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Physical Oceanography
Combined Homework and Laboratory Set 5
Due 8 March 2004

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


Stability in the Ocean Using Java Ocean Atlas

The goal of this laboratory session is to learn more about stability using data from a meridional section across the South Pacific and functions available in Java OceanAtlas.

OceanAtlas for Macintosh was developed by John Osborne at the NOAA Pacific Marine Environmental Laboratory, James Swift at the Scripps Institution of Oceanography, and Edward Flinchem of Seattle.

  • Do the work using the copy of OceanAtlas you used for the last assignment.
  • The software allows you to look at the internal structure of the oceans, including temperature, salinity, density, potential density, and stability frequency, as a function of depth and as cross-sections using hydrographic data collected by various expeditions to the Atlantic, Pacific, and Indian Ocean.

For this laboratory:

  1. If you did not download data files with Java OceanAtlas, download and unstuff the JOA_data_preWOCE.sit.hqx from JOA Data Files at the OceanAtlas ftp site. It contains data from all the oceans.
  2. Open Java OceanAtlas, then open S.Pacific 150_W.poa data file. It is in the JOA_data_preWOCE, Pacific data folder.
  3. Under Plots, choose Map, choose the southern hemisphere, then Print the map. If you like, you can select a part of the map and JOA will plot the expanded map.
  4. Under Calculations, choose Parameters... Check N under Buoyancy Frequency (Hz) and Sigma-0. Click OK. This will calculate the buoyancy frequency and store it in the worksheet.
  5. Under Plots, choose Contour, then choose BV3 on PRES; 0-6000.srf: Y-axis range 0-1000 with increments of 100, Style: Color fill w/contours; Offset: Distance; Markers: Observations. Create autoscale color bar by clicking on the linear plot. Click Plot. The program should now open a new window with the contour plot of stability frequency as a function of depth.
  6. Print the contour plot.
  7. Data points in the contour plot window are linked to data points and locations in the data window S. Pacific 150° W. Moving among the data points at the bottom of the data window also moved the pointer in the contour plot. Use this feature to find values for variables in the data window corresponding to features in the contour plot.
  8. In answering the questions below, you may also want to make other contour plots.

Questions:

  1. Which side of the plot is to the north, which to the south? Label the print of the plot.
  2. What is Sigma-0?
  3. Where is the maximum buoyancy frequency? Latitude, Longitude, Pressure.
    • Why is there a maximum at this location? That is, what physical processes in the upper ocean lead to the formation of a maximum in this location?
    • Where is the minimum?
    • Note these locations on the plot.
    • Comment on the feature at a depth near 60 decibar depth at station 230. What might cause such a prominent, isolated minimum?

  4. JOA uses pressure in decibars instead of depth in meters. Why?
    • How is pressure related to depth?
    • What is the approximate depth of the maximum buoyancy frequency?
    • What is the depth of the 1000 dbar level at station 231 using approximate density from that station (pick an approximate density typical of the upper 1000 decibars of the water column).

  5. Does the salinity or temperature gradient seem to be more important in determining buoyancy frequency along 150° W? Please give reasons for your answer, referring to the data or plots in support of your statements.
  6. Is there a mixed layer?
    • Please give reasons for your answer, referring to the data or plots in support of your statements.
    • Where is the maximum of the buoyancy frequency relative to the mixed layer? In answering this question review the definition of a mixed layer.
    • Describe two processes that might cause temperature of the mixed layer at mid latitudes to become cooler at the end of summer.
    • Why is there no mixed layer in some regions?

  7. What other processes seem to influence the buoyancy frequency along 150° W? Please give reasons for your answer, referring to the data or plots in support of your statements.
  8. Are there any regions in the plot where double-diffusion instability might be important?
    • If there are regions, where are they?
    • Which type of instability exists?
    • What will the instability do to the density profile?

  9. Why is salinity at the surface higher at lower latitudes for this section?

Updated on: 5 September, 2004

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