Water (Oceans)

The water cycle, the relief of the ocean floor, and how the temperature and salinity of sea water vary.

Key ideas

The hydrological cycle moves water between oceans, air, land, the ground and living things; the oceans hold about 91% of the earth’s water.
The ocean floor has four main divisions: continental shelf, continental slope, deep sea plain and oceanic deeps (trenches).
Minor features include mid-oceanic ridges, seamounts, flat-topped guyots, submarine canyons and coral atolls.
Ocean temperature falls from the equator to the poles and with depth: a warm top layer, a thermocline where it drops fast, and cold deep water.
Salinity is the salt dissolved in sea water; evaporation raises it, while rain, rivers and melting ice lower it.
The Arabian Sea is saltier than the Bay of Bengal, which gets much river water; the Red Sea is very salty.

Remember

Salinity	grams of salt in 1,000 g (1 kg) of water, in parts per thousand (‰)
Salt in a sample	salinity × kg of water = grams of salt
Salinity from a sample	grams of salt ÷ kg of water
Brackish water	below 24.7‰
Typical salinity	Indian Ocean 35‰, Atlantic 36‰, Red Sea 41‰
Surface temperature (book’s rule)	27 °C at the equator, 0.5 °C less per degree of latitude: 27 − latitude × 0.5
Ocean floor	shelf slopes 1° or less (average width 80 km; widest the Siberian shelf); slope 2°–5°; deep plains 3,000–6,000 m

Watch out

Mixing up the thermocline (temperature) and the halocline (salinity).
Thinking more rivers make a sea saltier: fresh water lowers salinity.
Calling a guyot pointed: it is a flat-topped seamount.

Worked example

Using 27 °C at the equator and a fall of 0.5 °C per degree of latitude, find the surface temperature at (a) 40° (b) 30°. Why is the highest temperature found slightly north of the equator?

Working: 27 − 40 × 0.5; 27 − 30 × 0.5

Answer: (a) 7 °C (b) 12 °C; the northern hemisphere has more land, which heats its oceans more.