Summary Of Chapter

1 The temperature and salinity of water in the ocean are determined while that water is at the surface. The temperature of surface water is determined by the relative sizes of the different terms in the oceanic heat budget equation; the salinity is determined by the balance between evaporation and precipitation (E-P) and, at high latitudes, by the freezing and melting of ice.

2 The heat-budget equation for a part of the ocean is:

Gs + 0V = 0b + 0h + Qe + Qy where Qs is the amount of heat reaching the sea-surface as incoming shortwave radiation, Qy is heat advected into the region in currents, Qb is the heat lost from the sea-surface by long-wave (back) radiation, Qb is the heat lost from the sea-surface by conduction/convection, is the net amount of heat lost from the sea-surface by evaporation, and Ql is the net amount of heat available to raise the temperature of the water.

3 The net radiation balance (Qs - Qb) is largely controlled by variations in £>s; these depend partly on the latitudinal variation in incoming solar radiation but also on the amount of cloud and water vapour in the atmosphere. The amount of long-wave radiation emitted from the sea-surface depends on its temperature. However, Qb is the net loss of heat from the sea-surface, and so is also affected by cloud cover, and by the water vapour content, etc., of the overlying air. Qs - Qh is generally positive.

4 Qb and Qc depend upon the gradients of temperature and water content, respectively, of the air above the sea-surface. Both Qh and Qe generally represent a loss of heat from the sea, and both are greatly enhanced by increased atmospheric turbulence above the sea-surface.

5 Within 10°-15° of the Equator, there is a net gain of heat by the sea all year, but outside these latitudes, the winter hemisphere experiences a net heat loss. The patterns of heat loss in the two hemispheres are very different, with Qb playing a greater role in winter cooling in the Northern Hemisphere. This is largely a result of cold, dry continental air moving over the warm western boundary currents, which also increases evaporative heat losses, Qe.

6 The formation of ice at the sea-surface greatly influences the local heat budget; in particular, it leads to an increase in the albedo and a substantial decrease in Qs, while Qb is not much affected. Thus, once formed, ice tends to be maintained.

7 The principle of conservation of salt, combined with the principle of continuity, may be used to make deductions about the volume transports into and out of semi-enclosed bodies of water or, alternatively (if these are known), about the evaporation-precipitation balance in the region concerned.

8 Water masses are bodies of water that are identifiable because they have certain combinations of physical and chemical characteristics. The properties most used to identify water masses are temperature (strictly potential temperature) and salinity, because away from the sea-surface they may only be changed through mixing, i.e. they are conservative properties. Deep and bottom water masses, and 'thick' upper water masses, are formed in regions of convergence, and where deep convection results from the destabilization of surface waters through cooling and/or increase in salinity. Whether a water mass can form in this way depends not on the absolute density of surface waters but on their density relative to that of underlying water.

9 Central water masses are 'thick' upper water masses that form in winter in the subtropical gyres. They are characterized by relatively high temperatures and relatively high salinities. The water mass that forms in the Sargasso Sea has a remarkably uniform temperature of about 18 °C. This '18 °C water' is an example of a mode water.

10 The most extensive intermediate water mass is Antarctic Intermediate Water (AAIW). which forms in the Antarctic Polar Frontal Zone. Like other intermediate water masses formed in subpolar regions (e.g. Labrador Sea Water), AAIW is characterized by relatively low temperature and relatively low salinity. Although Labrador Sea Water is referred to as an intermediate water mass, in some years it may be formed down to depths of 2000 m or more, and will contribute significantly to North Atlantic Deep Water (see 11). In contrast to AAIW and Labrador Sea Water. Mediterranean Water is characterized by relatively high temperature and relatively high salinity.

11 North Atlantic Deep Water is formed in winter, mainly through cooling of surface waters and ice-formation in the Greenland Sea. As in the Labrador Sea and the Mediterranean, the near-surface waters are more easily destabilized because isopycnals bow upwards as a result of cyclonic circulation. Deep convection seems to occur in small, well-defined regions ('chimneys') and produces a dense water mass that mixes at depth with a cold, highly saline outflow from the Arctic Sea. The resulting water mass circulates and accumulates in the deep basins of the Greenland and Norwegian Sea. Intermittently, it overflows the Greenland-Scotland ridge, and cascades down into the deep Atlantic, mixing with the overlying water masses. The densest overflow water eventually reaches the Labrador Sea where it mixes with overlying Labrador Sea Water to produce a slightly less dense variety of North Atlantic Deep Water than that which flows south at depth in the eastern basin of the Atlantic. There seems to be a 'sea-saw' relationship between the intensity of convection in the Labrador Sea and that in the Greenland Sea; this may be related to the North Atlantic Oscillation.

12 Antarctic Bottom Water is the most widespread water mass in the oceans. There are two types, the much more voluminous 'Circumpolar AABW', which flows north from lower levels in the Antarctic Circumpolar Current, and the extremely cold 'true' AABW, which forms around the Antarctic continent at various locations on the shelf. Ice-formation plays an important part in its production, particularly in coastal polynyas; cooling of surface waters by winds is also important, especially in 'open ocean' polynyas. Most of this extremely dense water never escapes the Antarctic region, but some formed in the Weddell Gyre spills over the Scotia Ridge and flows north in the western Atlantic and in the Indian Ocean. Like other deep water masses, it is affected by the Coriolis force and so flows as deep western boundary currents (as predicted by Stommel).

13 The deep water mass with the largest volume is Pacific and Indian Ocean Common Water. It is a mixture, being about half Antarctic Bottom Water, and half Antarctic Intermediate Water plus North Atlantic Deep Water.

14 Dense water tends to sink along isopycnic surfaces, and mixing along isopycnic surfaces occurs with a minimum expenditure of energy. However, mixing does occur between water of different densities. In certain circumstances, mixing also occurs as a result of double-diffusive processes occurring on a molecular scale (e.g. salt fingering). Generally, different water masses are envisaged as mixing at their boundaries, but 'meddies' are an example of eddies of one water mass being carried into the body of an adjacent water mass; eddies of Indian Ocean water, formed at the Agulhas retroflection, are another example.

15 Temperature-salinity diagrams may be constructed for different locations in the ocean. For a given location, the shape of the temperature-salinity plot is determined by the different water masses present and the extent to which they have mixed together; also (as in the case of central water masses) by variations within a particular water mass. Assuming that mixing occurs through turbulent processes only, temperature-salinity diagrams may be used to determine the proportions of different water masses contributing to the water at a given depth. Sequences of temperature-salinity diagrams may be used to trace the least-mixed (or core) layer of a water mass, as it spreads through the ocean.

16 For convenience, density (p) is usually written in terms of a (sigma), where a = p - 1000; temperature-salinity diagrams show equal-o contours. Temperature-salinity diagrams now generally use potential temperature (0) rather than in situ temperature (T) (hence also oe rather than a,), because 0 is corrected for adiabatic heating as a result of compression and so is more truly a conservative property than is T. Comparison of the trend of a 0-5 curve with contours of oe may be used to evaluate the degree of stability of a water column, but using 0 instead of T does not compensate for the direct effect of pressure on seawater density. This effect may be significant, especially as compressibility is affected by temperature and salinity.

17 Because it is a conservative property, potential vorticity may be used to track water masses. Dissolved oxygen and silica concentrations, which are non-conservative properties, may be used to identify as well as track water masses. The 'age' of a water mass (the time since leaving the sea-surface) is indicated by its concentration of dissolved oxygen, and may be determined with some accuracy from the concentrations of certain other substances that enter the ocean at the surface. These include radioactive isotopes such as ,4C (radiocarbon) and 3H (tritium) - which are formed both naturally and in nuclear tests - and chlorofluorocarbons. i.e. CFCs. Using radiocarbon data, residence times for water in the deep oceans have been estimated to be of the order of 250-500 years.

18 The 'global thermohaline conveyor' is a schematic representation of the vertical circulation (or 'meridional overturning circulation') of the global ocean. It encapsulates the idea of global convection being driven by the sinking of dense (cold, high salinity) water in the northern North Atlantic, with net heat transport in the topmost 1000 m or so of the ocean being southward in the Pacific and Indian Oceans and northward in the Atlantic Ocean. The overall net transport of freshwater within the oceans is believed to be in the opposite direction, i.e. southward in the Atlantic and northward in the Indian and Pacific Oceans.

19 The observational phase of the World Ocean Circulation Experiment (WOCE) took place during the 1990s. WOCE was designed to investigate large-scale fluxes of heat and freshwater, ocean variability and climate (both now the subject of the CLIVAR programme), and mechanisms of water-mass formation. The enormous amount of data collected during WOCE. combined with improvements in modelling techniques, have added greatly to our understanding of the ocean and its role in the global climate system. In the future, the rate of supply of oceanographic data will increase still further, through (for example) Argo floats, underwater autonomous vehicles and new satellites. This improved supply of data will enable assimilation of data into models to be an even more useful tool than it is at present. Plans for a multidisciplinary. Global Ocean Observing System (GOOS) are well advanced, and the programme may become operational around 2010.

Now try the following questions to consolidate your understanding of this and earlier Chapters.

QUESTION 6.14 Figure 6.45 shows the tracks of ALACE floats deployed at about 1000 m during WOCE, as recorded up until I999. Bearing in mind that the floats were initially launched at various locations along the transects shown in Figure 6.42, outline briefly what the tracks indicate Figure 6.45 Tracks of ALACE floats deployed in about the characteristics of current flow in ( I ) the equatorial region, the Pacific during WOCE. u|_|a,imd in the ocean g\n

Figure 6.46 Computer-generated 0-S diagrams for water colder than 4 °C (for use with Question 6.17). The elevation of the surface corresponds to the volume of water with given combinations of 0 and S, the volumes having been calculated for 0-S classes of 0.1 °C x 0.01. Diagram 1 is for the world ocean, 2 is for the Indian Ocean, and 3 and 4 are for the Pacific and Atlantic Oceans (not necessarily in that order). In 1 and 3, the elevation of the highest peak corresponds to 26 x 106km3 of water, In 2 it corresponds to 6.0 x 106 km3 of water, and in 4 to 4.7 x 106 km3.

Figure 6.46 Computer-generated 0-S diagrams for water colder than 4 °C (for use with Question 6.17). The elevation of the surface corresponds to the volume of water with given combinations of 0 and S, the volumes having been calculated for 0-S classes of 0.1 °C x 0.01. Diagram 1 is for the world ocean, 2 is for the Indian Ocean, and 3 and 4 are for the Pacific and Atlantic Oceans (not necessarily in that order). In 1 and 3, the elevation of the highest peak corresponds to 26 x 106km3 of water, In 2 it corresponds to 6.0 x 106 km3 of water, and in 4 to 4.7 x 106 km3.

QUESTION 6.15 As men tinned in Section ft. i.2, l lie re was .1 decline in the rate tit formation ot Labrador Sea Water in the early 1970s, when the Great Salinity Anomaly passed through the nonh-west Atlantic. Can you suggest why

QUESTION 6.16 As mentioned in lhe text, the Antarctic Convergence was renamed the Antarctic Polar Frontal Zone. What features of the ¿one mean that the use of the term 'frontal' is appropriate?

QUESTION 6.17 In Figure ft.4ft. diagram I is a computer-generated three-dimensional H \ diagram lor all ocean water vv ith B less than 4 C. and diagrams 2-4 show similar information for the three individual ocean basins In each diagram, the elevation of the surface corresponds to the volume of water vv ith given B-.S" characteristics.

(a) Diagram 2 corresponds to the Indian Ocean. Given ihe characteristics of North Atlantic Deep Water, can you say which of ihe remaining diagrams (3 and 4i corresponds to the Atlantic and which to the Pacific.'

tb> What is the water muss, distinguishable in all the diagrams, which has potential temperatures less than 0 C 1

tc> To what is tile peak (visible in diagrams I. 3 and. to some extent, 2i attributable?

QUESTION 6.18 In Section ft.,v2. we stated thai Pacific and Indian Ocean Common Water is approximately < Antarctic Bottom Water, w ith North Atlantic Deep Water and Antarctic Intermediate Water together making up the resi. Use Figure ft.47. along w ith the principles outlined in Section ft.4.2. to make your own estimates of the proportions of each of these water masses in Pacific and Indian Ocean Common Water

Figure 6.47 T-S diagram showing the characteristics of the water masses that mix together to form Pacific and Indian Ocean Common Water. (For use with Question 6.18.)

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