Sailors have known for many years that currents in the major ocean basins can be used to speed passage across the ocean. Benjamin Franklin even published a chart of the Atlantic Ocean that showed a "river-like" Gulf Stream flowing along the east coast of North America and, driven by the Prevailing Westerlies, across the North Atlantic to Europe. A return flow north of the equator is driven by the Trade Winds. Captains of early sailing ships, particularly the majestic 'Yankee Clippers', used their knowledge of these currents and the favorable winds to cut sailing times by several days. Traders not using this knowledge of ocean circulation soon went out of business because they could not compete. Now we understand that the Gulf Stream is highly complicated and variable (not at all like Franklin's "ocean river") with many fast moving meanders. We also now understand that currents flow below the surface of the ocean at intermediate, deep and bottom depths.
In this lesson we will begin our study of "motion-in-the-ocean", Part I -- ocean circulation -- the movement of wind-driven surface currents or density driven deep-water currents. Part II -- ocean waves and tides -- will be the subject of the next two lessons.
At the end of this lesson, you should: (1) Be able to describe how wind, blowing over the water surface, will transfer momentum to the water and, deflected by the Coriolis effect, create wind-drift currents that spiral; (2) Explain how the integrated mass transport from this spiral causes coastal and equatorial upwelling; (3) Demonstrate how the balance between the Coriolis effect and the down-slope component of gravity creates a Geostrophic current; (4) And explain how the vertical density distribution of the surface layer at two sites in the ocean can be used to calculate the Geostrophic current;.
Essentials of Oceanography, Thurman and Trujillio 11th Ed. Chapter 7, pages 197-233
To keep the timing of your learning consistent with the class schedule, this lesson has been divided into two parts. You may link to each below:
Part 1 contains a discussion of horizontal wind-driven circulation (including upwelling)
Part 2 contains a discussion of how horizontal currents are set up by sloping sea surfaces, how to determine this slope and how to estimate the Geostrophic Current