Chapter Six

Data analysis concepts and observational methods

Example 6B: Sections with horizontal axis other than distance

We have plotted vertical sections with horizontal axes based on space - either distance or latitude (or longitude). What if one wanted to examine changes over time with JOA?

Oceanographers are interested to learn more about how the ocean varies over time. With the advent of satellite remote sensed data, it has become possible to obtain approximately daily global coverage of some parameters - for example ocean surface temperature. Satellite-based observations cannot observe the ocean interior.

Various drifting or moored instruments can provide ocean interior data over the lifetime of the instruments, or the span of the lifetimes of their replacements. But the parameters these can sample are limited, though the list is continually growing.

Repeated hydrographic observations are an expensive and time-consuming, but potentially quite valuable, method of observing variations in the ocean interior over a wide range of parameters.

Currently the International Ocean Carbon Coordination Project (IOCCP; http://www.ioccp.org/) is carrying out repeats of selected basin-scale transects at approximately ten-year intervals. Some nations carry out annual or seasonal occupations of somewhat shorter, but valuable transects (for example, see the Pacific Ocean PR17 or the A01/AR07 listings at http://cchdo.ucsd.edu).

At a few key locations, oceanographers and national organizations have worked to provide ca. monthly observations. One of these is the Hawaii Ocean Time-series (HOT) measurements (see http://hahana.soest.hawaii.edu/hot/hot_jgofs.html). We have downloaded and prepared for JOA some of the HOT Station Aloha bottle data in the upper 300 meters, and provide these as the data file HOT_bottle_nutrient_data.joa.

Files that may be needed or created in this example:

  • HOT_bottle_nutrient_data.joa

Exercise 6b-01: Other Plots - Using Time As the X-Axis on a Contour (Vertical Section) Plot

  • Start JOA
  • File → Open…HOT_bottle_nutrient_data.joa

    The JOA Data Window default station display (at the bottom of the Data Window) is normally set to offset successive stations by distance, but since the HOT data are all at the same location, the display shows little.

  • Double-click on the station display at the bottom of the data window, which brings up the Configure Cross Section dialog box
  • Set up the Configure Cross Section panel as seen below:

    target

    Fig 6b-03 Configure Cross Section panel

  • After clicking OK the Data Window should appear like this:

    target

    Fig 6b-04 Station panel

    You can see that the HOT bottle nutrient data set contains many successive re-occupations of the same site, with somewhat uneven sampling in the upper 300 meters from one occupation to the next.

  • PlotsContour…
  • Set up a JOA vertical section (Contour) plot of salinity in the upper 300 meters by choosing PRES-0-300_srf.xml under Interpolation surfaces and Time under Offset as shown below:

    target}

    Fig 6b-05 Primary Contours panel of the Contour Plot dialog box

  • Create a custom salinity color bar:
    • With the SALT-Global_cbr.xml color bar chosen under Colorbars:
    • Double-click on the color bar to the right to bring up the Colorbar Editor
    • Change the lower and upper limits to 34.3 and 35.4, respectively
    • Click on the linear ramp generator after Create with shape:
    • Click OK

    target

    Fig 6b-06 Colorbar Editor panel

  • When you click Plot in the Contour Plot dialog box, you get this section of salinity versus time in the upper 300 meters at the HOT site (we stretched the section by click-dragging the re-size icon on its lower right):

    target

    Fig 6b-07 Contour Plot


NOTE:

Note that over the years there are fluctuations in salinity in the upper 300 meters at this site, for example a fresher period ca. 1996-1997 and some salty years during 1999-2003. Plus there are fluctuations at shorter time scales, perhaps seasonal and/or interannual.

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