Our planet is heated by solar radiation, incoming energy from the sun. Because the Earth is round, the angle of the surface relative to the incoming radiation differs with latitude. At low latitudes, near the equator, direct overhead sunlight received all year warms surface waters. At high latitudes, ocean waters receive less sunlight – the poles receive only 40 percent of the heat that the equator does. These variations in solar energy mean that the ocean surface can vary in temperature from a warm 30°C (86°F) in the tropics to a very cold -2°C (28°F) near the poles. In some areas, this surface temperature is relatively stable while in others, it fluctuates depending on the season (and thus the amount of sunlight received).
The temperature of ocean water also varies with depth. In the ocean, solar energy is reflected in the upper surface or rapidly absorbed with depth, meaning that the deeper into the ocean you descend, the less sunlight there is. This results in less warming of the water. Therefore, the deep ocean (below about 200 meters depth) is cold, with an average temperature of only 4°C (39°F). Cold water is also more dense, and as a result heavier, than warm water. Colder water sinks below the warm water at the surface, which contributes to the coldness of the deep ocean. The vertical structure in the ocean created by temperature differences has a large impact on how life is distributed in the ocean.
What is the Deep Sea? – Mountains in the Deep: Exploring the Central Pacific Basin
Here in Hawaiian Waters It Is...Cold! – Hohonu Moana: Exploring Deep Waters off Hawaiʻi
How is light distributed in the ocean?
Ocean Zones (pdf, 434 kb) – Learning Science through Ocean Exploration