What causes ocean currents and waves?
One 45-minute class period, plus time for student research
Classroom style or groups of 3-4 students
Ocean current, Ocean wave, Coriolis effect
More than 98% of cargo shipped to and from the United States is transported by water. In addition to accurate information on the geography of coastal areas, safe and efficient navigation of coastal waters requires up-to-the minute information on weather and sea conditions. Since these conditions can vary significantly from place to place and can change dramatically
in a short period of time, mariners need accurate real-time information to avoid groundings and collisions. NOAA’s Center for Operational Oceanographic Products and Services (CO-OPS) collects and distributes oceanographic observations and predictions to ensure safe, efficient and environmentally sound maritime transportation. The Center:
CO-OPS also manages a national network of Physical Oceanographic Real-Time Systems (PORTS®) located in major U.S. harbors. The PORTS® network provides real-time information such as water levels, currents, air gap (the clearance between the water surface and the bottom of a bridge), weather data, and other oceanographic information to help mariners avoid groundings and collisions. Visit http://tidesandcurrents.noaa.gov/products for more information on CO-OPS, PORTS®, other CO-OPS programs and their data products.
While CO-OPS deals mostly with currents along the coast and inside estuaries, other NOAA Program Offices are involved with measuring and understanding currents and circulation patterns in the open ocean. NOAA’s National Oceanographic Data Center (NODC) compiles information from the latest ocean current measurement programs that use current meters and drifters. Through the NODC Web site (http://www.nodc.noaa.gov/General/getdata.html), you can access a variety of data sets containing information on currents and other oceanographic measurements, such as beach temperatures, coastal buoy data, global temperature and salinity data, and photograph collections. For global current data obtained through satellite remote sensing systems visit NOAA’s Ocean Surface Current Analyses - Real Time Web site at http://www.oscar.noaa.gov/.
In this lesson, students will explore the relationships between currents, winds, and ocean waves.
The correct answers are:
- 3 feet
- Increasing the wind speed by 60 knots would increase the wave height to approximately 12 feet, while increasing the fetch length by 60 nautical miles (nm) would increase the wave height to less than 6 feet.
- A 60 knot wind would have to blow over a fetch of about 9 miles to produce a wave 10 feet high.
- The distance between the points is 524.6 nautical miles. The total time elapsed is 6 days, 10.25 hours = 154.25 hours. So the estimated current speed is:
524.6 nm ÷ 154.25 hr = 3.40 nm/hr = 3.40 knots
The estimated direction of the current is northeast.
- The distance between the points is 1,443.68 kilometers = 1.444 x 108 centimeters. The total time elapsed is 14 days, 2.92 hours = 338.92 hours = 1.220 x 106 seconds. So the estimated current speed is
1.444 x 108 cm ÷ 1.220 x 106 sec = 118.4 cm/sec
The estimated direction of the current is slightly east of due south.
- Since the latitude at the equator is zero, the formula for Coriolis acceleration suggests that the magnitude of this acceleration at the equator is zero.
- The latitude of Tijuana is about 32.5° N. A velocity of 10 meters/second is equal to 1,000 centimeters/second. So, the magnitude of the Coriolis acceleration is
(sin 32.5° • 1.5 x 10-4 • 1,000) cm/sec2
= 0.537 • 1.5 x 10-4 • 1,000 = 0.081 cm/sec2
The effect is very small.
- Even though the effect of Coriolis acceleration on soccer balls, walking humans, etc. is practically negligible, when it acts on very large masses over very long distances, the acceleration becomes significant.
The Bridge is a growing collection online marine education resources. It provides educators with a convenient source of useful information on global, national, and regional marine science topics. Educators and scientists review sites selected for the Bridge to insure that they are accurate and current.
www.vims.edu/bridge - In the “Site Navigation” menu on the left, click on “Ocean Science Topics,” then “Physics,” then one of the headings at the top of the page for links and resources about tides, waves, and currents.
Have students write a short essay on how the Coriolis force affects them personally, even though it only is significant at very large scales.
http://oceanexplorer.noaa.gov/explorations/03edge/background/edu/media/coriolis.pdf – Lesson on the Coriolis force from NOAA’s Ocean Explorer program, including “the Dishpan Analogy” explanation for this effect.
http://oceanservice.noaa.gov/education/tutorial_currents/ - Tutorial on tidal, coastal, and ocean currents.
http://oceanservice.noaa.gov/education/tutorial_tides/ – NOAA’s “Tides and Water Levels” Tutorial.
http://tidesandcurrents.noaa.gov – NOAA’s Center for Operational Oceanographic Products and Services (CO-OPS) Web page, with links to data and information about tides, water levels, currents, predictions, weather observations, forecasts, and harmonic constituents.
http://www.usm.maine.edu/maps/lessons/nr10.htm and http://www.usm.maine.edu/maps/lessons/nr11.htm – Lesson plans and activities about currents from the University of Southern Maine’s Osher Map Library.
http://www.eeb.ucla.edu/test/faculty/nezlin/PhysicalOceanography.htm — Online tutorial with additional details about ocean currents
Surface ocean waves are produced by winds. The height of these waves depends upon wind speed, the length of time the wind blows (duration) and the distance over which the wind blows (fetch). In 1952, Charles Bretschneider created a diagram that describes the relationship between these parameters and provides an easy way to predict the height of a wave produced by specific wind conditions. Figure 1 is an example of this kind of diagram (usually called a “Sverdrup-Munk-Bretschneider nomogram”). The y-axis describes Wind Speed; the x-axis describes Fetch Length; solid curved lines in the middle of the diagram show the Wave Height in feet (most Sverdrup-Munk-Bretschneider nomograms also include lines showing wave period and wind duration; these have been omitted from Figure 1 for clarity). When using the nomogram, be sure to match these lines with the correct labels!
There are a variety of ways to measure the velocity of a current. One of the oldest and simplest methods is to use a “drifter,” which can be any floating object (an ideal drifter is one that is not affected by wind; glass bottles partially filled with sand are a traditional type of drifter). To measure current velocity, an observer places the drifter into the water, measures the amount of time the drifter takes to move a known distance, and notes the direction of the drifter’s motion (since velocity is a vector quantity, and has dimensions of direction as well as speed). Next, the observer finds the speed of the current by dividing the distance the drifter traveled by the time it took to travel that distance. The speed of the drifter combined with the direction in which it moved is the current’s velocity.
Suppose a drifter is released near Charleston, SC from a research vessel whose position is 32°23’15” North latitude, 79°12’33”West longitude, at 0915 eastern standard time (EST) on May 11, 2004. A sailing yacht recovers the drifter at 1930 EST on May 17, 2004 in position 39°56’23” North latitude, 73°44’35” West longitude. What is the estimated velocity of the current that transported this drifter? In this case, it is sufficient to describe the direction component of the velocity vector as north, northeast, east, southeast, south, southwest, west, or northwest. State the speed component of the vector in knots (nautical miles per hour). [Hint: You can use the calculator at http://www.chemical-ecology.net/java/lat-long.htm to find the distance between two points whose latitude and longitude are known.]
If you would like to have a map of the area covered by the drifter, visit the Marine Geoscience Data System Web site (http://www.marine-geo.org/tools/maps_grids.php). Enter the latitude and longitude boundaries for the area you want the map to cover, then click on the “Map” button. In this case you would enter 40° as the northern boundary; -80° as the western boundary (note that longitudes west of the prime meridian are assigned a negative value, while longitudes east of the prime meridian are positive); -73° as the eastern boundary; and 32° as the southern boundary. The map will show the elevation (or depth) of Earth’s surface in the included area. You can download the map using the “Save Image As . . ” function of your Web browser.
You can use the Marine Geoscience Data System Web site (http://www.marine-geo.org/tools/maps_grids.php) to generate a map as described above. Enter 47° as the northern boundary; -126° as the western boundary; -120° as the eastern boundary; and 34° as the southern boundary.
where w is the angular velocity of Earth, v is the velocity of the moving object, and f is the latitude in degrees. Since the angular velocity of Earth is about 7.29 x 10-5 radians/sec, acceleration due to the Coriolis effect is about
(1.5 x 10-4 • v • sin f) cm/sec2
(note that radians have no units). What does this equation suggest about the magnitude of the Coriolis acceleration at the equator?