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Physical Oceanography
Combined Homework and Laboratory Set 6
Due 30 March 2004

Late homework will cost 15 points per week or part of a week it is late.


Ekman Currents

Today you will learn about the usefulness of Ekman's theory for ocean surface currents, and the wind-stress data necessary to calculate the currents.

  1. Wind Stress. Use the values of Corrected Zonal Wind for January 1988 in the North Atlantic from the NOAA Pacific Marine Environmental Laboratory's Live Access to the National Virtual Ocean Data System (NVODS). Click on Dataset Name. In the new window, select the following data sets:
    • Atlas of Surface Marine Data, Check Corrected Zonal Stress, then select Next. In the new window, select January 1989 from the pull-down menu for Select Time. Use default values for other options. Print the plot.
    • CDC Derived NCEP Reanalysis Products Surface Flux. Check Monthly Mean of Momentum Flux, U-Component, then select Next. In the new window, select January 1989 from the pull-down menu for Select Time. Use default values for other options. Print the plot.
    • COADS 2-degree standard. Check u-wind pseudoStress Mon. Mean at Surf, then select Next. In the new window, select January 1989 from the pull-down menu for Select Time. Use default values for other options.To convert to stress, multiply by the air density = 1.3 kg/m cubed. Print the plot.
    • ECHAM surface stresses. Check taux, then select Next. In the new window, select 15 January 1989 from the pull-down menu for Select Time. Use default values for other options. Print the plot.
    • Hellerman-Rosenstein wind stress climatology. Check Zonal wind stress, then select Next. In the new window, select January from the pull-down menu for Select Time. Use default values for other options. Print the plot.
    • Oberhuber heat flux climatology. Check Zonal wind stress, then select Next. In the new window, select January from the pull-down menu for Select Time. Use default values for other options. Print the plot.

    Using these maps:

    • What is the value of the stress from each map in the area of maximum wind stress shown in the Oberhuber map of average january zonal wind stress south of Newfoundland and east of New England?
    • Do the maps show the same patterns of wind stress in the Atlantic?
    • What are the most notable differences among the plots?
    • Where do the data shown in the plots come from? To find out, go to Live Access to the National Virtual Ocean Data System (NVODS). Click on the information symbol (circled i) at the beginning of each listing. Write the appropriate information on each plot.

  2. Ekman Pumping. Use the information from the Hellerman and Rosenstein mean zonal wind stress data set for January in the North Atlantic:
    • What is the maximum zonal wind stress along 36° N latitude (the latitude of the peak stress off New England) in Pascals? You will need to convert dynes per square centimeter to Pascals (Newtons per square meter).
    • Estimate the average stress along 36° N. You do not need to be very precise. I suggest you pick an eastward stress typical of this latitude in the Atlantic.
    • Using the stress, calculate the northward Ekman mass transport across 36° N.
    • Next, calculate the volume transport along 36° N from North America to Africa.
    • Give your answer in Sverdrups.
    • What is the direction of the transport?

  3. Ekman Pumping. Sketch the average zonal component of the wind in the North Atlantic from the equator to Iceland.
    • Sketch the Ekman transports driven by the winds.
    • What is the influence of the Ekman transports on the density in the upper kilometer of the water column?

  4. Coastal Upwelling You are planning a vacation to the beach. You decide to go to the southern hemisphere for a change in pace. You also decide to go to a beach near 25° S because you know that the descending air of the Hadley circulation in the atmosphere leads to clear sunny skies at this latitude.
    • Look at a map of mean sea surface temperature of the ocean at 25° S at the east and west coasts of the three southern-hemisphere continents. What are the values of temperature?
    • What is the temperature difference between the east and west coast of each continent at this latitude?
    • Describe at least two processes that might cause the difference in temperature?
    • Do you want to vacation at an east or west coast?

Updated: September 5, 2004

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