Guided Tour of Java OceanAtlas

Property-Property Plots

Property-property plots are essential to oceanographic data interpretation. As the science of oceanography developed in the late 1800s, and as physical/chemical observations became available from widespread regions, it became apparent that ocean waters had imprinted upon them clear and traceable patterns of characteristics. For example, waters from a region of low air temperatures and high precipitation would be cold and fresh, whereas those from the subtropics were warm and salty due to heating and evaporation there. The most basic oceanographic property-property plot is a property plotted against pressure (or depth) as the vertical axis. However, physical oceanographers have a special fondness for temperature-salinity plots (often potential temperature salinity plots for the reasons discussed above), because of the imprinting from the source regions and because isopycnals at appropriate reference pressures can be plotted on them.

For example, we show in Figure 9 a potential temperature vs. salinity plot prepared from a global selection of data (a long section from many cruises extending from the Ross Sea to the Bering Sea in the Pacific Ocean, across the Arctic Ocean and Nordic Seas, and then from Iceland to the Weddell Sea in the Atlantic Ocean), colored by a specially-made color-contour bar with equal divisions of sigma-0 from 22.7 to 27.95.

Gt_fig-09 Figure 9 Property-Property plot of temperature vs. salinity

Each transition of the bands of color in Figure 9 marks an isopycnal. You can visualize which waters along this section would most readily mix, that is, if they were brought together.

Many other property-property correlations are of interest to oceanographers. In Java OceanAtlas we permit you to plot any observed or calculated parameter versus any other, and to color the data points by any color bar which corresponds to a parameter named in the Data Window.

  • Select 'Property-property...' from the 'Plots' menu. Set up a theta-salinity plot, by selecting THTA for the Y-axis and SALT for the X-axis. Select 'Color Legend', which adds the current color bar to the plot, and 'Connect observations', which will add a line connecting the observations at the station current in the Data Window. Also, because this is a THTA versus SALT plot, we can add isopycnals - lines of constant density - to the plot (same holds for TEMP versus SALT): click on the 'Advanced' tab near the top of the dialog box, and near the bottom of the Advanced panel click on 'Isopycnals', leaving the 'Pressure' slider set to '0' decibars. See Figure 10 for a picture of the dialog box with these selections.
  • Gt_fig-10a
    Gt_fig-10b Figure 10 Property-Property Plot dialog boxes set up for the Potential Temperature versus Salinity plot for the Atlantic 11°S data shown in Figure 11.
  • Click 'Plot' in the Property-Property Plot dialog box, and you should see the plot shown in Figure 11.
  • Gt_fig-11 Figure 11 Potential Temperature versus Salinity plot for the Atlantic 11°S data. Note that the dots on this plot are colored in accord with the color bar in the Data Window.
  • Now again select 'Property-property...' from the Plots menu. Set up a THTA (Y-axis) - SIO3 (X-axis) plot. This time, experiment with the tick-mark options by clicking on the 'Advanced' tab near the top of the dialog box. Set the silicate (X-axis) range from 0 to 140, with an axis increment of 20 and 3 minor ticks between each labeled axis tick, and set the potential temperature (THTA, Y-axis) increment to 5 and 4 minor ticks between each labeled axis tick. See Figure 12 for the Property-Property Plot dialog box set up this way, and see Figure 13 for the resulting plot.
  • Gt_fig-12a
    Gt_fig-12b Figure 12 Property-Property Plot dialog box set up for the Potential Temperature versus Silicate plot for the Atlantic 11°S data shown in Figure 13.
    Gt_fig-13 Figure 13 Potential Temperature versus Silicate plot for the Atlantic 11°S data.

You now should have on your monitor screen a Data Window, a map, one 'profile' plot, and two 'property-property' plots.

There is much that can be said about the potential temperature - salinity and potential temperature - silicate plots:

First, on the -S plot, examine the values on the labeled isopycnals. The warmer or the fresher the water, the smaller the value - the less the density - and the colder or saltier the water, the higher the value - the greater the density. Less dense water always overlies more dense water, but in portions of many vertical profiles, although the density profile is stable, the influence of temperature on density opposes that of salinity, with one of the changes being stronger and thus creating a stable profile. For example, warm, salty water may lie on top of colder, fresher water. In that case the influence of that specific temperature difference in making the deeper, colder water denser is greater than that of the salinity difference in making the deeper, fresher water less dense. This is exactly the case in the -S plot of the Atlantic Ocean 11°S section data. Remembering that less dense water will always lie on top of more dense water, the isopycnal labels indicate the vertical sense of the data in the -S plot: warm, mostly salty tropical surface water - at some stations the surface water is fresher than at others, perhaps due to rainfall - overlies colder, fresher water known as Antarctic Intermediate Water (AAIW), with a cold/fresh AAIW extremum near about 4'C and a salinity of 33.4. The AAIW has spread northward from its source in the Antarctic Circumpolar Current zone in winter under cold air temperatures, and in a region where there is ample precipitation, so it is relatively cold and fresh. Beneath the AAIW - i.e. at densities immediately greater than that of the AAIW - the water becomes saltier, with not very much temperature change. This layer originates in the North Atlantic Ocean from a blend of cold, dense waters from the Labrador Sea and dense overflows into the North Atlantic from the Nordic Seas, and so is called North Atlantic Deep Water (NADW). The Atlantic Ocean is the saltiest of the major oceans - due in part to the influence of the Mediterranean Sea - and so its dense products tend to be salty as well. Underneath the NADW the water again becomes colder and fresher. These abyssal waters originate as a blend of very cold, relatively fresh, dense bottom waters from the Weddell Sea with Antarctic Circumpolar Deep Water and NADW, and are known away from the Antarctic as Antarctic Bottom Water (AABW).

The perceptive observer may note something odd: In the -S plot (Figure 11) the coldest, freshest AABW is very slightly 'less dense' than some of the water about halfway between the coldest/fresh extremum and the NADW extremum. [You can see this more clearly if you jump ahead to Extracting Selections to learn about extracting selections from plots and try it on this feature.] How can this be? The answer is simple enough, though not so easy to explain: Density of seawater is a strong function of pressure. But the effect of temperature on density is also a strong function of pressure. It is an observed fact that warmer water resists compression better than does colder water, all other factors being equal. (Imagine that the increased motion of the water molecules of the warmer water - temperature is partly a measure of the motion of molecules - enables them to better resist being compressed.) And so if you have two parcels of water at the sea surface of equal density at the sea surface, but one is warm and salty and the other is cold and fresh - the two parcels would lie along one of the isopycnal lines in Figure 11 - but then move them adiabatically to 4000 decibars, the colder, fresher parcel will be compressed more: it will then be denser than the warm, salty parcel. Thus to avoid confusion when comparing the densities of water samples, the densities must be compared at a pressure appropriate to the comparison. The isopycnals in Figure 11 were calculated with a 'reference pressure' of 0 decibars, i.e. with respect to sea surface pressure. But the South Atlantic deep waters lie near 4000 meters, and so a reference pressure of 4000 decibars would be more nearly appropriate. Double-click on the -S plot, and in the isopycnals portion of the dialog box move the 'pressure' slider to '4000'; then click 'OK'. You will see a dramatic change in the slopes of the isopycnals, which are now drawn with respect to 4000 decibars pressure, and now it will be clear that in the deep and bottom water along 11°S the AABW underneath the NADW is truly more dense than the NADW, as indeed it must be. This effect of temperature on the compressibility of seawater seems subtle, but it is a relationship crucial to physical oceanography.

There is no density implication to the potential temperature - silicate relationships (Figure 13), but recalling that the temperature values there are of course identical in the -S plot (Figure 11), it is easy to see that the warm upper layer waters are depleted in dissolved silica, with a marked increase of silicate into the AAIW, followed by an abrupt decrease of silicate in NADW, and then a very sharp increase of silicate into the AABW, with a bottom maximum. This pattern makes good oceanographic sense: silicate is used by diatoms to make their shells, and is often depleted in the upper layers by plankton uptake. Antarctic-source waters found in the South Atlantic have a higher silicate concentration than do North Atlantic waters because silicate is cycled in great abundance in the Antarctic zone by planktonic activity, and the AAIW and AABW layers arise partly from the deeper layers where the shells from diatoms dissolve. Also, the deep Antarctic circumpolar waters receive the deep waters of the Pacific Ocean, which are very strongly enriched in dissolved organic nutrients by biological decay during their long passage as intermediate and deep layers in the Pacific. The North Atlantic Ocean is not directly connected to this huge nutrient cycling machine and so has lower nutrient concentrations in general, in nearly every layer, than do the other oceans.

If you created the -S and -silicate plots in accord with this tutorial, then the data points on your plots are colored in accord with the dissolved oxygen concentration of those samples, as in Figures 1.11 and 1.13. The very low oxygen concentrations of much of the water from 5-15'C is caused by organic decay and a lack of local wintertime exchange of gases at the sea surface. But oxygen concentrations in the NADW, especially, are high. The Labrador Sea Water and the Nordic Sea overflows which helped to form NADW obtained their characteristics at the winter sea surface in their regions of origin, where they were cooled, and where, because cold waters have a greater capacity to hold dissolved gasses than do warm waters, they attained very high oxygen concentrations. Other factors in helping NADW maintain its high oxygen concentrations are that (1) the Nordic Sea outflows mix in the northern North Atlantic Ocean with relatively high oxygen water, and (2) the formation and spreading rate of NADW, relative to the capacity of the North Atlantic biological production to deplete its oxygen by decay of organic matter 'raining' into the NADW from above, is much higher than is the case for the Pacific Ocean deep water (which as noted above has a strong influence on the Antarctic zone intermediate and deep waters). NADW is sometimes thought of as the 'ventilator' of the deep World Ocean.