The temperature 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 atmosphere as water vapour
Water warms up more slowly than air but can hold more heat – water needs 4 times as much energy to raise its temperature by 1ºC as the same mass 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 energy 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 thermocline, 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 gases, such as carbon dioxide
- Water temperature can affect the productivity of organisms living in it.
Water expands when it warms up – heat energy makes its molecules move around more and take up more space. Because the molecules are more spread out, the density goes down. When water cools, it contracts and becomes denser.
Temperature and salinity both affect the density of water, resulting in water moving up or down through the ocean layers and moving as currents around the ocean.