Ocean temperature

The ocean temperature plays a crucial role in the global climate system, ocean currents and for marine habitats. It varies depending on depth, geographical location and season. Not only the temperature differs in seawater but also the salinity. Warm surface water is generally saltier than the cooler deep or polar waters.^[1] In polar regions, the upper layers of ocean water are cold and fresh.^[2] Deep ocean water is cold, salty water found deep below the surface of Earth's oceans. This water has a uniform temperature of around 0-3 °C.^[3] The ocean temperature also depends on the amount of solar radiation falling on its surface. In the tropics, with the Sun nearly overhead, the temperature of the surface layers can rise to over 30 °C (86 °F). Near the poles the temperature in equilibrium with the sea ice is about −2 °C (28 °F).

There is a continuous large-scale circulation of water in the oceans. One part of it is the thermohaline circulation (THC). It is driven by global density gradients created by surface heat and freshwater fluxes.^[4][5] Warm surface currents cool as they move away from the tropics. This happens as the water becomes denser and sinks. Changes in temperature and density move the cold water back towards the equator as a deep sea current. Then it eventually wells up again towards the surface.

Ocean temperature as a term applies to the temperature in the ocean at any depth. It can also apply specifically to the ocean temperatures that are not near the surface. In this case it is synonymous with deep ocean temperature).

It is clear that the oceans are warming as a result of climate change and this rate of warming is increasing.^{[6]: 9 [7]} The upper ocean (above 700 m) is warming fastest, but the warming trend extends throughout the ocean. In 2022, the global ocean was the hottest ever recorded by humans.^[8]