These exercises were created by Dr. Karen Grove (© 1998 Grove) for use in the Introduction to Oceanography course offered by the Department of Geosciences at San Francisco State University. Other educators are welcome to use these exercises in their own classes. Please send comments to kgrove@sfsu.edu. Course home page: http://geosci.sfsu.edu/courses/geol102/home.html Commercial use is prohibited.
Voyage 5: The Origin and Evolution of Ocean Waves
Instructions: First print copies of the Lag-time questions and Real-time questions. Next read the text and examine the images on the computer and write answers to the questions on your paper copies. When you are satisfied with your answers, type them into the computer and submit You may also wish to print the text and images as a study aid.
Don't forget to check out the course web site where you can obtain preparatory information before each class and summaries that are posted after the class meets. Click here to bring up a new browser window with the course site. You can also use this site to display the text and images part of the voyage on a second window so that you don't have to print anything.
The waves that break on the beach were created by wind blowing over water. As they travel away from the area where they were generated they evolve into long, smooth-crested waves called "swell waves." Swell waves can travel for very long distances across the ocean without losing the energy they acquired from the wind. Local wind can also generate waves. These are called "sea waves" and have short, choppy shapes. So sea waves are generated locally and swell waves are generated far away in the open ocean. This voyage will deal with the generation and evolution of swell waves.
Figure 1. This image of wave height and direction is typical for the North Pacific Ocean, where most of the waves that hit our shores are generated. The colors correspond to wave height: blues are the smallest waves and reds-pinks are the largest waves. The arrows point in the direction that the waves are moving. The data in this image were acquired by instruments on buoys and ships that measure wave height. Computers are used to interpolate between the points of observation and to produce a map of wave heights for the entire ocean.
Click here to go to the FNMOC web site with real-time wave data from the North Pacific (NOTE: this site may take a minute or two to load). When you get to the FNMOC site, you will see a list of contents on the left side of the page. Under "Oceanography", click on "WW3", which will bring up the FNMOC Wave Watch 3 (WW3) page. Click on one of the small maps beneath NORTH PACIFIC (these are the same maps with different colors. I like the one that looks more black). You will next see a table of products. Click on the top dot in the leftmost column (significant wave height row and 000 column). This will bring up a map that shows wave heights in the North Pacific right now. The other dots in the table are predictions of what the waves will look like over the next 12-144 hours (012, 024, etc.) Also shown on this web site are wave data and predictions for the other oceans.
Swell waves can travel for long distances in deep water without losing the energy they acquired from the wind. But when they travel into shallow water their shape and direction changes. In shallow water, the waves slow down, their crests can bend and change direction, and their vertical profile steepens. They become so steep that they fall over themselves, breaking and losing most of their energy in the surf zone.
A group at the Scripps Institution of Oceanography uses computer models to predict how deep water swell waves will change direction and height as they enter shallow water along the California coast. In southern California, islands and an irregular coastline cause large changes in the swell waves from place to place. In this part, we will look at examples from their web site.
Figure 2. Use this map as a reference for coastal locations shown in Figures 3, 4, and 5. The map shows the California coastline between San Francisco and the Mexican border (red line at bottom of map). Note Point Reyes and San Francisco Bay near the north (top) end of the map and the Channel Islands off the coast near the south (bottom) end of the map.
Figure 3. This is a map of predicted wave heights during the winter along the coast between Point Conception (just west of Santa Barbara) and the Mexican border (south of San Diego). See Figure 2 for this location. In southern California, islands and an irregular coastline cause large changes in the swell waves from place to place. The black areas offshore are the Channel Islands (for example, Santa Cruz and Catalina). The predictions are made by a model that takes the deep water swell height and direction measured at offshore buoys and calculates how the height will change as the waves travel into shallow water.
NOTE: In all images, land is shown in black and the colors in the ocean correspond to different wave heights in shallow water. Blue colors indicate small waves and yellow to red colors indicate large waves; green colors indicate intermediate-sized waves (see scale at the bottom of the image). The inset compass diagram shows height (Hs, in feet), period (Tp, in seconds), and direction (Dp, in degrees) of swell waves in deep water. Only the primary swell direction (largest wave height) is shown on the compass diagram. All wave information is shown in a chart called Deep Water Swell. For your information, 0 degrees = north; 90 degrees = east; 180 degrees = south; 270 degrees = west. The wave direction is the direction the waves are coming FROM. The significant wave height is the average height of the highest one-third of all waves during a particular time period.
Figure 4. This image is for the same location as Figure 3, but shows a pattern of predicted wave heights that is common in the summer, rather than the winter. Note that the primary deep water swell direction differs from that in Figure 3.
Figure 5. This is a map of predicted wave height along the northern California coastline between Point Arena and Ano Nuevo (just north of Santa Cruz). See Figure 2 for this location. The coastline is straighter than in southern California and there are fewer offshore islands. However, headlands such as Point Reyes cause variations in wave height and direction along this part of the California coast as well.
Now let's see go to the Scripps real-time site to see the current wave height prediction along our coast today. Click here to go to today's model for the coastline near San Francisco.