Lessons Index:
6. CHART MAKING FOR NAVIGATORS
Osher Map Library
University of Southern Maine
Charting Neptune's Realm:
From Classical Mythology to Satellite Imagery
An exhibition at the Osher Map Library and Smith Center for Cartographic Education, University of Southern Maine, Portland, 4 April 2000 to 11 January 2001
Donald S. Johnson, guest curator
SURFACE CURRENTS
Osher Map Library Lesson
Charting Neptune's Realm
Hope McVane
B. Background Information
The Osher charts you will be using show Ben Franklin's A Chart of the Gulf Stream and John Elliot Pillsbury's Chart of the Gulf Stream. They used sea surface temperatures to identify this great river within the sea. Notice that the charts differ in age by approximately one hundred years. Comparing and contrasting the charts would lead to interesting discussions about the differences in technology during that time period. An extension of this activity would be to find another chart of the Gulf Stream created during the 1990's to view the difference in technology within the last one hundred years.
Ocean currents are defined as any continuous horizontal flow of water along a broad path in the ocean. Surface currents travel parallel to the surface and are caused by our prevailing winds. Temperature readings along with prevailing winds help early sailors find currents and then use them to their advantage in traveling across oceans. Density currents, also called thermohaline circulation, are caused by differences in density of ocean water. These are discussed in more detail in 'The Gulf Stream, Osher Map Library Lesson 5'.
Surface currents are driven by two main winds, the trade winds and the westerly winds (see 'Riding the Wind, Osher Map Library Lesson 3'). Trade winds blow from the northeast in the northern hemisphere and from the southeast in the southern hemisphere. The Westerlies blow from the southwest in the northern hemisphere and the northwest in the northern hemisphere. In looking at the prevailing winds and the currents one can determine the correlation between the prevailing winds and the surface currents. An example of this is that the Gulf Stream is powered by the South Westerlies. Students will be able to predict the warm and cold currents by looking at the prevailing winds and the direction from which they are blowing. Warm currents usually head away from the equator. Cold currents usually flow towards the equator.
Surface currents usually occur in the upper layers of the ocean, the mixed layer. The Gulf Stream, a surface current driven by the Southwesterlies, does not always stay in the same path. It will wander slightly, slow down and/or speed up. On average, the Gulf Stream is 240 km wide and 1.6 km deep and can move 100,000,000 cubic meters of water a second. Off the coast of New England, there is a cold-water plug between the coast and the Gulf Stream. This cold water is from smaller currents coming from the Labrador Current. Therefore New England does not receive the benefits of the warm water brought up by the Gulf Stream. However, the warm water affects the weather of the British Isles and Iceland. Without the Gulf Stream, these places would have much cooler climates. On the eastern side of the Gulf Stream we find warmer water and a relatively calm area called the Sargasso Sea. This is known for the large amounts of sargassum, or seaweed that floats there on the surface.
Rings, circular movement of water in the ocean, of cold and warm water often break off from the Gulf Stream. These are called eddies and have different characteristics depending on the side of the Gulf Stream that they break from. Eddies that break off on the western side spin clockwise and contain warm water- a warm core ring. Eddies that break off on the eastern side contain cold water and spin counterclockwise. These are called cold core rings and are surrounded by the warm Sargasso Sea.
Examine the charts of the global prevailing winds and ocean currents. You will see an alignment of the winds and currents. Notice how the Gulf Stream follows the South Westerly winds, while the Brazilian current is driven by the North Westerly winds. The trade winds drive the north and south equatorial currents. Polar Easterly winds aid the Canary Current in the Atlantic Ocean.
In this activity, students will be able to identify the cold-water plug between the New England coast and the Gulf Stream while identifying the Gulf Stream within the Atlantic using sea surface temperatures. They will determine a correlation between the prevailing winds and major surface currents by comparing maps of both.
Return to Lesson 10
Index
Return
to Charting Neptune's Realm index of lessons
Return
to Osher Map Library's complete list of lessons on the web