Question 611

tat The water mass represented h\ the top 600m or so ol the H-ji curve is (Eastern) North Atlantic Central Water.

(i) What is the water mass below that, between about HiK)in and 1200m depth, and what is unusual about it at this location'?

(hi What is the deepest water mass represented hy the 8-.S"curve?

(b) Imagine that two water masses w ith 0=2 C. 5= 35.04 and 0 = 8.5 C. S = 36.00 mix togethci in approximately equal proportions. What is particularly interesting about the resulting mixture? i Begin h\ plotting the data points lightly onto Figure 6.34.)

The phenomenon identified in Question 6.11(b) - i.e. two water masses mixing together to form water with a higher density than either of the original contributions - is known as cabelling. It may be important in the production of deep and bottom water masses.

Water mass analysis has been invaluable to oceanographers attempting to build up a three-dimensional picture of large-scale flow within the oceans. Currents at depth are often too slow and/or too variable for their average motions to be easily determined directly. However, although temperature-salinity diagrams enable us to identify both the depth of the least-mixed (core) layer of a water mass and the direction in which it is spreading, they tell us nothing about the rate at which the water is moving. For this, we need to track water masses using a 'time-coded' tracer, or a non-conservative property of the water mass. How this may be done is discussed in Section 6.5.

There is, however, another conservative property of ocean water that we have not so far mentioned in connection with the tracking of water masses: its potential vorticity. As discussed in Section 4.2.1, away from regions of strong current shear, planetary vorticity/is very much greater than relative vorticity £ and so. to a first approximation, potential vorticity is given by f ID where D is the thickness of the layer under consideration. In regions where water masses of significant vertical extent are forming, the water column is well mixed, i.e. there is a pycnostad in which isopycnic surfaces are widely spaced (Section 6.3.1). If we take D to be the distance between two selected isopycnic surfaces, then within the water mass f ID is relatively small (Figure 6.35); furthermore, it will remain so as the water mass spreads away from the source region, enabling it to be tracked. In Figure 6.36, the depth at which the potential vorticity within a particular water mass is at its lowest has been identified, and the salinity of the water at that depth has been contoured.

Figure 6.34 8-S curve for station SuroitWO, to the east of the Azores. The numbers on the curve are in hundreds of decibars, and so approximate to hundreds of metres. The topmost part of the curve, above 100dbar, corresponds to the thermocline. Equal-density lines are lines of equal ae. (Note the detail visible on this curve, which was computer-generated using 9 and S data obtained by continuous profiling techniques. By contrast, the curve in Figure 6.33 was obtained from measurements made mostly at intervals of about 100 m or more.)

Figure 6.34 8-S curve for station SuroitWO, to the east of the Azores. The numbers on the curve are in hundreds of decibars, and so approximate to hundreds of metres. The topmost part of the curve, above 100dbar, corresponds to the thermocline. Equal-density lines are lines of equal ae. (Note the detail visible on this curve, which was computer-generated using 9 and S data obtained by continuous profiling techniques. By contrast, the curve in Figure 6.33 was obtained from measurements made mostly at intervals of about 100 m or more.)

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