DataStreme Ocean Benchmark Investigation 7A

DEEP- AND SHALLOW-WATER WAVES

Do Now:

  1.  Image 1 (Real-Time Image 1), Image 2 (Real-Time Image 2) and Image 3 (Real-Time Image 3) files.

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The first part of this investigation focused on the characteristics of shallow-water, transitional, and deep-water ocean waves. In this part of the investigation, we examine the influence of wind on the direction and height of ocean waves. Over the long-term average, the direction of wave movement largely mirrors the prevailing surface winds that produce the ocean gyres. But on any given day, wind direction may be much different than the long-term average due to weather systems (e.g., storm systems) traveling over the ocean. These wind shifts and related variations in wind speed alter wave characteristics, including the direction of wave movement and wave height.

Image 1 (Real-Time Image 1) is a global map showing significant wave heights and wave direction provided by Oceanweather, Inc. and valid for 06Z on 16 March 2004. Either online viewing or a color copy of this image is preferred for this investigation although a black-and-white copy is adequate. The wind field is derived from Oceanweather's Global Ocean Wave Model that is initialized using actual wind observations obtained by buoys plus remote sensing of the state of the sea by satellite. As noted on page 150 of your DataStreme Ocean textbook, significant wave height is defined as the average height of the highest one-third of waves observed. Arrows represent the direction of wave movement and are oriented perpendicular to the advancing wave crests.

  1. Significant wave heights are displayed in feet and meters using a color-coded background. (Refer to the color scale under the map. In black-and white, the lighter the shading of gray generally means the greater the significant wave height.) On the date and time of Image 1, the shading indicated that the regions of highest significant wave heights covered a greater portion of the North Atlantic Ocean in the latitude belt from [(0 to 30 degrees) (30 to 60 degrees)]. Since the waves are wind generated, we can assume that in regions of highest significant wave heights, surface winds were relatively [(weak) (strong)].

  2. According to Image 1, in both the Northern and Southern Hemispheres, the regions of highest significant wave heights covered a greater portion of the ocean surface in the latitude belt from [(0 to 30 degrees) (30 to 60 degrees)]. This difference in significant wave heights can be attributed to [(more) (fewer)] storm systems at those latitudes where the highest significant wave heights exist.

  3. According to Image 1, waves in the Southern Hemisphere between about 40 degrees S and 60 degrees S were moving generally toward the [(east) (west)]. The movement of waves at these latitudes indicates winds blowing generally from [(east to west) (west to east)].

  4. Image 1 indicates that, viewed from above, waves in the North Atlantic Ocean between 30 and 60 degrees N were generally moving toward the [(east) (west)] while waves in the North Atlantic between 0 and 30 degrees N were generally moving toward the [(southwest) (northeast)]. These patterns of wave movement are consistent with the surface winds associated the planetary-scale prevailing westerly and northeast trade wind belts.

  5. Image 2 (Real-Time Image 2) is a map of North Atlantic Ocean basin showing in greater detail significant wave heights and wave directions valid for 06Z on 16 March 2004 (the same time as the Image 1 map). Centered at about 33 degrees N, 61 degrees W is a broad wave pattern exhibiting [(clockwise) (counterclockwise)] motion as seen from above. The dark blue (dark gray) in the same region indicates significant wave heights which are relatively [(high) (low)].

  6. Image 3 (Real-Time Image 3) displays the Marine Observations and model data for the North Atlantic for the same time as the Image 1 and Image 2 depictions. The isobars drawn on Image 2 reveal an atmospheric high pressure system centered at about 35 degrees north, 50 degrees W. As is typical with Northern Hemisphere atmospheric high pressure systems, the wind pattern associated with the high displays [(clockwise) (counterclockwise)] motion as seen from above and the wind speeds near its center are relatively low.

  7. The wind of this atmospheric high was the likely cause of the ocean wave pattern and relatively low significant wave heights described in Item 16 above. Comparison of Image 2 and Image 3 shows that the center of the atmospheric high was somewhat to the [(west) (east)] of the center of the significant wave height pattern described in Item 16 above. This occurred because the atmospheric high was progressing rapidly eastward across the ocean surface. Wind-driven waves must build over time and are persistent once developed, causing their pattern to lag behind the advancing atmospheric conditions which caused them.

You may wish to further explore global wave analysis maps by going to the DataStreme Ocean Homepage and under "Physical & Chemical," click on "Oceanweather/Marine Data/Obs."

Optional: The University of Delaware has a wave calculator (Java applet) that enables you to determine how various wave properties interact. Go to: http://www.coastal.udel.edu/faculty/rad/. Scroll down to "Wave Calculator." First, click on "calculate" to determine the wavelength of the wave for which data already appear in the windows at the left. The calculator shows that the deep-water wave with a five-second period has a wavelength of 28 m. Next, check to see if the numerical values appearing on the Wave Analyzer Slide Chart you used in the print portion of this investigation are realistic by changing the period value in the calculator window to the value you determined and clicking on "calculate". Does the resulting wavelength fit the slide chart's value for the deep-water wave it depicts?

Want to learn more about waves? Try different combinations of wave height (m), period (seconds), and local depth (m), to calculate a number of wave characteristics including wavelength and wave celerity. If you really like waves, explore other measures determined by the calculator.


Self-Assessment Revisited: Respond to the statements below. Compare your responses now with those you made at the beginning of the investigation. Has learning taken place?

 

True, False
(T or F)

Confidence
(H, M, L)

Waves are water movements only on the water surface.

   

Waves break in shallow water when the wave height becomes greater than about 1/7 the wavelength.