Ocean Temperature

The temperature of the sea governs marine life, coastal climate and the great weather systems of the planet. Unlike the land, the ocean heats and cools slowly and unevenly. Its temperature also falls in a distinctive way with depth. Understanding ocean temperature (its source, its layers and its distribution) is core physical geography.

The chief source of the ocean's heat is the Sun: the radiant energy received at the surface is called insolation. Because water mixes and evaporates, heat spreads differently than on land.

Going down from the surface, the ocean has three temperature layers:

• The first (surface) layer: warm water about 500 m thick, 20–25°C in the tropics. It is well developed all year in the tropics.
• The thermocline: the middle zone where temperature falls rapidly with depth. This sharp transition layer is the key feature of ocean temperature.
• The deep layer: very cold water extending to the ocean floor.

In the polar regions there is only one cold layer from surface to bottom.

Across the surface, ocean temperature is controlled mainly by latitude:

• it is highest near the equator (about 27°C, actually a little north of it) and decreases towards the poles (down to about −1.9°C),
• the average surface temperature is about 26.7°C,
• the rate of decrease is roughly 0.5°C per degree of latitude, and
• the Northern Hemisphere is slightly warmer than the Southern because it has more land.

Warm and cold ocean currents modify this pattern. Warm currents (like the Gulf Stream) raise coastal temperatures, while cold currents (like the Labrador) lower them.

Water has a higher specific heat than land. It needs more energy to warm up, and it releases its heat slowly. Land heats and cools quickly. The land-ocean temperature contrast is therefore greater in winter than in summer, because in winter the continents cool far more than the oceans do.

This difference shows clearly on temperature maps. Isotherms are lines joining places with the same temperature. In January, the Northern Hemisphere winter, the oceans are warmer than the landmasses. The isotherms therefore bend poleward (north) over the oceans and equatorward (south) over the continents. Warm currents strengthen this bending. The North Atlantic Drift, a warm current that continues the Gulf Stream, pushes the January isotherms far to the north over the North Atlantic.

Temperature also varies from one side of an ocean to the other at the same latitude, because the prevailing winds drag surface water across the basin:

• Tropics: the trade winds push warm surface water westward. The western sections of the oceans are warmer than the eastern.
• Temperate latitudes: the westerlies drive warm water eastward. The eastern sections are warmer than the western.

The yearly swing of surface temperature is not the same everywhere:

• Hemispheres: the annual range is greater in the Northern Hemisphere than in the Southern, because the north has more land.
• Basins: the annual range is greater in the Atlantic than in the Pacific. The Atlantic is narrower and flanked by more land, so nearby continents impose their larger seasonal swings on it.

With depth, temperature falls, but not evenly:

• the surface receives the insolation and is warmest,
• temperature drops sharply through the thermocline, and
• below it, the water is uniformly cold into the deep ocean, where temperature changes very little.

The daily (diurnal) range of surface temperature is tiny (about 1°C). The annual range is also small over the open ocean. Water heats and cools slowly, so the sea moderates the climate of nearby coasts.

Scientists summarise the heat of the upper ocean with the Ocean Mean Temperature (OMT). It is the mean temperature of the water column from the surface down to the depth of the 26°C isotherm. That depth is not a fixed value. It varies with region and season. In the south-western Indian Ocean it lies at about 129 m during January–March. OMT measured during January–March is used to predict whether the coming monsoon rainfall will be above or below the long-term mean.