How does the temperature of ocean water vary?

The temperature of ocean water varies by location – both in terms of latitude and depth, due to variations in solar radiation and the physical properties of water.

During the Océano Profundo 2018: Exploring Deep-Sea Habitats off Puerto Rico and the U.S. Virgin Islands expedition on NOAA Ship Okeanos Explorer, we dove to a depth of 5,000 meters (16,405 feet) on Mona Seamount, where the average ocean water temperature was 2.2°C (36°F). This was the deepest dive of the expedition and was also one of the deepest dives ever to reach the seafloor in this part of the world. Despite the depth of the dive, the site was not devoid of life, as we saw sponges, corals, anemones, and even some fishes. Video courtesy of the NOAA Office of Ocean Exploration and Research, Exploring Deep-sea Habitats off Puerto Rico and the U.S. Virgin Islands. Download larger version (mp4, 72.7 MB).

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.