Ocean Currents, Waves, and Tides

Ocean waters are constantly on the move. How they move influences climate and living conditions for plants and animals, even on land. Currents are large streams of moving water that flow through the ocean. Currents flow in complex patterns affected by wind, the water's salinity and heat content, bottom topography, and the earth's rotation.

Ocean currents play an extremely important role in the global transfer of water, heat, organisms, nutrients, potential pollutants, and sediments. From the time explorers set sail to discover new lands to the present day when deep-sea submersibles venture deep into the oceans depths, knowledge of how water flows within the sea has been sought. Today we use satellite and computer technology, combined with years of experience to investigate how water moves within the ocean.

The sun heats air near the equator more than it heats air at other latitudes. Pressure differences form because of these differences in heating. For example, the air that is heated near the equator is warmer and less dense than surrounding air. Warm, less dense air rises and creates an area of low pressure near the equator. Pressure differences in the atmosphere cause the wind to blow.

The Global Conveyor Belt

Great Ocean Conveyor Belt In a cycle known as the global conveyor belt, warm water near the equator is pushed by normally strong winds toward the poles and begins to cool. In a few regions, such as the North Atlantic, cold, salty water sinks to the ocean floor. This water then travels in the deep ocean back towards the equator and begins to rise to replace the water that is being pushed away from the equator by the winds. This cool water rising to the surface is known as upwelling. This entire process, which may take a thousand years to complete, helps regulate the climate of the Earth as heat is transported from the equator to the polar regions and cooling water is transported to the equator.

The sun ultimately creates winds in the atmosphere and ocean currents. Because the equator receives more direct rays from the sun throughout the year, the equatorial oceans "heat up" more than the oceans at higher latitudes in both the north and the south. This creates a temperature imbalance, which creates a pole ward flow of heat by way of the atmospheric winds and the oceanic currents. Unequal heating of the atmosphere on land and the oceans creates winds and circulation.

Surface Currents

Surface currents and deep currents form convection currents that move ocean water.

Horizontal, streamlike movements of water that occur at or near the surface of the ocean are called surface currents. Surface currents can reach depths of several hundred meters. These currents also reach lengths of several thousand kilometers and can travel across oceans. The Gulf Stream, is one of the strongest surface currents on Earth. The Gulf Stream transports at least 25 times more water each year than is transported by all of the rivers in the world combined. The Gulf Stream resembles a fast-moving, deep-blue river within the ocean. It carries warm water from the Gulf of Mexico to the Caribbean Sea, then northward along the coast of the United States, Near Cape Hatteras, North Carolina, it curves eastward across the Atlantic.

Surface currents are controlled by three factors: global winds, the Coriolis effect, and continental deflections. These three factors keep surface currents flowing in distinct patterns around Earth. The sun causes winds to blow, and winds cause surface currents to form. Therefore, the major source of the energy that powers surface currents is the sun.

Global Winds

Imagine you have just been served a hot bowl of soup. To cool the soup you blow across the top gently and notice ripples across the surface as your breath pushes the soup. Global winds have a similar affect on the oceans water, pushing the water along, this process causes surface currents.

Near the equator, the winds blow mostly east to west. Between 30° south latitude and 60° south latitude, winds blow mostly west to east. Regardless of direction they will eventually double back to complete a circle. As the Earth rotates, the paths of the winds and currents curve in relation to Earth's surface. This effect of Earth's rotation on the direction of winds and currents is called the Coriolis effect.

Northern Hemisphere - currents curve right

Southern Hemisphere - currents curve left

Surface Currents and Climate

Currents affect climate by moving cold and warm water around the globe. A surface current warms or cools the air above it, influencing the climate of the land near the coast. Warm water will increase the temperature and bring moisture into cool dry areas, while cold water currents will bring refreshing cool dryer air into hot, humid areas. Remember cold air holds less moisture than warm air.

Deep Currents

Below the surface we find another type of current with cold waters. Deep currents are caused by differences in density rather than surface winds. We know ocean water is different from freshwater because it contains salt.

Decrease in Temperature+An Increase in Salinity=An Increase in Water Density

Water moves from the equator toward the poles. The water will gradually cool. As ice forms near the poles, the salinity of the liquid water increases from the salt left behind during freezing. The water density changes. The temperature decreases, the salinity increases and the water has become more dense. This dense water will begin to sink, and this cold water will flow back along the ocean floor as a deep current. Deep currents move much slower than surface currents. It could take as much as 1,000 years for water to make the round trip from pole to equator!

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