Just like a forest, an ocean has different layers.

Southern Ocean Cross Section. Wikimedia Commons: Hannes Grobe


The temperature of the ocean 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 changesOcean temperature layers
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.
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