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36 0 36 2 salinity, S

Figure 6.3f A temperature-salinity diagiam showing the effect ol mixing three water types (I II and III) to give a mixture R with T= 3 ;C and S = 35. The method used to determine ihe relative proportions ol I, II and III in the mixture is shown here graphically and described in the text a + b e + tf e+ f

Figure 6.3f A temperature-salinity diagiam showing the effect ol mixing three water types (I II and III) to give a mixture R with T= 3 ;C and S = 35. The method used to determine ihe relative proportions ol I, II and III in the mixture is shown here graphically and described in the text

salinity. S

Of course, mixing occurs between adjacent water masses (e.g. the various upper water masses shown in Figure 6.15), but the mixing that is manifested in a T-S diagram plotted from data collected at a single hydrographic station is more often primarily from water masses which are one above the other. Let us consider how the distributions of temperature and salinity change as three such water masses mix together in the oceans, and how the T-S curve changes as a result.

Imagine three homogeneous water masses as follows: one at fairly shallow depth (200-600 m), one at intermediate depth (600-1000 m) and one fairly deep (1000-1400 m). We will assume that the intermediate and deep water masses have the same temperature but different salinities - this approximates to what is often found in the oceans. Temperature and salinity profiles are shown in Figure 6.32(a) and (b), while the T-S relationships are shown in part (c). The diagrams in 1 (at the top) illustrate the situation before any mixing has occurred, while 2 and 3 show subsequent stages as mixing progresses. As mixing occurs between moving water masses, the diagrams in 1, 2 and 3 should be seen as representing the situation at three different locations, although from the point of view of the moving water masses they correspond to three different stages in time.

Initially (stage 1), the three water masses are homogeneous and may be represented on the T-S diagram by three points (i.e. they are water types). As mixing progresses (stage 2), the sharp interfaces between the water masses become transition zones, so that the 'corners' of the temperature and salinity profiles become rounded off and water with characteristics between those at 400 m and 800 m, and between those at 800 m and 1200 m, appears on the T-S diagram. A layer of the intermediate water with its original temperature and salinity is still discernible at 800 m. This is known as core water and it shows up on the T-S diagram as a sharp point. As the core water continues to be affected by mixing both above and below, the sharp angle on the T-S diagram is eroded away, and the final T-S plot has the curved shape shown in stage 3. The changing shape of the T-S plot, from a sharp point to a curve, thus corresponds to the modification of the 'least-mixed layer' of the water mass as it spreads away from its site of formation (as shown by Figures 6.14 and 6.18 for Mediterranean Water and Antarctic Intermediate Water, respectively).

Stage 3 of the hypothetical scenario we have been considering is similar in many ways to what was actually observed at Meteor Station 200. at 9° S in the Atlantic Ocean. We can use the T-S curve for this station, shown in Figure 6.33, to apply the principles illustrated in Figures 6.29 and 6.31.

QUESTION 6.10 First locate the position ol Meteor Station 200 on Figure 6.19. Now assume that at this station, the water between 400 m and 1800 m depth is ihe result of the mixing of Antarctic Intermediate Walei with North Atlantic Deep Water (below u i and with water that has the I S characteristics represented h\ the 400m point on the 7 Vcurve (above it), (at K it possible to identity an eroded core ol Antarctic Intermediate Water m Figure 6.33?

(hi What is the percentage ol \ntarclic Intermediate Water present in the water ai K00 m deplh (to the nearest \(\'i i' (Assume that the charactcrislics ol Antarctic Intermediate Water and North Atlantic Deep Water correspond to the i eiitrcs ol the blue rectangles i

(a) TEMPERATURE PROFILES

(b) SALINITY PROFILES

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