The ocean is in constant motion. You can see this for yourself when you watch waves crash onto shore. If you go swimming, you may even feel an ocean current pulling you along. Surface currents, such as the Gulf Stream, move water across the globe. They are powered by Earth's various wind patterns. The ocean also has deep, underwater currents. These are bigger than surface currents, but slower. Underwater currents mix the ocean's waters on a global scale. They are driven by a process known as thermohaline circulation.

Global Oceanic Circulation

Thermohaline circulation moves a huge current of water around the globe, from northern oceans to southern oceans, and back again. Currents slowly turn over water in the entire ocean, from top to bottom. Warm surface waters move downward and cold, nutrient-rich waters are forced upward. The whole process is something like a giant conveyor belt. A conveyor belt is a continuously moving band of material that moves objects from one place to another, and then circles back on itself. You may have seen conveyor belts at airports, where they are used to move passenger luggage.

The term thermohaline combines the words thermo (heat) and haline (salt). Both heat and salt are factors that influence the density of seawater. Density is the amount of matter in a specific volume of material. The more matter something has, the more it weighs. The ocean is constantly shifting and moving in reaction to changes in water density. To best understand ocean-water dynamics, or how water moves, there are a few simple principles to keep in mind:

• Water always flows down toward the lowest point.

• Water's density is determined by the water's temperature and salinity (amount of salt).

• Cold water is denser than warm water.

• Water with high salinity is denser than water with low salinity.

• Ocean water always moves toward an equilibrium, or balance. For example, if surface water cools and becomes denser, it will sink. The warmer water below will rise to balance out the missing surface water.

Top Layer, Thermocline and Deep Ocean

The ocean can be divided into several layers. The top layer collects the warmth and energy of sunlight, while the bottom layers collect the rich, nutrient-filled sediment of decayed plant and animal matter.

The top ocean layer is about 100 meters (330 feet) deep. Enough sunlight reaches that depth for phytoplankton to carry out photosynthesis. Phytoplankton are microscopic plants. They make up the first part of the marine food chain and are essential to all ocean life.

The middle layer is called the thermocline. The ocean's temperature and density change very quickly at this layer. The thermocline is about 200 to 1,000 meters (656 to 3,300 feet) deep.

Below the thermocline is the bottom layer, or deep ocean, which averages about three kilometers (two miles) in depth.

Movement in the Depths

As phytoplankton die, they sink and collect on the ocean floor. Thus, nutrients continuously move from the ocean's surface to its depths. However, this process is not one-way. In certain regions of the ocean, deep water upwells, or rises to the surface. As it rises, it brings nutrients back up to the surface.

The ocean slowly turns over from top to bottom in a continual global loop. Like a conveyor belt, thermohaline circulation moves nutrients from one part of the ocean to another.

Let's start in the northern Atlantic Ocean and follow the conveyor belt as it moves water around the planet.

In the seas near Greenland and Norway, the water is cold. Some of it freezes, leaving salt behind. The cold, salty water becomes dense and sinks to the ocean floor. This water is known as the North Atlantic Deep Water, and it is one of the primary driving forces of the conveyor belt.

The force of the sinking, cold water pushes the existing North Atlantic Deep Water south, toward Antarctica, in a slow-moving underwater current. When it reaches Antarctica, the water flows east with the Antarctic Circumpolar Current. This huge and powerful current circles the continent.

Parts of the Antarctic Circumpolar Current flow northward and move into the Indian and Pacific Oceans. As the deep, cold water travels through the oceans, it mixes with warmer water. The water eventually becomes warm enough to rise. This creates a slow upwelling that brings nutrients to the surface.

In the Pacific, the surface water flows into the Indian Ocean. It then travels around southern Africa, and back into the Atlantic. The warm waters eventually travel back to the North Atlantic Deep Water, completing the global loop.

It takes about 500 years for the conveyor belt to turn over the ocean's waters and make one complete trip around Earth.

The deep water in the Greenland Sea flows along toward the lowest point on the floor of the North Atlantic Ocean. The water collects in a basin, the same way river water flows into a lake or pond. That basin is the North Atlantic Deep Water.

Other seas also feed their waters into the North Atlantic Deep Water. Among them are the Mediterranean Sea, which enters the Atlantic Ocean through the Strait of Gibraltar.

Overturning Near Antarctica

Once the conveyor belt reaches the southern part of the globe it is driven back north by the Antarctic Circumpolar Current.

Western winds are very strong in the Antarctic. They help create the intensely powerful Antarctic Circumpolar Current, which moves a huge amount of water very quickly around the continent of Antarctica.

Overturning occurs in the waters around Antarctica. It happens when the extremely cold Antarctic surface water sinks, causing nutrient-rich deep water to rise. Overturning continually moves huge amounts of water from the ocean bottom to the surface.

The Antarctic Circumpolar Current and overturning make the waters around Antarctica an ideal habitat for many marine mammals. Many types of whales, for instance, migrate to the waters around Antarctica every year. They feed on phytoplankton and other tiny sea creatures churned up by overturning waters.

Threat to the System

Ocean temperature plays a key role in the conveyor belt. Thus, climate change caused by human activities could threaten the system. If one part of the conveyor belt were to break down, nutrients would not be distributed to start the food chain. Organisms such as phytoplankton need those nutrients to thrive. Severe climate change slows phytoplankton from forming the first link in the marine food chain. If the first link is threatened, all life in the oceans is threatened.