Ocean Temperatures

The sea-surface temperature (SST) was first mapped by Alexander von Humboldt in 1817. Modern maps show long-term values of the SST varying from the sea's freezing point of -1.9°C near polar ice, to over 30°C in the Persian Gulf and the Red Sea in July (Figure 11.2). Temperatures offshore tend to be cooler than inland in January (summer) but warmer in July, around South Africa for instance (Figure 11.3). These differences of temperature cause coastal breezes (Chapter 14).

Generally, the isotherms depend chiefly on latitude (Table 11.1), though there is a variation near the coasts of continents where isotherms bend polewards on the east coasts (especially at 20-30°S, implying extra warmth) and vice versa on the west (Table 11.2); ocean currents are the main reason, as explained in Section 11.5. The effect is most against South America, and least around Australia. East-coast places in Australia are hardly, warmer than those at the same latitude on the west, because the west

coast of Australia lacks a cold current of the kind found off Chile and south-west Africa (Section 11.5). Yet all three continents are similar in having remarkably dry west coasts (Table 11.2), as would be expected with a cold SST there.

Temperatures are below freezing near the poles, so there is sea-ice. It is only a few metres thick and extends from Antarctica to 63-70°S in March and 55-65°S in September, the area increasing from about 4 million square kilometres to 20 million in winter. Even if temperatures are not quite so cold, they can be lethal: a person can last no longer than about two hours in water at 5°C, or four hours at 13°C.

At the other extreme, there is the world's largest 'warm pool' whose annual mean surface temperature exceeds 28°C. It is larger than Australia and centred in the western Pacific ocean just north of the equator, near Papua New Guinea (Figure 11.2). The pool extends eastward as far as 170°E into the Pacific during a La Niña but almost 6,000 km further to 140°W in an El Niño year. The high temperature and vast area make it the most active breeding ground for tropical

Figure 11.3 Monthly mean SST and screen temperatures around and over southern Africa, in January and July.

Table 11.1 The effect of latitude and season on the monthly mean sea-surface temperature and annual rainfall of South Pacific islands

Latitude

Elevation

Place

(°S)

(metres)

Cook Island

9

1

Pitcairn Island

25

264

Easter Island

27

20

Lord Howe Is.

32

0

Waitangi

44

44

Macquarie Island

55

6

Maximum monthly mean temperature l°C) 28 (April) 24 (Feb) 24 (Feb) 23 (Feb) 15 (Feb) 7 (Jan)

Minimum monthly mean temperature Rainfall

27 (August) 3,655

19 (August) 2,630

18 (August) 1,134

16 (August) 1,685

Table 11.2 A comparison of monthly mean temperatures (°C) and rainfall (mm) at coastal cities on the west and east sides of three continents

Temperature (°C) Daily range (K) Precipitation (mm)

Table 11.2 A comparison of monthly mean temperatures (°C) and rainfall (mm) at coastal cities on the west and east sides of three continents

Temperature (°C) Daily range (K) Precipitation (mm)

Continent

Coast

Place

Jan

July

Jan

July

Jan

July

Around 23°S

S. America

West

Antofagasta

21

14

7

6

0

5

East

Rio de Janeiro

26

21

6

7

125

41

S. Africa

West

Walvis Bay

19

15

8

13

0

0

East

Maputo

25

18

8

11

130

13

Australia

West

Carnarvon

27

17

8

11

20

46

East

Brisbane

25

15

6

11

163

56

Around 34°S

S. America

West

Santiago

21

9

17

12

3

76

East

Buenos Aires

23

10

12

8

79

56

S. Africa

West

Cape Town

21

12

10

10

15

89

East

Durban

24

17

6

11

127

85

Australia

West

Perth

23

13

12

8

8

170

East

Sydney

22

12

8

8

89

117

cyclones (Chapter 13) and the engine for the 'Walker circulation' across the Pacific ocean (Chapter 12). The high SST leads to rapid evaporation (Figure 4.11), and rainfalls there average about 3 m/a, and exceed 5 m/a in some parts of the warm pool (Figure 10.3).

There are considerable week-to-week fluctuations in the pattern of sea-surface temperatures. Figure 11.4 shows the short-term irregularity of SST off the east Australian coast. The small-scale structure, such as the two isolated loops below the 24°C isotherm, is due to ocean eddies which evolve over the course of a few days. In addition, there are seasonal and interannual variations.

Seasonal Variation

Sea-surface temperatures vary over the year (Figure 11.5 and Figure 11.6). The average difference between the extremes (the annual range) is usually only a degree or two at the poles and at the equator, and most (i.e. 5-6K) at 30-40°S (Figure 3.4; Table 11.1). For instance, the range offshore at Sydney (at 34°S) is from about 22.6°C in March to 17.2°C in August, i.e. 5.4 K. Larger ranges are found in seas that are surrounded by large land masses and in coastal waters where there is a wide continental shelf, e.g. 10.5 K offshore from Cabo Corrientes in Patagonia. Even this is less than the range inland, because of the ground's relatively small thermal inertia (Section 3.3).

A large annual range may occur at sea when the ocean currents (Section 11.5) change seasonally. For instance, the south equatorial eastern Pacific and Atlantic are about 5K cooler in winter than in summer (Figure 11.5), because the cold currents affecting these regions are stronger in winter. In this exceptional case, the annual temperature range offshore can be higher than on the coast.

The annual extremes of sea-surface temperature occur about 2-3 months after the extremes of radiation (Table 11.1) because of the ocean's thermal inertia, due to the large volume affected as a result of stirring within the ocean's mixing layer. Temperature variations are less at greater depth, as in the case of temperatures

Plate 11.1 The oceanographic research vessel Franklin of the CSIRO Division of Oceanography, Hobart. This photograph was taken during experiments in the warm pool of the western Pacific, when a boom forward of the bows carried instruments for measuring the fluxes of heat and moisture from the water surface upwards, and of momentum downwards. The boom also carried an instrument to be lowered into the water for measuring the profiles of temperature and salinity within the upper 3 metres. Other instruments on the foremast repeated the flux measurements, and measured rainfall. Shortwave and longwave radiation were determined by instruments above the wheelhouse. Other instruments were towed from the afterdeck to determine the temperature and salinity profiled to 300 m depth.

Plate 11.1 The oceanographic research vessel Franklin of the CSIRO Division of Oceanography, Hobart. This photograph was taken during experiments in the warm pool of the western Pacific, when a boom forward of the bows carried instruments for measuring the fluxes of heat and moisture from the water surface upwards, and of momentum downwards. The boom also carried an instrument to be lowered into the water for measuring the profiles of temperature and salinity within the upper 3 metres. Other instruments on the foremast repeated the flux measurements, and measured rainfall. Shortwave and longwave radiation were determined by instruments above the wheelhouse. Other instruments were towed from the afterdeck to determine the temperature and salinity profiled to 300 m depth.

below ground (Figure 3.16). There is no annual variation beyond two or three hundred metres under the surface (Figure 11.6).

El Niño Episodes

There is a variation of SST from year to year, especially in the tropical Pacific ocean. This was mentioned in Note 10.M, in conection with the SST off Peru, which is usually much lower than anywhere else so close to the equator (Section 11.2). However, the temperature there rises by several degrees for twelve months or so every few years, on account of a shifting of the easterly winds and ocean currents along the equator of the Pacific ocean (Note 11.C). For example, the temperature was 24°C, instead of the usual 16°C, at one point off Peru between September 1982 and January 1983. Such El Niño episodes are discussed further in Chapter 12.

Icebergs

Some 5,000 icebergs break off from Antarctic glaciers and ice shelves each year, and the total

Figure 11.4 Sea-surface temperatures off the eastern Australian coast on 6 April 1975.

Figure 11.4 Sea-surface temperatures off the eastern Australian coast on 6 April 1975.

volume of this salt-free ice is about a thousand cubic kilometres. Some icebergs are huge, 2040 m high and ten times that below the waterline. A berg seen in 1956 was 200x60 km in area. Another was 150 km long, which broke off the Ross ice shelf (at 77°S, south of New Zealand) in 1987. It was tracked by satellite for twenty-two months, until it became three pieces. One iceberg was initially about 30 km across and then was observed drifting some 10,000 km, nearly round Antarctica. Some bergs drift slowly north, especially in the Atlantic Ocean, where they have been seen at 28°S (Figure 11.7).

Relationship to Rainfall

The relatively cloudless skies over the cool eastern half of a southern tropical ocean (Figure

8.14) lead to little rainfall P (Figure 10.3) but a high evaporation rate Eo, so that many islands in these areas have a 'marine desert' climate. The evaporation rate exceeds 2 m/a in some places (Figure 4.11) and the difference (P-Eo) is negative, so that there is a continuous loss of pure water (Figure 11.8), leaving the sea surface with a high salt concentration. This in turn helps maintain the currents which lowered the SST in the first place (Sections 11.3 and 11.5), so there is positive feedback, providing another illustration of the tight coupling between atmosphere and ocean.

Climate Change

The oceans' temperature is a major factor in climate change (Chapter 15). Firstly, the water's enormous thermal capacity slows up any sudden alteration of temperature. Secondly, the oceans dissolve much of the atmosphere's carbon dioxide, which governs the greenhouse effect (Note 2.L) and hence global temperatures. In this connection, there is a positive feedback: warm water dissolves less of the gas than cold water, so an increase of SST causes a release of carbon dioxide, thereby increasing greenhouse heating and hence accelerating global warming. Thirdly, the deep waters of the oceans already store about fifty times as much carbon dioxide as the global atmosphere (Figure1.3 ), and can hold much more. Fourthly, there appears to be an interaction between global temperatures and the largest oceanic circulations. The pattern of temperatures drives the ocean currents, and they carry heat which alters the temperatures— again an example of atmosphere—ocean coupling.

Warming of the oceans in the course of climate change causes expansion of surface water. For instance, heating just the top 500 m by 3K would increase the volume by 0.06 per cent, so that the level would rise by perhaps 0.3 m (i.e. 0.0006x500). Current indications are of a rise

Figure 11.5 The difference between January and July mean sea-surface temperatures.

Figure 11.5 The difference between January and July mean sea-surface temperatures.

by around 0.2 m per century, though that may be due partly to the melting of glaciers.

Temperature Profile

Ocean temperatures also vary in a vertical direction, forming layers like those of the atmosphere (Figure 1.10), but upside-down. The main difference is the stability of the ocean below the surface mixed layer. That mixed layer has a depth of only 20 m or so off Peru near the equator, but sometimes 1,000 m west of Vancouver (49°N). Deep convection of (relatively dense) cold surface water to cause stirring within the layer occurs especially at high latitudes and in winter.

The mixed layer comprises the warm-water sphere of the ocean. Lower down is a zone with a notable drop in temperature (Figure 11.6), the 'thermocline', corresponding to the inversion at the top of the PBL (Section 7.6). It is a few hundred metres thick, with the same stability as an atmospheric inversion, so that fluctuations of surface temperature or oxygen concentration, for instance, do not penetrate below. The thermocline is most pronounced during the summer (Figure 11.6) and in the tropics (Figure 11.9). It is especially evident at about 20°S in the Indian ocean, just north-west of Australia, where there is a difference of 18 K within 400

temperature: °C 4 6 8 10 12 14 16 18

_

j

i may

i june

july ---

i i august

march

cipril À

j

-

i

i

i

" i

Figure 11.6 The seasonal effect on temperature profiles in the Pacific ocean at 50°N, 145°W.

m. On the other hand, the thermocline hardly exists near the poles.

Remarkably cold 'deep water' lies beneath the thermocline, and beyond that there is the abyssal (or bottom) layer, a few hundred metres deep on the sea-bed. It has a temperature of about 0°C, even in the Tropics. These layers are discussed further in Section 11.5.

11.3 SALINITY

There are normally about 34.5 grams of salts dissolved in each kilogram of sea water, written as 34.5%o. The salts consist mainly of chloride (55 per cent) and sodium (31 per cent) ions, which form sodium chloride (i.e. common salt) if the water is evaporated away in evaporation ponds. The amount of sodium chloride in the sea represents about 12 per cent of saturation, beyond which the water can dissolve no more. Other salts include sulphate (8 per cent), magnesium (3.7 per cent), calcium (1.2 per cent) and potassium (1.1 per cent). The total salt concentration (or salinity) affects climates by altering the density of the sea, thus changing the pattern of pressures which govern the ocean currents and hence the transport of heat around the world.

Figure 11.9 shows that the salt concentration varies across the thermocline and around the world. Seas near estuaries or melting icebergs may be covered by a layer of lighter fresh water, and a low salinity is found anywhere that precipitation greatly exceeds evaporation, e.g. at high latitudes (Figure 10.6). Values are only 33.5% south of 60°S, around Antarctica.

The salinity of the surface is greater at 37°S off south-west Australia, where evaporation is more than precipitation. The highest salinity in the south Pacific ocean (36.5%) occurs east of Tahiti (20°S), where rainfalls are less than 250 mm/a (Figure 10.3) but the evaporation rate is high (Figure 4.11). Likewise, salinity in the Atlantic approaches 37% off the north-east coast of Brazil.

Effects of Salinity

High salinity increases the water's density (Note 11.B) and lowers the freezing point. In addition, the salt in sea water removes a curious feature of pure water—that it is most dense at 4°C, not at freezing point. The consequence is that sea water is less likely than fresh water to become ice, for the following reason. Cooling of the surface induces convection (Note 11.B and Section 11.5), i.e. vertical mixing with warmer layers below. This ceases in pure fresh water when the surface reaches 4°C because further cooling makes the water relatively light so that

Figure 11.7 Limits of ice around Antarctica. The zones signify, respectively, (A) occasional iceberg sightings, (B) occasional sea-ice in winter and spring, (C) occasional sea-ice in summer and autumn, and (D) permanently frozen.

Figure 11.7 Limits of ice around Antarctica. The zones signify, respectively, (A) occasional iceberg sightings, (B) occasional sea-ice in winter and spring, (C) occasional sea-ice in summer and autumn, and (D) permanently frozen.

it remains on top, to freeze. The ice then insulates the water below from further cooling, and so a thin layer forms over water still at 4°C. On the other hand, cooling the surface of sea water induces convection which continues until the whole depth has cooled to its freezing point at about -2°C. The resulting convection to the bottom of the ocean creates global circulations within the oceans (Section 11.5).

The dissolved salt also reduces the water's vapour pressure, e.g. a saturated solution has a vapour pressure of only 75 per cent of that of pure water, and therefore the figure for the sea is 97 per cent (i.e. 100-0.12x[100-751). This means a slight reduction in the evaporation rate (Note 4.E).

Figure 11.8 The difference between the annual mean monthly rainfall and evaporation (in millimetres) over the oceans of the globe. Positive values (mm/mo) indicate rainfall greater than evaporation, and vice versa.

salinity: %o

Figure 11.9 Profiles of temperature and salinity at various latitudes in the Indian ocean.

salinity: %o

temperature: °C

Figure 11.9 Profiles of temperature and salinity at various latitudes in the Indian ocean.

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment