SO231 General Oceanography

Geostrophic Currents and Ocean Circulation

Fall 2000

For the next two labs we will use the TS-PLOT program with a series of data sets around the world.  Your instructor will determine which data sets you will use.  You will do two different sets of calculations that will be turned in as homework; you instructor will give you specifics of the assignment and how the results will be used. Likewise you will not have to know the specifics of any of the current systems we will look at in the lab, but you must understand geostrophic currents and be able to predict them based on oceanographic conditions. 

Bring the book to both labs; you will need it to do the homework problems and to compare the results we see with the theory in the book.

Homework before the first lab: Solve Question 3.7 on page 51 of the text.  Do this in a spreadsheet, and bring your spreadsheet to class on a disk.  In designing your spreadsheet, consider that you will use the spreadsheet to solve a problem for the Kuroshio current.

Data set kurosh.rom: this data set consists entirely of data collected in August 1981, so it gives a synoptic view of the area around Japan at essentially one instant in time.  The Kurishio current is a warm current in the western Pacific analogous to the Gulf Stream in the Atlantic.

Look at the surface temperatures.  Where is the Kuroshio current?  Why don't the surface temperatures clearly show this current? (Try temperature-depth plots for the N-S line of stations off the southern coast of Japan).  You can also use the MCSST program to look at average August surface water temperatures near Japan.

 

 

We will calculate the Kuroshio current as a geostrophic current, with a reference level at 1500 m.  Go to View, Map Surface, Average Density. Accept the two depth levels at 0 and  1500 m.   Only those stations that went to 1500 m or deeper will be color coded.   The program will give you the average density (sigma tee) between the surface and your reference level.  Record the two densities for use in requirement 1b below.  Which way does the sea surface slope?  Which way will the geostrophic current flow?

 

Homework Requirement 1a. Look at the eastern of the two north-south profiles with stations over 1500 m deep, located at about 138½°E.  Click on each station and record the average densities at each of the nine stations along the profile between the surface and 1500 m.  Sketch the sea surface, show the direction of the pressure gradient force(s), and show the direction of the geostrophic current(s).

Homework Requirement 1b. Calculate the geostrophic current, and the difference in height of the sea surface, using the numbers you recorded above and the method explained on pages 50-51 of the text and question 3.7.

Data set pacific.rom: this data set consists primarily of August data.  We will look at an east-west transect along the equator.

What happens to the thermocline along the equator in the Pacific? (Use View, Profile contours, temperature; restrict depth to 750 m.)

Which way will the winds blow along the equator?  What will the Ekman transport be?  What will happen to the surface water?

What happens to the average density of the top 1500 m of the water column along the equator? (Use View, Map, Average Density, reference levels 0 and 1500 m.)

Which way will the sea surface slope along the equator?  Which way will the pressure gradient force be oriented?

What will the Coriolis force do along the equator?

Which way will the geostrophic current flow along the equator?  At what depth will it be?  Why?

Data set pac-eq.rom:  Look at Fig. 5.1 on page 123 to help with this region near the equator in the central Pacific south of Hawaii.  We will concentrate on the data on the north-south transect near 150°W.

Draw a north-south profile across the equator, (Use View, Profile Contour) for temperature.  Then rescale to show the upper 500 m of the ocean.  Where is the smallest mixed surface layer?   Why?

Locate the equatorial divergence.  At what latitude is it located?  What characterizes it here?

There is another divergence.  At what latitude is it located?  

Which divergence shows a greater displacement of the thermocline?

Look at a map surface for average density (Use View, Map Surface, Average density).  Pick 0 and 1000 m reference levels.

 

Along the north-south profile, where will the highs and lows in the sea surface topography be located?  How do these relate to the divergences you found earlier?

 

  

What geostrophic current(s) will result?

 

  

Does this pattern look like the currents shown in the book on page 123?

 

  

 

Data set s-polar.rom:  use with fig.5.22 on p.152

 

Look at the stations south of 70° latitude.  Which way does the sea surface slope?  Which way will the geostrophic current run?  What current is this?  What winds drive it?

 

 

 

Where is the West Wind Drift, as determined by the slope of the sea surface?  Which way does the current flow?

 

 

 

Where is the Antarctic Polar Front, and how could you recognize it?

 

 

 

Can you pick out the Antarctic Divergence?  What happens there?

 

 

Data Set redsea.rom: this data set consists entirely of stations collected in the month of July.  Note that some stations consist solely of deep data (below 1500 m).

Arabian Sea water: 26°C, 36.0 PSU

Indian Equatorial water: 28°C, 34.6 PSU

South Indian central water: 24°C, 35.1 PSU

Red Sea Water intermediate water: 22°C, 40.4 PSU

Common water: 1.5°C, 34.7 PSU

Antarctic bottom water: 0.2°C, 34.7 PSU

Homework Question 2. Rescale the T-S diagram to 34-36 PSU, and 0-30°C.  Plot the following water masses on the diagram: Arabian Sea, Common water, and Indian equatorial.  Plot station S10E65 (S 9.97, E64.83) on the diagram.  Post the depths beside the plot (To do this, go to options, T-S Depths, before plotting, or rescale the plot after you check the option).  Print the diagram (if your computer does not have a printer attached, get your neighbor to print a copy for you) so that you can answer the following questions: 

How does density change with depth at this station? Increases.

 

Calculate proportions of the three water masses at each of the following depths.  See the discussion on pages 189-190 of the text book, or question 6.19 on p.208.

 

Depth

ASW

IEW

CW

129
228
391
2969

 

What is the name of the feature centered at 228 m depth?

Is there any water at this station that could not be obtained by mixing the three water masses given?  How might you explain this fact?

Can you see any evidence of Red Sea intermediate water moving southward into the Indian Ocean?  What is the depth of this water within the Red Sea, and what would its density make it do if it entered the northern Indian Ocean?

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Can you see any water within this region that could not be a mixture of the water masses listed above?  What characteristics of a water mass could not be obtained by mixing the masses listed above?

Data set redsea.rom.  Use with fig.5.10, p.136  This data set consists of July data.

Look at the density distribution in the upper 1000 m of the ocean, along the coast of Somalia and Arabia.  Which way will the sea surface slope, and the geostrophic current flow?  Is this to be expected at this time of the year?

Can you find the equatorial convergence?

Data set redsea-3.rom.  Use with fig.5.10, p.136  This data set consists of March data.

Can you find the equatorial convergences?

How does this situation compare with the July data in terms of the sea surface slope and geostrophic current along the coast of Somalia?