Wave dispersion illustrated by a large North Pacific storm: November 2-3, 2010

(data from Surfline and the Coastal Data Information Program)

Wave height: the vertical distance from the trough (low point) to the crest (high point) of a wave, measured in feet or meters.

Wavelength: the horizontal distance between two successive wave crests, measured in feet or meters.

Wave period: the time between two successive wave crests, measured in seconds.

On about November 1, 2010, an intense storm system developed over the Gulf of Alaska, whipping up huge waves. The waves began to spread away from the storm, and as the waves traveled south, gigantic surf soon began to hit the coasts of Canada, Washington, Oregon, and California. The waves had a range of wavelengths and wave periods, and thus dispersed from the storm area at different speeds.

Image 1 below shows the expected wave heights from this storm on November 2. The color scale at the bottom shows the heights of the waves in feet.

Image 2 below shows the expected wave periods from this storm on November 2. The color scale at the bottom shows the wave periods in seconds.

Image 3 below shows some of the early waves from the storm arriving along the coast of southern California. The time is 12:36pm on November 2. A sensor buoy—the CDIP Harvest Buoy off of Point Conception—detected the waves. The buoy recorded the height in feet (Hs(ft)), the period in seconds (Tp(s)), and the compass direction (Dp) of the waves. (The waves of interest here are the North Pacific swells, the first row of numbers under where it says “Deep Water Swell.” You can ignore the much smaller South Pacific swells shown in the second row of numbers.) Notice the very large waves hitting the coast north of Point Conception (the scale bar gives the expected wave heights). Notice how the waves along the coast south and east of Point Conception are much smaller. This is because of the change in the direction of the southern California coastline sheltered the coast from the full fury of these waves as they approached from the northwest.

Image 4 below shows the storm waves at 7:06am on November 3: about 18 hours later than Image 3 above. You can see, in the numbers under “Deep Water Swell,” that the compass direction (Dp) of the waves has not changed; the waves are still coming in from the northwest. But notice that both the wave height and the wave period have changed. These are all waves from the same storm, but they arrived in southern California at different times.

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