In order to identify the water mass, or water masses, present at a given location in the ocean, a set of observations of temperature and salinity for successive depths at that location are plotted on a graph with temperature on the vertical axis and salinity on the horizontal axis, and the points are joined up in order of increasing depth. Today, this is generally achieved using data acquired by continuous profiling. The result is known as a temperature-salinity (T-S) diagram.
If a water mass is completely homogeneous, it will be represented on a T-S diagram by a single point, and will be described as a water type. Observations clustering around such a point indicate the presence of this water type. For example, a number of observations of newly formed Deep Water in the north-western Mediterranean would all plot at or around a point corresponding to 7"= 12.8 °C and 5 = 38.4.
Figure 6.29 shows what happens when two water types of differing temperature and salinity are mixed, and illustrates the following principles.
1 Whatever the relative proportions of the two water types, the point representing the temperature and salinity of any mixture must lie on the straight line joining the two water types on the T-S diagram.
2 Following on from 1. the actual position of the point representing the mixture will be determined by the relative proportions of the two water types. In the example shown in Figure 6.29, the point for the mixture, R, lies closer to type II than type I. so the mixture must contain a larger proportion of type II. The actual proportions of the two water types that are present in the mixture can be determined by measuring the lengths of the segments a and b, as shown in Figure 6.29. (Note that we have been using the term 'water type' rather than 'water mass', because we are assuming that mixing is occurring between two bodies of water each represented by a single temperature and salinity value, rather than by a range of values.)
Figure 6.29 A temperature-salinity diagram showing the effect of mixing water type I having T) and S, with water type II having fM and S,:. The resulting mixture ft (having 7>and So) will be represented by a point on the line between I and II, ibe position of which will be determined by
S, the relative proportions of the two water types in the mixture.
Although T~S diagrams can be used to predict the temperatures and salinities that would result from water masses mixing together, the usual application of the method is to determine ihe relative proportions of different (known) water masses contributing to the water we are interested in. and for which we know the temperature and salinity (from measurements). The following question illustrates how this can he done.
QUESTION 6.9 If water type !, with a temperature of 5 C and a salinity of 35.5 mixes with water type II with a temperature of 2°C and a salinity of 34.5, to give a mixture with T-S characteristics of 3 "C and 34,85, what are the proportions of water types I and II in the mixture? (You can use l;igure 6.29 by lightly adding scales of your own choosing and then plotting the data directly onto it.)
Before moving on to consider how T-S diagrams can be used to interpret more complex situations, we should note that not all straight segments of T-S curves reflect mixing between water masses - they may indicate variations n i til in one w ater mass. Such variations might result from waters of slightly different T-S characteristics forming at different times of year and sinking to different depths, according to their densities.
Alternatively, surface conditions may \ ary within the source region during the period (usually winter) when the water mass forms. Water mixing down along sloping isopycnic surfaces w ill eventually become vertically stratified (Figure 6JO(a() and at a given hydrographtc station, the water mass will be represented by a more or less straight line on a T-S diagr am (Figure 6.30(b)). The water mass used as an example in Figure 6.30 is North Atlantic Central Water (Section 6,3 I). and because of the way that they form. Central Waters are generally recognizable as segments of T-S plots w ith this characteristic slope source region
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