The ocean is constantly moving. You can see this when waves crash onto shore. Swimmers can feel it when an ocean current pulls them along. Surface currents move water across the planet. They are powered by ocean winds. The ocean also has deep, underwater currents. These are bigger than surface currents, but slower. Underwater currents slowly mix the waters of the entire ocean. A process, or series of steps, drives these currents. This process is known as thermohaline circulation.
Earth's Ocean Currents
Thermohaline circulation moves a huge current of water around Earth. 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. Surface waters move downward. Then, deep 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 word thermohaline combines the words thermo (heat) and haline (salt). Both heat and salt affect 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 moving in reaction to changes in water density. Below are a few simple points about water. They will help you understand how ocean water moves:
• Water always flows down toward the lowest point.
• Water's density depends on two things: one is its temperature, and the other is its salinity (amount of salt).
• Cold water is denser than warm water.
• Salt is heavy. Therefore, 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.
The Ocean's Three Layers
The ocean can be divided into several layers. The top layer collects the warmth and energy of sunlight. The bottom layers collect rich, nutrient-filled sediment. This sediment is made of decayed plant and animal matter. It provides nutrition for countless lifeforms.
The top ocean layer is about 100 meters (330 feet) deep. Enough sunlight reaches it for phytoplankton to carry out photosynthesis, through which sunlight is turned into food. Phytoplankton are like microscopic plants. They make up the first part of the ocean food chain, of which each level feeds on the level below. In other words, big fish eat small fish, and small fish eat plants. All ocean life depends on phytoplankton. They are the base of the food chain.
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.
Turning Over the Ocean's Water
As phytoplankton die, they sink and collect on the ocean floor. So, nutrients continuously move from the ocean's surface to its depths. However, their travel is not one-way. Over time, deep water rises to the surface. As it rises, it brings nutrients back up with it.
Over time, the ocean slowly turns over from top to bottom. Like a conveyor belt, thermohaline circulation moves nutrients from one part of the ocean to another. How does this happen?
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. It is pushed along by the Antarctic Circumpolar Current. This huge and very strong 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 Ocean and Pacific Ocean. As the deep, cold water travels, 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. In time, the warm waters travel back to the North Atlantic Deep Water, completing the circle.
It takes about 500 years for the conveyor belt to make one complete trip around Earth. During this time, all the ocean's water is turned over.
Ocean temperature plays a key role in the conveyor belt. So, climate change could harm the system. Climate change is caused by human activities, like burning fuels. If one part of the conveyor belt breaks down, nutrients will not be distributed to start the food chain. Phytoplankton need those nutrients. Without them, they cannot grow. Then, the first link in the ocean food chain will not be formed. If the first link is endangered, all ocean life is endangered.