Shop Windows to the Universe

Our Glaciers: Then and Now activity kit helps you see the changes taking place in glaciers around the world. See all our activity kits and classroom activities.
A schematic showing the ocean "conveyor belt", where surface waters sink, enter deep water circulation, then resurface after slowly flowing through the deep ocean.
Click on image for full size
Windows to the Universe original image

The Deep Waters of the Ocean

Have you ever taken a vacation near an ocean? Maybe you went swimming or surfing in the water, maybe you went deep sea fishing or on a glass bottom boat ride. These activities all take place in the surface waters of the ocean. But did you know that most of the water in the ocean (90% by volume) is actually found below the surface layer of the ocean? Most people will never see the deep waters of the ocean...they are just too deep! The pycnocline, characterized by a rapid change of density, separates the surface layer of the ocean from the deep ocean. Typical deep ocean water has a temperature of about 3 degrees Celsius and a salinity measuring about 34-35 psu.

So, where do these deep waters come from? The biggest source of deep water is surface water that sinks in the North Atlantic Ocean. The Gulf Stream current brings highly saline water northward (salinity is high in mid-latitudes where evaporation is high and precipitation is low). Cold water from the north (Labrador and Irminger currents) meets the Gulf Stream and the water gets mixed. This cold, salty mixture can be dense enough to sink into the depths of the ocean.

While surface currents are driven mainly by wind, deep water circulation is driven by gravity. Cold and salty ocean surface water in the North Atlantic and elsewhere will slowly sink downward until it reaches a level of equal density. If the water is more dense (colder and/or saltier) then any other water in the deep ocean, it will sink all the way to the sea floor. Once the water reaches a level of equal density, the water spreads out. So in this way the deep ocean is stratified into horizontal layers, with water of higher and higher density sinking to deeper and deeper layers. The water that sinks in the North Atlantic flows all the way past the equator into the Southern Hemisphere. The water then flows past Antarctica and into the Pacific and Indian Oceans. Here, some of the deep waters are warmed and so rise again to the surface and to the surface circulation system. This conveyor belt cycling (pictured above) of the ocean waters is a simplification of the global ocean circulation. Still, it helps to show the basic idea of ocean water circulation from the surface to the depths of the ocean back to the surface again. In reality, deep water circulation is interrupted or diverted by ridges, rises and other underwater obstacles.

Deep water circulation is driven by density differences. Density is in turn dependent on the temperature and salinity of the water sample. Because of this dependence, deep water circulation is sometimes referred to as thermohaline circulation. Thermohaline circulation is very slow. Once water sinks from the surface, it can spend a long time away from the surface until that water rises again to become part of surface circulation. Some scientists estimate that the residence time of deep water, the time spent away from the surface, can be about 200-500 years for the Atlantic Ocean and 1,000-2,000 years for the Pacific Ocean.

Deep water circulation and surface currents are both ways in which heat distribution happens on Earth. These two types of water movement keep the equatorial region from overheating and the poles from becoming permanent frozen regions.

Deep waters contain a lot of nutrients such as phosphates, nitrates, and carbonates. Regions of upwelling of these deep waters are important for ocean productivity at the surface. When the nutrient-enriched deep water does well up to the surface, high plankton productivity and abundant fish can be found.

Last modified September 27, 2001 by Jennifer Bergman.

Shop Windows to the Universe Science Store!

The Spring 2010 issue of The Earth Scientist, focuses on the ocean, including articles on polar research, coral reefs, ocean acidification, and climate. Includes a gorgeous full color poster!

Windows to the Universe Community



You might also be interested in:

Cool It! Game

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


Wind is moving air. Warm air rises, and cool air comes in to take its place. This movement creates different pressures in the atmosphere which creates the winds around the globe. Since the Earth spins,...more

Deep Water and the CO2 cycle

The deep ocean waters are under pressure and are much colder than layers of the ocean which are closer to the surface. Dissolved carbon dioxide seems to be absent from the deep ocean water and as a result...more


Carbonate is a name for rocks and minerals which contain a certain form of carbon/oxygen compound known as CO32-. (CO32- is also known as the molecule carbonate). Limestone is an example of a calcium carbonate,...more

Thermohaline Circulation: The Global Ocean Conveyor

The world has several oceans, the Pacific, the Atlantic, the Indian, the Arctic, and the Southern Ocean. While we have different names for them, they are not really separate. There are not walls between...more

Scientists to Investigate Role of Equatorial Pacific Ocean in Global Climate System

In early March, an international team of scientists will set sail aboard the drill ship JOIDES Resolution on the first of two Integrated Ocean Drilling Program (IODP) expeditions to the equatorial Pacific...more

Life in the Deep Ocean

The deep ocean is very cold, under high pressure, and always dark because sunlight can not penetrate that far. The only light comes from bioluminescence – a chemical reaction inside the bodies of some...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF