The oceans

The oceans cover 361 million square kilometres, or 71% of the surface area of the globe, almost two and a half times the land area. To the surface observer this immense area seems almost featureless compared with the land, with only icebergs and waves to give a vertical dimension. However, beneath the water surface the ocean floor shows all the orographic richness of the land.

Fig. 1.12 shows the percentage of the Earth's surface in different height bands, relative to mean sea level. The first striking feature is the greater average depth of the ocean compared with the land's altitude. Much of the oceans are more than 3000 m deep, while little of the land surface is above 3000 m in altitude. This discrepancy also appears in the extremes of orography: Mount Everest is 8848 m in altitude, but the deepest point in the Marianas Trench, east of the Philippines, is 11 022 m below sea level. Large areas of the ocean have relatively little variation in depth; these regions are known as abyssal plains. They are usually deeper than 3000 m.

Fig. 1.12. Variation of the elevation of the Earth's surface. The mean elevation of the land is 0.84 km, while the mean depth of the ocean is 3.8 km.

Fig. 1.12. Variation of the elevation of the Earth's surface. The mean elevation of the land is 0.84 km, while the mean depth of the ocean is 3.8 km.

'Mountainous' bathymetry separates these deep, oceanic plateaux. For example, the Mid-Atlantic Ridge begins north of Eurasia and essentially splits the Atlantic Ocean into two halves. This ridge then continues eastwards across the Southern Ocean, curving northward into the east Pacific as the East Pacific Rise. A spur extends northwards from this main ridge into the western Indian Ocean.

The character of the oceanic perimeters varies considerably. In places the continental shelves adjacent to the coast are hundreds of kilometres across, for example, off part of western Europe and northern Australia. Elsewhere the shelf may be only a few tens of kilometres wide; a good example is off the west coast of South America. The continental slope, which joins the shallow coastal zone to the deep ocean, can be relatively steep in such regions, with average gradients greater than 1 in 10. In §1.3.2 we will see that these various bathymetric structures are important in guiding the oceanic circulation, particularly the deep water flow.

1.3.1 Chemical composition of the oceans

The oceans are, by volume, 96.5% water. The water molecule has properties that are important for the movement of heat, momentum and climatically active gases (including water itself) between the atmosphere and the oceans. The remaining 3.5% of the oceanic solution - dissolved salts, particles, organic material and gases - plays an inordinately large role in such climatic processes and the circulation in the ocean. We will here consider some chemical properties of water and the dissolved salts; more details of the chemical and biological processes in the ocean which contribute to the climate system will be the subject of Chapters 3 and 4.

Water, or H2O, is a very special molecule. Other compounds with similar molecular weight, such as methane, CH4, or ammonia, NH3, are gases at room

Table 1.3. Some physical properties of liquid water

Latent heat of fusion Latent heat of vaporization Specific heat Surface tension Maximum density Heat conductivity (at 290K) Molecular viscosity (at 293K)

3.33 x 105 Jkg-1K-1 2.25 x 106 Jkg-1K-1 4.18 x 103 Jkg-1K-1 7.2 x 109 Nm-1 1.00 x 103 kgm-3 5.92 x 10-2 Js-1m-1K-1 1.0 x 10-2 Nsm-2

temperature. Water, by contrast, is a liquid, and is readily found in its solid state (ice) below 0°C. The reason for this unusual behaviour lies in the molecular structure, already seen in Fig. 1.8. A water molecule is composed of an oxygen atom, bonded to two hydrogen atoms separated by an angle of 105°. As a result of the re-arrangement of the atomic electron orbitals (see Appendix B), the oxygen atom accrues a small net negative charge, while the hydrogen atoms gain a small positive charge. This makes the molecule dipolar and allows groups of molecules to form aggregates, with oppositely charged portions of the molecules adjoining each other, held together by coulombic forces. These structures resist break-up and permit water to be in a less energetic state than other, similar molecular weight, compounds at ordinary temperatures. Such unusual bonding leads to a number of important physical properties. These are summarized in Table 1.3, but are worth elaboration because we shall see their effects in later chapters.

The latent heat of fusion (the energy required to melt 1 kg of ice) and the latent heat of vaporization (that needed to evaporate 1 kg of water) are among the highest for any substance. This has important implications for the climate system as, conversely, this energy is released to the environment when water changes state to a more ordered structure. For instance, when water vapour condenses to form water droplets in a cloud, the energy latent within the vapour is released as heat and contributes to the driving energy of the cloud-producing process.

Related to these properties is the specific heat, highest of all solids and liquids except ammonia. This is the amount of energy required to increase the temperature of one kilogramme of the substance by 1°C. Dry air requires less than a quarter of the energy water needs to heat 1 kg by 1°C, and when the thousand-fold difference in density is taken into account, it is quickly seen that the ocean will be much slower in responding to heating, or cooling, than the atmosphere. Climatically, this is an extremely important property, as not only does it explain the smaller annual range in temperature of maritime climates, but it also points to the ocean's ability to act as a flywheel for longer term climatic change. Energy can be both stored and released over decades, or even centuries, by the ocean while the atmosphere reacts to energy changes with time delays of only a few weeks.

The heat conductivity and molecular viscosity of water are also strongly affected by the inter-molecular forces, being unusually high and low respectively. These parameters give the mixing ability of the liquid with respect to heat and molecular motion. However, mixing within water principally occurs because

Table 1.4. Concentration of major ions in sea water

Constituent

Ion

Average concentration in sea water of salinity 35 psu

Average concentration in river water (%)

Chloride

Cl-

19.350

0.0078

Sodium

Na+

10.760

0.0063

Sulphate

SO42-

2.712

0.0012

Magnesium

Mg2+

1.294

0.0041

Calcium

ca2+

0.412

0.0150

Potassium

K+

0.399

0.0023

Bicarbonate/

hco3-/

0.145

0.0588

carbonate

CO32-

of stirring by eddies within the fluid, which are on a much bigger scale than the molecular processes for which conductivity and viscosity are appropriate. Therefore these properties are not significant for the processes with which we are concerned. A final physical property of note that derives from liquid water's unique structure is the high surface tension. This is related to the force needed to break the air-water interface: a high value is detrimental to the speed of gas and particle exchanges between the air and water. This will be of importance in later chapters.

Another consequence of the molecular structure of water is its dissolving power. Sea water is a mixture of many compounds; the main ingredients, apart from water itself, are shown in Table 1.4. The addition of these salts has its own effect on the properties of the mixture. The freezing point of sea water is about -1.8°C, rather than 0°C. This lowering of the freezing point of water upon the addition of salts underlies the salting of roads in winter when near-freezing temperatures are expected. The density is also affected by the addition of salt. A typical surface sea water density is about 1026 kgm-3, an increase of 2.6% above that for pure water (see Table 1.3; note that the density of air near sea level is only about 1.2 kgm-3). The density of sea water is a complicated function of temperature, salinity (the proportion, by weight, of dissolved salts)5 and pressure (see, for example, Gill 1982, Appendix 3). However, because of the strength of the inter-molecular forces within water near its freezing point, it is found that salinity has most effect on density at low temperatures, while temperature exerts the predominant influence at higher temperatures.

The main input of particulate or dissolved material to the oceans occurs through riverine input; there is a small contribution from wind-blown (aeolian) deposits and precipitation. The globally averaged riverine chemical composition is distinctly different from the sea water composition shown in Table 1.4. Bicarbonate (HCO3-) is the dominant riverine anion and calcium (Ca2+) the most prevalent cation. Neither sodium (Na+) nor chloride (Cl-) contribute large percentages to the total dissolved ion concentration.

5 Since 1982 a salinity scale based on the electrical conductivity of sea water has been used. The average salinity of the oceans is 35 x 10-3, or 35 practical salinity units (psu) in this scale (a dimensionless number). Salinity values in psu are essentially identical to a measure of parts per thousand (%c) by weight.

Sea water appears to be of remarkably stable composition; the salinity may vary but the proportions of the different salts remain almost constant. Therefore, for a considerable time, perhaps hundreds of millions of years, the riverine input has been in balance with processes that remove the salts, such as sedimentation and ejection into the atmosphere. The excess chloride in sea water, in comparison with the riverine source, is thought to have come from volcanism early in the Earth's history. Volcanic eruptions would have emitted large quantities of the very soluble gas HCl. Dissolution of this gas in the sea forms a very weak hydrochloric acid solution. The input of bicarbonate over time has neutralised this, to leave sea water as slightly alkaline (pH c. 8.0). Another non-riverine input to the oceans which has contributed to the concentration of trace constituents is submarine hydrothermal activity on mid-ocean ridges. These sites are sources for dissolved gases like helium, and some metals, for example manganese.

The chloride ion concentration in sea water may be explained by early vol-canism, but sodium is not a large component of volcanic gases. Sodium must therefore attain its abundance by other means. This leads us to an important concept in the chemistry of the environment, namely the concept of elemental cycling. Many chemical elements cycle repeatedly through various parts of the Earth's outer crust, atmosphere and oceans in such a way that the concentration in each component of the system is stable over long periods of time. The carbon cycle is the best known of these, and will be considered in detail in Chapter 3. Other environmentally important cycles include sulphur, calcium, sodium and lead. The time taken for an atom to complete one cycle is determined by the average residence time of an atom of the element concerned within the different components of the cycle. In the ocean, for instance, this is defined as the total amount of the element in the ocean divided by the riverine and aeolian input per year. This definition implicitly assumes the cycle is in long-term balance.

To explain the abundance of sodium in sea water, relative to calcium, we can use the concept of residence time. Calcium has a much shorter residence time in the oceans than sodium. This is because it is a major constituent in the skeletons of marine organisms and is therefore easily lost to the ocean through settling of dead organisms onto the sea floor. It is therefore taken out of the oceans into the sediments sufficiently fast for more sodium, from riverine inputs, to accumulate in the oceans than calcium.

Cycling can also be applied to important molecules, as well as elements. The hydrological cycle is merely the cycle of water through the climate system. This is illustrated in Fig. 1.13. The residence time of water in the ocean is 3220 years. This can be compared with the time taken for material injected into the deep ocean to mix thoroughly around the globe (typically 1000 years). By contrast, the residence time for water vapour in the atmosphere is only 10 days and the mixing time is of the order of a month.

1.3.2 Ocean circulation

A schematic of the global ocean circulation is shown in Fig. 1.14. This is known as the thermohaline circulation because it is driven by density contrasts. The basic structure consists of deep water being carried towards the Pacific Ocean, which upwells along route and is transported back in near-surface currents to

Fig. 1.13. Reservoir sizes in the hydrological cycle. The accuracy of several of the components is poor, making it difficult to accurately close the cycle. [Reprinted with permission from Chahine (1992), Nature, 359,373-9.]

Fig. 1.13. Reservoir sizes in the hydrological cycle. The accuracy of several of the components is poor, making it difficult to accurately close the cycle. [Reprinted with permission from Chahine (1992), Nature, 359,373-9.]

Fig. 1.14. Schematic of the thermohaline circulation of the global ocean - the Conveyor Belt. The broken arrows represent the major surface components of the circulation. The continuous line denotes the deep water circulation, emanating from source regions denoted by open circles. Slow upwelling in the Atlantic, Indian and Pacific Oceans closes the circuit.

the downwelling regions in the North Atlantic (principally the Norwegian-Greenland Sea and the Labrador Sea) and the Weddell Sea. This cycle, known as the conveyor belt, is very important in the climate system. We will see in later chapters that it provides a stabilising effect on climate, because of its long time scale (see the last section), but can also cause abrupt climatic change in the space of a few decades if it is disturbed in certain ways.

The conveyor belt mechanism is naturally a gross simplification. The mean surface circulation is shown in Fig. 1.15. It has several shared features in each basin. Sub-tropical gyres rotate anticyclonically in each of the main ocean basins. The western margins of each of these gyres have strong poleward currents, such as the Gulf Stream in the North Atlantic. Poleward of these gyres there is some evidence in the Northern Hemisphere for cyclonic sub-polar gyres, where the westerly winds change to polar easterlies. In the Southern Hemisphere the water is able to flow around the entire globe, driven by the strong westerly winds at these latitudes. Sub-polar gyres exist in the Weddell and Ross Seas, poleward of this Antarctic Circumpolar Current.

The surface flow in the tropics consists of strong westward flowing currents at, and near, the equator. These are extensions of the tropical arm of the sub-tropical gyres. Between these two westward currents a counter-current,

Fig. 1.14. Schematic of the thermohaline circulation of the global ocean - the Conveyor Belt. The broken arrows represent the major surface components of the circulation. The continuous line denotes the deep water circulation, emanating from source regions denoted by open circles. Slow upwelling in the Atlantic, Indian and Pacific Oceans closes the circuit.

flowing eastwards, is usually found. There is also typically a strong eastward current below the surface on the equator, the equatorial under-current. The equatorial currents are intimately coupled with the atmosphere and will be discussed further in Chapters 2 and 5.

The deep circulation, shown in Fig. 1.16, conveys water that has sunk in the polar regions throughout the world oceans. In the Greenland and Norwegian Seas during winter the surface waters are strongly cooled making them denser. This dense water then overturns, probably in very localized regions up to a few tens of kilometres in diameter. In the Southern Hemisphere, particularly in the Weddell Sea, ice formation leaves a greater concentration of salt in the water beneath, as salt tends to be expelled from the ice lattice as it forms. This dense water also sinks. These two distinct types of water, or water masses, then travel equatorwards from the polar regions, to form the deep waters of the world's oceans. This deep water circulation is driven by subtle differences in temperature and salinity.

The water at intermediate depths also comes from the sinking of water masses, but those formed in less extreme circumstances. One such important contribution comes from the Mediterranean Sea. Intense evaporation raises the salinity of this basin above that of the North Atlantic. This dense, saline water sinks to form the deep water of the Mediterranean basin and spills out over the sill at the Strait of Gibraltar. Here, surface water flows into the basin from the North Atlantic, to compensate for the intermediate water outflow and the Mediterranean's evaporation. To conserve mass locally, the deep water flows out into the Atlantic beneath. This warm, salty water is evident in the intermediate layers of the North Atlantic for thousands of kilometres, contributing about 6% of the North Atlantic's salinity. Under some climatic situations, although probably

Fig. 1.15. The global surface current system. Cool currents are shown by dashed arrows; warm currents are shown by solid arrows. The map shows average conditions for winter months in the Northern Hemisphere; there are local differences in the summer, particularly in regions affected by monsoonal circulations. [Fig. 3.1 of Open University Course Team (1989). Reprinted with permission from Butterworth-Heinemann.]

Fig. 1.16. The deep circulation of the global ocean. The two main sources of deep water are shown by open circles. The deep water originating from the North Atlantic is slightly less dense than the deep water of Antarctic origin; hence its path is shown by dashed arrows until it merges with the latter (whose path is shown by a continuous line).

Fig. 1.16. The deep circulation of the global ocean. The two main sources of deep water are shown by open circles. The deep water originating from the North Atlantic is slightly less dense than the deep water of Antarctic origin; hence its path is shown by dashed arrows until it merges with the latter (whose path is shown by a continuous line).

not at the present time, it may also act to pre-condition the water entering the Norwegian-Greenland Sea, that will later overturn and produce deep water, by making it denser than it would otherwise be.

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