The temperature1 of the ocean determines what form the water takes. Most of the ocean is liquid water, but if it gets cold enough, it turns to solid ice, or if it gets hot enough, it can pass into the atmosphere2 as water vapour
Water warms up more slowly than air but can hold more heat3 – water needs 4 times as much energy to raise its temperature by 1ºC as the same mass4 of air does – so the ocean plays an important part in taking up energy from the Sun and stopping the Earth getting too hot.
Variation in temperature
The temperature of the ocean, especially the surface, varies from place to place and from season to season. Ocean temperature depends on the amount of solar energy5 absorbed.
Tropical oceans receive a lot of direct overhead sunlight for much of the year, so the water is warm. Summer is the only time polar regions receive sunlight, and even then, it is never directly overhead, so water in these places tends to be cold. The amount of sunlight that hits the temperate regions (between the tropics and the poles) varies between summer and winter. The variation in solar energy absorbed means that the ocean surface can vary in temperature from a warm 30°C in the tropics to a very cold -2°C near the poles.
The temperature of the ocean also varies from top to bottom, giving a vertical structure to most of the ocean. There is an upper layer of water, up to 200m deep, that is warmed by the Sun and has the same temperature from top to bottom. Below that is a layer called the thermocline6, reaching down in places to 1000m, which is colder at the bottom than at the top. The deep ocean below the thermocline, making up 80% of the ocean, is the same very cold temperature throughout.
What happens when temperature changes
Some properties of water change with temperature:
- Cold water is denser than warm water, so it tends to sink.
- Cold water holds more dissolvable gases7, such as carbon8 dioxide
- Water temperature can affect the productivity of organisms living in it.
Water expands when it warms up – heat energy9 makes its molecules10 move around more and take up more space. Because the molecules are more spread out, the density11 goes down. When water cools, it contracts and becomes denser.
Temperature and salinity12 both affect the density of water, resulting in water moving up or down through the ocean layers and moving as currents around the ocean.
Activity idea
In this activity on water temperature, students look at what happens when hot and cold water meet.
- temperature: A measure of the degree of hotness or coldness of an object or substance. Temperature is measured with a thermometer calibrated in one or more temperature scales. Kelvin scale temperature is a measure of the average energy of the molecules of a body.
- atmosphere: 1. The layer of gas around the Earth. 2. (atm) A non-SI unit of pressure equivalent to 101.325 kPa.
- heat energy (heat): Heat energy: the transfer of energy in materials from the random movement of the particles in that material. The greater the random movement of particles the more heat energy the material has. Temperature is a measure of the heat energy of a material.
Heat: the flow of energy from a warm object to a cooler object. - mass: The amount of matter an object has, measured in kilograms.
- solar energy: The energy received by the Earth and from the Sun. Also called solar power.
- thermocline: A layer of water that has a large variation in temperature from top to bottom. In the ocean, it often separates surface waters from deep waters, both of which have similar temperatures throughout.
- gases: The state of matter distinguished from the solid and liquid states. Gases have the ability to diffuse readily and to become distributed uniformly throughout any container.
- carbon: A non-metal element (C). It is a key component of living things.
- heat energy (heat): Heat energy: the transfer of energy in materials from the random movement of the particles in that material. The greater the random movement of particles the more heat energy the material has. Temperature is a measure of the heat energy of a material.
Heat: the flow of energy from a warm object to a cooler object. - molecule: Two or more atoms bonded together. The molecule of an element has all its atoms the same. The molecule of a compound has two or more different atoms.
- density: How tightly a certain amount of matter (atoms or molecules) of a substance is compacted in a given volume. Density is commonly measured in grams per millilitre (g/ml) or cubic centimetre (g/cm3).
- salinity: The amount of chemicals dissolved in water. In seawater, the main chemical is sodium chloride (salt), but there are many others in smaller quantities.