the easiest databases to access is on the website. Once there, chose your area from the WW3 menu. You will find a series of choices with “significant wave height” at the top of the list. Significant wave height is the average height of the largest one-third of waves. This will give you an overview. I also like to look at swell heights and direction.

This website will not give you all the answers, as the scale is too small to go into a great deal of detail, but it does provide a good snapshot. Alternatively, The U.S. National Oceanic and Atmospheric Administration’s site, which also is recommended for weather forecasts, provides charts of wave heights. Check for the Caribbean and for the Atlantic. Do check out the units as some are posted in meters and some in feet.

Swell can be a problem mid-Atlantic, but once you’ve reached your destination, it can still haunt you. Lying in a beautiful anchorage that is suddenly spoilt by swell arriving unannounced is a common problem in many areas. As waves enter shallow water, they start to build in height. We consider water to be shallow when it is about half the wave length of the wave. As wave length in the ocean can be hundreds of meters in length, a wave will start feeling the bottom when it comes over the continental shelf. This slowing and steepening goes some way to explaining why even with a relatively calm sea offshore, once at anchor, the yacht will start to roll.

Waves also will bend around headlands and islands and come at you from different directions than the wind, so the lee of an island is not always as protected as you might hope it to be. In the Caribbean, you sometimes get a swell that propagates from a North Atlantic storm a thousand miles away.

Using wave models provided on the sites listed above can help you in planning itineraries. It is also possible to get wave heights in GRIB file format. Many local forecasts also include wave height and direction.

Chris Tibbs is a meteorologist who previously worked as a skipper in the yachting industry. He is the author of the Onboard Weather Handbook (McGraw-Hill).