Geography · Ch 18 — Water (Oceans)
Temperature of Ocean Waters
Temperature of Ocean Waters
Ocean water, like land, is heated by solar energy — but it heats up and cools down far more slowly than land does. This section looks at how that temperature varies across the ocean surface (spatially) and with depth (vertically), and what causes those variations.
Factors Affecting Temperature Distribution
Four main factors control the temperature of ocean water:
- Latitude — Surface water temperature decreases from the equator towards the poles, because the amount of insolation (incoming solar radiation) decreases as we move poleward.
- Unequal distribution of land and water — The oceans of the northern hemisphere receive more heat than those of the southern hemisphere, because they are in greater contact with large landmasses.
- Prevailing winds — Winds blowing from land towards the ocean push warm surface water away from the coast, causing cold water from below to rise up (upwelling). This creates longitudinal variations in temperature. On the other hand, onshore winds pile up warm water near the coast, raising the temperature there.
- Ocean currents — Warm currents raise temperatures in cold areas, while cold currents lower temperatures in warm areas. For example, the Gulf Stream (a warm current) raises temperatures near the eastern coast of North America and the west coast of Europe, whereas the Labrador Current (a cold current) lowers temperatures near the north-east coast of North America.
These factors influence ocean temperatures locally. A related point: enclosed seas in low latitudes record relatively higher temperatures than open seas, while enclosed seas in high latitudes have lower temperatures than open seas.
Horizontal and Vertical Distribution of Temperature
The temperature-depth profile of ocean water shows how temperature decreases with increasing depth. The profile reveals a boundary region between the surface waters and the deeper layers. This boundary usually begins around 100–400 m below the sea surface and extends several hundred metres downward. This region, where there is a rapid decrease of temperature, is called the thermocline. About 90 per cent of the total volume of ocean water lies below the thermocline, in the deep ocean, where temperatures approach 0°C.
In middle and low latitudes, the temperature structure of the oceans can be described as a three-layer system from surface to bottom:
| Layer | Characteristics |
|---|---|
| First layer | Top layer of warm oceanic water, about 500 m thick, with temperatures between 20°C and 25°C. In the tropical region it exists throughout the year, but in mid-latitudes it develops only during summer. |
| Second layer (thermocline) | Lies below the first layer; characterised by a rapid decrease in temperature with depth. It is 500–1,000 m thick. |
| Third layer | Very cold, extending up to the deep ocean floor. |
In the Arctic and Antarctic circles, surface water temperatures are close to 0°C, so the temperature change with depth is very slight. Here, only one layer of cold water exists, extending from the surface to the deep ocean floor.
Key Temperature Values
The average temperature of surface water of the oceans is about 27°C, and it gradually decreases from the equator towards the poles. The rate of decrease with increasing latitude is generally 0.5°C per degree of latitude. The average temperature is around 22°C at 20° latitudes, 14°C at 40° latitudes, and 0°C near the poles. …