This is a simple temperature-depth ocean water profile. You can see temperature decreases with increasing depth. The thermocline are layers of water where the temperature changes rapidly with depth. This temperature-depth profile is what you might expect to find in low to middle latitudes.
Click on image for full size
Windows to the Universe original image

Temperature of Ocean Water

Most of the solar radiation (light and heat) that hits the ocean is absorbed in the first few tens of meters of water. Waves and turbulence mix this heat downward quickly. The surface layer of the ocean is well mixed from the top to the bottom of that layer. The temperature of the surface waters (the mixed layer) varies mainly with latitude. The polar seas (high latitude) can be as cold as -2 degrees Celsius (28.4 degrees Fahrenheit) while the Persian Gulf (low latitude) can be as warm as 36 degrees Celsius (96.8 degrees Fahrenheit). The average temperature of the ocean surface waters is about 17 degrees Celsius (62.6 degrees Fahrenheit).

There is a boundary between surface waters of the ocean and deeper layers that are not well mixed. The boundary usually begins around 100-400 meters and extends several hundred of meters downward from there. This boundary region, where there is a rapid decrease of temperature, is called the thermocline. 90 % of the total volume of ocean is found below the thermocline in the deep ocean. Here, temperatures approach 0 degrees Celsius. So even though surface waters can be a comfortable 20 degrees Celsius (good for swimming in!), the majority of our ocean water has a temperature between 0-3 degrees Celsius (32-37.5 degrees Fahrenheit).

The density of ocean water continuously increases with decreasing temperature until the water freezes. Ocean water, with an average salinity of 35 psu, freezes at -1.94 degrees Celsius (28.5 degrees Fahrenheit). At very high latitudes, ocean water can reach these low temperatures and freeze. Dissolved salts in the water tend to be rejected by the forming ice so that sea ice is only about 1 % salt. An interesting tidbit for those of you interested in survival tactics - because of the lessened amount of salt, melted sea ice would be fit to drink even if sea water is not! Sea ice formation at high latitudes ultimately drives circulation of the deep waters of the ocean.

At the surface of the ocean, thermometers placed on buoys can take temperature measurements. In the last decade, the Argo program has taken on the task of monitoring the state of ocean surface waters across the globe. The Argo program deploys floats that measure salinity and temperature throughout the surface layer of the ocean. Over 3,000 free-drifting floats have been deployed all over the ocean and each float is programmed to sink 2,000 meters down, drifting at that depth for about 10 days. The float then makes its way to the surface measuring temperature and salinity the whole time. Data is transmitted to a satellite once the float reaches the surface, so that scientists and the public have access to the state of the ocean within hours of the data collection. At a greater depth in the water, in situ measurements are often made with a CTD instrument (CTD = conductivity, temperature, depth), where the instrument is placed in the ocean water from a ship or a platform. These instruments are used by the Bermuda Institute of Ocean Sciences (BIOS), where they have been tracking ocean measurements like temperature, salinity and oxygen concentrations for over 55 years.

The Argos and BIOS program have both published results that confirm that the ocean is warming. Surface water temperatures obviously change from season to season and year to year, but the whole ocean has warmed about 0.1 degree Fahrenheit (0.055 degree Celsius) in the past 30-50 years. This may not seem like much of a temperature change, but it is quite significant. Think about a pot of water heating on a stove. A small pot of water will heat quickly, while a large pot of water at the same heat setting will heat very slowly. This is due to a difference in heat capacity. The ocean has an enormous heat capacity because of its large size, thus, it is like an enormous pot of water, and it takes a great amount of heat to warm the ocean. The fact that the ocean has warmed significantly in 30 to 50 years is remarkable and concerning.

Last modified February 16, 2011 by Jennifer Bergman.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Radiation

Radiation comes in two basic types: electromagnetic radiation transmitted by photons, and particle radiation consisting of electrons, protons, alpha particles, and so forth. Electromagnetic radiation,...more

Surface Ocean Currents

The water at the ocean surface is moved primarily by winds. Large scale winds move in specific directions because they are affected by Earth’s spin and the Coriolis Effect. Because Earth spins constantly,...more

Polar Oceans

There are two oceans in Earth’s polar regions: the Arctic Ocean, which occupies most of the north polar region, and the Southern Ocean, which surrounds the continent of Antarctica in the south polar region....more

Salinity

About 70% of the Earth is covered with water, and we find 97% of that water in the oceans. Everyone who has taken in a mouthful of ocean water while swimming knows that the ocean is really salty. All water...more

Climate Change and Sea Level Rise

The low-lying coast of Bangladesh in South Asia is home to millions of people, yet the amount of sea level rise predicted for the 21st Century is expected to change that, flooding homes with seawater and...more

Tidewater

Have you ever walked along a beach at low tide? Maybe you've had the opportunity to explore the world the sea water revealed when the water receded? Everyone likes to look for uncovered sea shells or the...more

Robots Watch out for Poisonous Plankton!

Tiny algae that float in the sea may look harmless but certain species are toxic and, in large numbers, they are able to kill fish, poison seafood and even choke swimmers. Now special underwater robots...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA