The ocean is constantly moving. 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 waters of the entire world's oceans. A process known as thermohaline circulation drives these currents.
Global Water Currents
Thermohaline circulation moves a huge current of water around the globe. This current travels 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 that circles back on itself. You may have seen conveyor belts at airports. They are used to move passenger luggage.
The term thermohaline combines the words thermo (heat) and haline (salt). Both heat and salt influence the density of seawater. Density is the amount of matter in a specific volume of material. The higher the density, the greater the weight of that material.
The ocean is constantly shifting and moving in reaction to changes in water density. To best understand how ocean 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, Middle and Bottom Ocean Layers
The ocean can be divided into several layers. The top layer collects the warmth and energy of sunlight. The bottom layers collect the rich, nutrient-filled deposits 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, through which sunlight is turned into food. Phytoplankton are microscopic plants. They make up the first part of the ocean food chain, each level of which feeds on the level below. All ocean life depends on phytoplankton.
The middle ocean layer is called the thermocline. In this layer, the ocean's temperature and density change very quickly. 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. It averages about three kilometers (two miles) in depth.
Rising and Falling Oceans
As phytoplankton die, they sink and collect on the ocean floor. Thus, nutrients continuously move from the ocean's surface to its depths. However, their travel is not one-way. In certain regions of the ocean, deep water rises to the surface. As it rises, it brings nutrients back up with it.
The ocean slowly turns over from top to bottom in a constant 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. From there we will 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. It is one of the main driving forces of the conveyor belt.
The force of the sinking cold water pushes the North Atlantic Deep Water south 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 Antartica.
A great deal of overturning of water happens in the ocean around Antarctica. It happens when the very cold Antarctic surface water sinks. This sinking forces the nutrient-rich deep water to rise.
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. Over time the water becomes warm enough to rise. This 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 make one complete trip around Earth. During this time, all the waters of the world's oceans are turned over.
Temperature Changes
Ocean temperature plays a key role in the conveyor belt. Thus, climate change caused by human activities, like burning fuels, could harm the system. If one part of the conveyor belt were to break down, nutrients will 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 ocean food chain. If the first link is endangered, all life in the oceans is endangered.
