Oceanography: Relief of the Oceans, Temperature, Salinity and Density, Currents, Waves, Tides, Sea-Level Change, Tsunamis and Cyclones
2. Composition: temperature, salinity and density
Temperature. The ocean is heated from above, so it is layered. A warm, wind-stirred mixed layer (tens to a couple of hundred metres) overlies the thermocline, where temperature falls rapidly with depth down to about 1,000 m; below lies cold deep water at about 2–4 °C everywhere. Surface temperature falls from the equator towards the poles, is higher in the Northern Hemisphere than the Southern at the same latitude, and has a small annual range because of water's high specific heat. Salinity is the total dissolved salt, expressed in parts per thousand (‰) or practical salinity units; the ocean average is about 35‰. Sodium chloride makes up about 78 per cent of the salts, followed by magnesium chloride and magnesium sulphate, and — the principle of constant proportions — the ratios between the major ions are almost the same everywhere, whatever the total.
Salinity rises with evaporation and with the formation of sea ice, and falls with precipitation, river inflow and melting ice. So surface salinity is not highest at the equator, where rainfall is heavy, but in the subtropics near 20°–30° under the dry subtropical highs; it is low at the equator, lower still in polar waters. Enclosed seas show the extremes: the Red Sea exceeds 40‰ (high evaporation, no rivers), while the Baltic is brackish, with surface values well under 10‰ in its northern gulfs. Among inland water bodies Lake Van in Turkey, the Dead Sea and the Great Salt Lake are far saltier still. Density increases with salinity and pressure and as temperature falls; a zone of rapid density increase with depth is the pycnocline. Water that is cold and salty enough sinks: this is the source of North Atlantic Deep Water in the Greenland and Norwegian seas and of Antarctic Bottom Water, the densest water mass, in the Weddell Sea.
3. Circulation: warm and cold currents
Surface currents are driven chiefly by the planetary winds and shaped by the Coriolis force, the configuration of the coasts and differences in density. V.W. Ekman showed that wind-driven water is deflected with depth in a spiral, so that the net transport of the wind-driven layer is at 90° to the wind — to the right in the Northern Hemisphere and to the left in the Southern. Ekman transport away from a coast draws up cold, nutrient-rich water (coastal upwelling). Under the subtropical highs, the trades and westerlies turn the surface water into great gyres, clockwise in the Northern Hemisphere and anticlockwise in the Southern. The gyres are asymmetric: Henry Stommel (1948) showed that the Earth's rotation concentrates flow along the western boundary, so western boundary currents such as the Gulf Stream and Kuroshio are narrow, deep, fast and warm, while eastern boundary currents are broad, slow, shallow and cold.
| Ocean | Warm currents | Cold currents |
|---|---|---|
| North Atlantic | Gulf Stream, North Atlantic Drift | Labrador, Canary |
| South Atlantic | Brazil | Benguela, Falkland |
| North Pacific | Kuroshio, North Pacific Drift, Alaska | Oyashio, California |
| South Pacific | East Australian | Humboldt (Peru) |
| Indian Ocean | Agulhas, Mozambique; the monsoon currents reverse seasonally | West Australian |
Cold eastern-boundary currents with upwelling coincide with the great coastal deserts — the Atacama beside the Humboldt, the Namib beside the Benguela — because cool water chills and stabilises the air above it and suppresses rain, while fogs are common. The meeting of warm and cold currents off Newfoundland (Gulf Stream and Labrador) and Japan (Kuroshio and Oyashio) creates fog and some of the richest fishing grounds. The Antarctic Circumpolar Current, the West Wind Drift, is the largest current by volume and the only one that circles the globe. The north Indian Ocean is unique: its currents reverse with the monsoon, flowing clockwise under the south-west monsoon in summer and anticlockwise under the north-east monsoon in winter, with the Somali Current off East Africa among the fastest in the world in July. Beneath the surface, density differences drive the slow thermohaline circulation, the global "conveyor belt" described by Wallace Broecker.
4. Waves, tides, sea-level change, tsunamis and cyclones
Waves are generated by wind; their height grows with wind speed, the duration of the wind and the fetch (the distance of open water over which it blows). In deep water the water particles move in near-circular orbits and it is energy, not water, that travels. As a wave enters water shallower than about half its wavelength it "feels bottom", slows, shortens and steepens until it breaks. Long, regular waves that have left their generating area are swell. Tides are the periodic rise and fall of sea level caused mainly by the Moon's gravitational pull, with the Sun contributing a little under half as much. When Sun, Moon and Earth are in line — new moon and full moon — the pulls combine into spring tides of maximum range; when they are at right angles — the first and third quarters — they partly cancel into neap tides. Because the Moon advances in its orbit, the tidal day is about 24 hours 50 minutes, so semidiurnal high tides come about 12 hours 25 minutes apart. Tides may be semidiurnal, diurnal or mixed; the range is greatest in funnel-shaped inlets — the Bay of Fundy exceeds 15 m, and in India the Gulf of Khambhat has the largest range, while the Hooghly carries a tidal bore.
Sea-level change is either eustatic — a worldwide change in the volume of ocean water or of the ocean basins — or relative and local. Glacio-eustatic change follows ice volume: at the Last Glacial Maximum, about 20,000 years ago, sea level stood roughly 120 m lower, exposing the continental shelves; thermal expansion of warming water is a second eustatic component. Isostatic change is local movement of the land — glacio-isostatic rebound is still lifting Scandinavia and the Hudson Bay region after the ice load was removed — and tectonic movement can raise or drown a coast. The evidence is read in raised beaches and marine terraces (emergence) and in drowned river valleys (rias), fjords and submerged forests (submergence). The IPCC AR6 estimates that global mean sea level rose about 0.20 m between 1901 and 2018, at an accelerating rate of about 3.7 mm a year in 2006–2018.
A tsunami is a train of very long waves set off by a sudden displacement of the sea floor — above all a large subduction earthquake with vertical movement, but also submarine landslides and volcanic collapse. In the deep ocean its speed is given by √(g × d): over water 4,000 m deep that is about 200 m/s, or some 700 km/h, with a wavelength of well over 100 km and a height of well under a metre, so ships do not notice it. Near the coast it slows and piles up to many metres; the sea often first draws back. The Sumatra–Andaman earthquake of 26 December 2004 (magnitude about 9.1) drove the tsunami that struck Tamil Nadu, the Andaman and Nicobar Islands and Sri Lanka, and led India to set up the Indian Tsunami Early Warning Centre at INCOIS, Hyderabad, operational since 2007. The ocean hazard of a cyclone is the storm surge — a dome of sea water pushed ashore by wind and low pressure. It is worst at the head of the Bay of Bengal because the continental shelf is broad and shallow and the coastline funnels the water, which is why Bangladesh and coastal Odisha and West Bengal have suffered the deadliest surges on record.
Key takeaways
- Shelf (to about 200 m, 0.1°), slope (2°–5°, canyons), rise (fans such as the Bengal Fan), abyssal plain, ridges about 65,000 km long, guyots and trenches; the Challenger Deep is nearly 11,000 m deep.
- Average salinity is about 35‰, highest in the subtropics (evaporation minus precipitation), over 40‰ in the Red Sea and very low in the Baltic; density rises with salinity and cold, and cold salty water sinks to drive the thermohaline circulation.
- Ekman transport is 90° to the wind; gyres turn clockwise in the north; western boundary currents (Gulf Stream, Kuroshio) are narrow, fast and warm; cold eastern currents with upwelling border coastal deserts; north Indian Ocean currents reverse with the monsoon.
- Spring tides at new and full moon, neap tides at the quarters, high tides about 12 h 25 min apart; the Gulf of Khambhat has India's largest tidal range.
- Eustatic change is global (ice volume, thermal expansion; 120 m lower at the Last Glacial Maximum), isostatic change local; tsunamis travel at √(gd) and are invisible at sea; storm surges are worst at the shallow, funnel-shaped head of the Bay of Bengal.
Practice questions (16)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
The gently sloping, shallow, submerged edge of a continent is the:
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Answer: C — Continental shelf
The continental shelf is continental crust under shallow water, usually to 130–200 m, sloping at about 0.1°. The rise is a sediment apron at the foot of the slope, and the abyssal plain lies at 3,000–6,000 m.A submarine volcano with a flat top, planed by waves before it subsided, is called a:
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Answer: B — Guyot
A guyot is a flat-topped seamount: its summit was eroded near sea level and then sank as the ocean crust cooled and subsided. An atoll is a ring-shaped coral reef, and a canyon is a valley cut into the continental slope.The deepest known point of the world ocean, the Challenger Deep, lies in the:
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Answer: A — Mariana Trench
The Challenger Deep in the Mariana Trench of the western Pacific is nearly 11,000 m deep. The Sunda Trench is the deepest in the Indian Ocean, and the Puerto Rico Trench the deepest in the Atlantic.Surface salinity of the open ocean is highest near 20°–30° latitude rather than at the equator because:
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Answer: D — Evaporation is high and rainfall low under the subtropical highs
Salinity follows evaporation minus precipitation. The equator is hot but very rainy; the subtropical highs have clear skies, dry trade winds and strong evaporation, so the surface water there is saltiest.The average salinity of ocean water is about:
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Answer: C — 35 parts per thousand
The world ocean averages about 35‰ — 35 grams of dissolved salts per kilogram of sea water — of which sodium chloride is about 78 per cent. Values near 7‰ are found in the brackish Baltic.Which of the following are cold ocean currents? Select all that apply.
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Answer: A — Canary Current; C — Benguela Current; D — Humboldt (Peru) Current
The Humboldt, Benguela and Canary are cold eastern-boundary currents with upwelling off South America, south-west Africa and north-west Africa. The Kuroshio is the warm western-boundary current of the North Pacific.Why are the Gulf Stream and the Kuroshio narrower, deeper and faster than the currents on the eastern sides of the same oceans?
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Answer: B — The Earth's rotation intensifies flow along the western boundaries of ocean basins
Stommel (1948) showed that the variation of the Coriolis effect with latitude compresses the return flow of the subtropical gyres against the western boundary — western intensification. Eastern boundary currents are correspondingly broad and slow.According to Ekman's theory, at what angle (in degrees) to the wind direction is the net transport of the wind-driven surface layer? Type a number.
Numerical answer — type the value.
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Answer: 90
Integrated over the whole Ekman spiral, the net transport is 90° to the right of the wind in the Northern Hemisphere and 90° to the left in the Southern. Surface water alone moves at about 45°, which is a common distractor.Assertion (A): Coastal deserts such as the Atacama and the Namib lie on the western margins of continents in the subtropics. Reason (R): Cold currents and upwelling along these coasts cool and stabilise the lower air and suppress rainfall. Choose the correct option.
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Answer: A — Both A and R are true, and R is the correct explanation of A
Both are true and R explains A: the Humboldt and Benguela currents chill the air from below, creating a stable inversion that allows fog but not rain, reinforcing the dryness of the subtropical high.Spring tides occur when:
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Answer: A — The Sun, Moon and Earth are in a straight line, at new moon and full moon
At syzygy — new and full moon — the tide-producing forces of Sun and Moon add together and the tidal range is greatest. At the quarters (quadrature) they partly cancel and produce neap tides. The name has nothing to do with the season.Successive high tides at a place on a semidiurnal coast come about 12 hours 25 minutes apart, not 12 hours, because:
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Answer: D — The Moon moves forward in its orbit, so the Earth must rotate a little more to face it again
The Moon advances about 13° a day along its orbit, so a point on the Earth returns under the Moon after about 24 hours 50 minutes — the tidal day — and two high tides fall within it, roughly 12 hours 25 minutes apart.Along the Indian coast, the largest tidal range is found in the:
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Answer: A — Gulf of Khambhat
The funnel shape of the Gulf of Khambhat (Cambay) amplifies the tide to a range of several metres — up to about 11 m at its head — the largest in India, which has drawn interest in tidal power there. The Kerala coast has a small range.Which of the following are eustatic changes of sea level? Select all that apply.
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Answer: C — The fall of sea level during the Last Glacial Maximum as water was locked in ice sheets; D — The rise of sea level from the thermal expansion of warming ocean water
Eustatic changes alter the volume of water in the world ocean and are felt everywhere: glacio-eustatic lowering and thermal expansion both qualify. The rebound of Scandinavia is glacio-isostatic and the faulted block is tectonic — both are local movements of the land.A tsunami crosses an ocean 4,000 m deep. Using speed = √(g × d) with g = 10 m/s², what is its speed in metres per second? Type a number.
Numerical answer — type the value.
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Answer: 200
Speed = √(10 × 4,000) = √40,000 = 200 m/s, which is 720 km/h. Tsunamis are shallow-water waves because their wavelength is far greater than the ocean depth, so their speed depends only on depth.Assertion (A): Tsunamis are generated mainly by strong winds blowing over the open ocean for several days. Reason (R): In the deep ocean a tsunami has a very long wavelength and a small height, so ships barely notice it. Choose the correct option.
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Answer: D — A is false, but R is true
A is false: tsunamis come from sudden displacement of the sea floor by earthquakes, landslides or volcanic collapse — wind makes ordinary waves and swell. R is true: wavelengths exceed 100 km while deep-water heights are often under a metre.Storm surges are especially destructive at the head of the Bay of Bengal mainly because:
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Answer: D — The continental shelf is broad and shallow and the coast funnels the water
A shallow, wide shelf lets wind-driven water pile up instead of returning at depth, and the converging, low-lying coast of the Ganga–Brahmaputra delta concentrates it; the Bay's surface water is in fact relatively fresh because of river inflow.