SIO 210 Topic 1: Introduction to ocean circulation.

Lynne Talley, 2000
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Reading and study questions


Geography: Pickard and Emery, chapter 2, especially 2.1 and 2.21

Introductions to circulation: Tomczak and Godfrey, chap. 1 or Pickard and Emery, chap. 7.1

Surface circulation: P&E figs. 7.9, 7.26, 7.31, 7.45 or Tomczak and Godfrey maps in each chapter on different oceans

Broecker, W.S., 1991. The great ocean conveyor. Oceanography, 4, 79-89.

Study questions:

1. How much of the earth is covered with water? In what latitude ranges is the earth covered by water all the way around? What effect does the presence of continental boundaries at most latitudes have on the circulation? What happens where there is no continental boundary?

2. How deep is the ocean on average? How does this compare with the average zonal dimension of the three major oceans? What effect might this difference between the horizontal and vertical dimensions have on the relative magnitude of the horizontal and vertical velocities? (Another factor affecting the vertical velocities is vertical stratification.)

3. What are the forcing mechanisms for the ocean?

4. In the northern hemisphere, what is the flow pattern associated with the subtropical gyre? Is pressure high or low in its center? What differs for the subtropical gyre in the southern hemisphere? Are these gyres driven mainly by the wind or by heating/cooling? Answer the same questions for the subpolar gyre as well.

5. Memorize the names of the major western boundary currents found at the surface. Name a major southern hemisphere current which has no northern hemisphere counterpart.

6. What is the range of horizontal velocities for the ocean? What is the typical vertical velocity?

Study calculations:

1. Consider a layer at the bottom of the ocean, so no water can exit down. Assume that the layer is 5000 km in the east-west directions and 5000 km in the north-south direction. Assume that the layer is 500 m deep. Assume that the layer is closed on the north, east and west sides by continents (e.g. bottom-most layer of the North Atlantic). Suppose the average flow northward into the layer is 1 cm/sec. All of this water must upwell. Assume it upwells at the same rate everywhere. Calculate the average vertical velocity.

2. (a) Suppose that the volume transport associated with the wind-driven circulation's western boundary current (such as the Gulf Stream) is 50 x 10^6 m^3/sec (where ^ is my only way to write superscript in this html file). Suppose that this is carried in a current of uniform speed which is 50 km wide and 1 km deep. Calculate the average velocity of the current.
(b) Suppose that the velocity associated with the thermohaline circulation's Deep Western Boundary Current (DWBC) which flows southward under and inshore of the Gulf Stream is approximately 0.2 m/sec. Suppose that this current is 50 km wide and 0.5 km high. What is the volume transport associated with this current?

Further reading

Gill text, sections 1.2 and 1.6

Additional sources for viewgraphs (figure or data for the figure):

Barnier, B., L. Siefridt and P. Marchesiello, 1994. Thermal forcing for a global ocean circulation model using a three-year climatology of ECMWF analyses. J. Marine Systems, 6, 363-380. (net surface heat flux; gif plot data)

Gordon, A.L., 1986. Interocean exchange of thermocline water. J. Geophys. Res., 91, 5037-5046. 339-359. (source of popularized conveyor belt idea)

Schmitz, W. J., 1995. On the interbasin-scale thermohaline circulation. Rev. Geophys., 33, 151-173. (updated conveyor belt ideas)

Schmitz, W. J., 1996. On the world ocean circulation: volumes I and II. Woods Hole Oceanographic Institution Technical Report WHOI-96-08. (nice 3-D figure of thermohaline circulation)