Thus, according to our calculations, the rale Of inflow through the Straits of Gibraltar is about 1.7b x M/'mV, while ihe rate of ouillow is 1.71 x lO^m-V

An alternative method would have been to use Equations 6.2 and 6.3 and hence obtain two equations relating V] and VS w hich could then he solved.

(b) Dividing the total volume hy l',. the rate of inflow, gives: 3 8 x I0h x 10" m-1

it i


365 x 24 x 3600

That is. it would lake about 70 years for all the water in the Mediterranean to he replaced once.

Question 6,6 Important regions of convergence in mid-latitudes are the centres of the subtropical gyres, where antieyclontc winds lead to Fkrnart transport towards the centres of the gyres {Figure 3.24(c) and (d)). The high-latitude region of convergence that should have come to mind is, of course, the Antarctic Polar Frontal Zone, previously known as (he Antarctic Convergence (Figure 5.31),

Question 6.7 (a) The cross-sections in Figure 6.17 indicate the presence of a large volume of Labrador Sea Water because they show water with very similar temperature and salinity characteristics occupying a region from about 100 m depth down to about 2000 m, in the central pan of the gyre. (As discussed in the text following rhis question, mode waters such as Labrador Sea Water are by definition characterized by such pyenostads. but this is an unusually thick one.)

(h) The contour patterns in Figure 6.17 are consistent w ith a doming of isotherms (isopycnals). divergence of surface water and upwelling, under the influence of cyclonic winds (as shown in Figure 3.24). The fact that relatively dense water has been brought close to the surface means that the uppermost layers are easily destabilized by extra cooling (or an increase in surface salinity).

(The distribution in (c) indicates oxygen dissolving in surface water which is being mixed down into up we I led water which is older' and so somewhat depleted in oxygen. That is why concentrations in (he centre of the gyre are not as high as around the edge. 1

Question 6.8 Mehwater is fresh and therefore of low density. The formation of a layer of low-density water at the surface increases the stability ot the upper water column and so tend*- to damp Out" vertical convection.

Question 6.9 See Figure AS for the result. Measurement of segments it and h shows thai they are in the proportion of 1 ;2. which means that the mixture contains twice as much of water type 11 as of water type I I i.e. water type 1 makes up 33ri of the mixture). You could have obtained the same result mathematically by using values of 7(or 5), By proportion.

h $-2 ( 34.85-34..M_ 1 a ~ 5 - 31 °r 35.5~34.85 i~ 2

Figure AS The completed T-S diagram for Question 6 9 snowing water types I and II mixed in proportions of 1 2, to give a resultant mixture with a temperature of 3"C and a salinity of 34.B5.

However, the graphical met hud is useful because it helps you to estimate proportions of contributing water masses 'by eye': the closer the position of a mixture to a particular water mass, the more of that water mass must be in the mixture

salinity, S

Queslion 6,10 If you find Meteor Station 200 on Figure o. IS), you will sec that ai this location (9°St warm, saline upper water flows above cooler, fresher A At W. below which flows relatively saline NADW and finally, at depth. AARW. All these features may be identified in the T-S curve in Figure 6,33. (Remember that in Figure ft. 19 the 10 "C and 0°C isotherms and the 34.8 isohaline are not actually boundaries between water masses, although they give fairly good indications of ihe relative positions of the different water masses.)

(a) Yes, the water at 800m is ihe eroded core of Antarctic Intermediate Water I see Figure A 9).

(b) If you draw construction lines as shown in Figure AM (cf. Figure 6.31) and measure the segments n and b. you get

So. the percentage of Antarctic Intermediate Water at 800 m depth is about

The graphical method enables you to find out the proportion of one water mass without having to find it for all three However, for completeness, the proportions of the other water masses contributing to the mixed w ater at 800 m are about 25fv for North Ailantic Deep Water and about 207c for the water at 400 m depth.

Question 6.11 (a)(i) The water mass below North Atlantic Central Water is of high salinity (reaching nearly 35.8) and is warm (9-lO°C). This, combined with the latitude and the depth of the water mass, indicates clearly that it is Mediterranean Water. The trend of the 0-5 curve in this region is more or less parallel to the ce contours, reflecting the spread of Mediterranean Water at depths where it is neutrally buoyant. Interestingly, there seems to be a 'double core' of Mediterranean Water. This could be because Mediterranean Water of slightly different densities is spreading out along two different isopycnic surfaces (corresponding to o9 ~ 27.6 and ~ 27.55), allowing a local intrusion of slightly fresher, cooler water between 900 and 1000 m depth.

(ii) The deepest water mass represented by the 0-5 curve is North Atlantic Deep Water, clearly identified by its temperature-salinity combination of ~2-4 °C and -35.0 (as well as by the geographical location of the station). If the 0-5 curve had been plotted for sufficiently great depths, some influence of Antarctic Bottom Water would have been detected.

(b) The two water masses are of approximately the same density. However, if you plotted them on Figure 6.34, you would have seen that when they mix together, the resulting water mass (on the line joining the two points) is denser than either of the original contributions (with ae ~ 28.05 as opposed to 28.00).

Figure A9 The completed T-S diagram for Question 6.10. The ratio a Vindicates that the proportion of Antarctic Intermediate Water in the core is about 55%.

Question 6.12 Yes, the two water masses can be distinguished. The lowest silica concentrations (< 40|imol L'), corresponding to North Atlantic Deep Water, are seen between 1500 m and 3000 m. Below that, concentrations increase again, reaching 125 pinol l"1. in Antarctic Bottom Water. The way the contours of silica concentration relate to the sea-bed topography strongly suggests that both North Atlantic Deep Water and Antarctic Bottom Water are flowing along the western boundary of the ocean (albeit in different directions), as deep western boundary currents (cf. Figure 6.21). The same can be said of the contours of oxygen concentration in Figure 6.37(a).

Question 6.13 If. as is now thought, the Gulf Stream is driven partly by the sinking of water in the subpolar seas to the north-east of the Atlantic (Section 4.3.1). a reduction in the rate of formation of deep water would cause the Gulf Stream to slow down (and/or penetrate less far north). Currently, the waters of the Gulf Stream/North Atlantic Current give up large amounts of heat (and moisture) to the atmosphere over the northern Atlantic, and this is carried to north-west Europe in the prevailing westerlies. If this heat-supply declined markedly, the climate of north-west Europe would become much colder (in particular, winters would become longer and harsher).

Question 6.14 (1) In equatorial regions, flow is predominantly zonal and fast (though not as fast as that in the Antarctic Circumpolar Current, cf. Question 5.8). (Remember these floats are at about 1000 m depth.)

(2) Within the subtropical gyres, flow is much slower and apparently more random - some floats in the eastern Pacific have hardly moved from their launch sites, which can still be distinguished because they are along a line of longitude (cf. Figure 6.42).

You may also have spotted some locations where floats seem to have been somehow "trapped", for example those apparently in a gyre just to the north of the Falklands Plateau.

Question 6.15 As discussed in Section 6.3.1, Labrador Sea Water is formed when the density of surface water in the Labrador Sea is increased through cooling and an increase in salinity, as a result of cold, dry winds blowing off the adjacent continent in winter. A large volume of low salinity water moving into the area would reduce the density of the upper part of the water column and make it much less likely that it could be destabilized in this way. (We can assume that the low salinity water was not exceptionally cold.)

Question 6.16 Features you may have thought of are (i) the convergence of surface waters, (ii) sloping boundaries between denser and less dense water (cf. sloping isotherms/isohalines on Figures 5.31 and 6.26), (iii) swift currents flowing along these boundaries (Figure 5.32), and (iv) a tendency for meanders and eddies to form from these currents (Section 5.5.2).

Question 6.17 (a) The high-salinity ridge evident in diagram 4 corresponds to North Atlantic Deep Water. Diagram 4 is therefore for the Atlantic, while diagram 3. with much lower salinities, is for the Pacific.

(b) The low-temperature water mass visible on all the diagrams (though clearest in 4. for the Atlantic) is Antarctic Bottom Water.

(c) The peaks in diagrams 1. 2 and 3 correspond to the huge volume of water making up Pacific and Indian Ocean Common Water.

Question 6.18 Our answer is illustrated in Figure AlO. The results obtained here differ from those of the researchers who first published estimates of these proportions. They obtained the following values: Antarctic Bottom Water, 60c/c: North Atlantic Deep Water. 249c. Antarctic Intermediate Water, 16%. The discrepancy probably arises because these researchers allowed for the fact that the temperature of Antarctic Bottom Water is raised by heat flow through the sea-bed.


Figure A10 Completed T-S diagram for the answer to Question 6.18. The lengths of the segments a-f are given in millimetres.

Antarctic Intermediate Water


Question 6.19 As discussed in Section 6.1.1. the surfaces of the oceans and continents emit long-wave (back-) radiation which is absorbed by clouds and water vapour and other gases including carbon dioxide. As a result of this warming, these all emit radiation of a longer wavelength in all directions, including back to the Earth's surface. This reduces the net amount of long-wave radiation emitted by the sea-surface, and hence reduces the heat-loss term Qh. Because the concentration of carbon dioxide in the atmosphere is increasing, the amount of heat absorbed in the atmosphere and re-emitted back towards the surface of the Earth is also increasing, so the net heat loss from the sea-surface as Qb is probably decreasing (along with heat lost from the continents as long-wave radiation).

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