Currents: Water on the Move (PDF)
Can you name Earth’s most important rivers? Would you guess the Nile, the Amazon, the Mississippi, or maybe the Yangtze? Did you know there are rivers that flow through the world’s oceans? And that they are more important in shaping our environment than the major rivers on land? These mighty bodies of moving water are called currents.
Ocean currents move water continuously along specific pathways, often over very great distances. This happens both on the surface and in the deep ocean.
Currents driven by the wind travel thousands of miles across the ocean’s surface. In the process, they move heat from warmer to cooler areas. Ocean waters also move vertically, mixing waters of different temperatures and salinities (amounts of salt).
Currents influence temperature, climate, plants, and animals in the ocean and on land. They carry organisms, nutrients, and pollutants across long distances and up and down through the ocean’s depths. Both short-term weather patterns and long-term changes in climate, such as global warming, are affected by currents.
Ocean in Motion
Most of the currents in the upper half-mile of the ocean are caused by winds that blow year-round in one direction across Earth. Wind pushes directly on the ocean surface, causing the ocean’s top layer to move. Surface winds also set in motion other currents that extend to the ocean bottom, far below the direct influence of the wind.
However, most ocean currents do not occur at the surface and are not directly caused by wind. Deep ocean currents may move across great horizontal distances in the same way as surface currents. They also may run through the ocean vertically, although vertical currents are weak, and a body of water may take centuries to go from the surface to the bottom, Generally, the deeper a current, the more slowly it moves.
Ocean Currents Map
When wind-driven surface currents run into continents, the water must turn and flow toward either the poles or the equator (Earth’s mid-section). The effect of Earth’s rotation causes moving objects on Earth to follow curved paths (a scientific principle known as the Coriolis effect), which in turn causes the currents to turn and move in huge, oceanwide, looping circles called gyres.
Scientists Studying Currents
Surface currents have been studied by sailors for hundreds of years. Early ships depended on winds and currents. Understanding currents was important to people who used the ocean for fishing, trade and travel.
Scientific studies of currents date back more than 100 years. In early studies, researchers set afloat drift bottles and other types of floating markers form ships or into offshore currents. Their movement was then charted with the help of other scientists, sailors, or even beachcombers who would report finding them.
For nearly 40 years scientists at Scripps, and around the world, have been involved in intensive efforts to understand currents in relation to world ocean circulation patterns. Most recently, the relationship between the ocean and the atmosphere and its effect on climate has become a major environmental issue.
The development of a worldwide network of communication satellites, and the invention of computers able to process huge amounts of information, set the stage for the current generation of instruments.
Drifters were invented that recorded the speed and direction of currents from the motion of the drifter within the current. Drifters able to take many different direct measurements simultaneously were also developed. They could record ocean temperature, salinity, pressure, and other variables in addition to current speed and direction.
Satellite transmission devices fitted to drifters “talked” directly to overhead satellites, which instantly relayed data to labs on land. The wealth of new data was used to make complex models of water circulation and density patterns and the movement of heat in the ocean. These models were used to study possible climate change.
The New Generation
Scripps scientists, along with colleagues in the United States and 22 other countries, are taking part in a major program called Argo that will deploy 3,000 drifters (SOLO floats) into the world ocean possibly by the end of this year. These instruments can collect and store many kinds of data, and then transmit them to satellites on a regular schedule. They also are capable of automatically descending to a depth of 6,560 feet (2,000 meters), drifting for ten days, and then returning to the surface while recording temperature, salinity, and other data about water conditions on the way up. Scientists hope that this fleet of drifters will be able to produce a new, real-time picture of the world ocean every ten days.
Properties of Water
In general, currents in the deeper ocean are caused by the mixing of bodies of water at different temperatures, salinity levels, pressures, and densities, although this may not be the direct cause.
Salinity is the scientific term for the amount of saltiness in water. All water has some dissolved salts in it, but the ocean has much more salt than fresh water.
Ocean water is always evaporating into the atmosphere, but the salt remains in the ocean. Thus, the ocean remains salty even though fresh water is continually being added by rainfall and by rivers and runoff from land.
Density is the scientific term for the weight of water. Salt water weighs more than fresh water, so it is heavier. The more salt the water contain, the denser it is. Because it weighs more, saltier water sinks.
Temperature can vary a lot in the ocean. Shallow tropical waters can reach close to 90°F (31° C), while water at the poles will be close to freezing (28.4°F, -2° C). Seawater freezes at a lower temperature than fresh water (32° F, 0° C) because of its salt content.
The warmest water in any area is at the surface, and the coldest water is at the bottom. This is partly because the sun warms surface water. However, cold water is heavier (denser) than warm water, so cold water naturally moves toward the bottom and warm toward the top.
Pressure is a measure of the weight of the water and atmosphere pushing downward at any given point. There is 15 pounds per square inch of pressure at sea level. In the deepest parts of the ocean, pressure is around 15,000 pounds per square inch, an increase of 1,000 times.
Density is a measure of weight, technically the weight of a liquid divided by how much space it talkes up. Salinity, temperature, and pressure work together to create density.
Movement happens when two opposing forces are not in balance. When one part of the ocean is denser than the surrounding area, water movement will take place. This creates currents. Because of the many factors involved, the flow of currents is very complex
Of Toys and Tennis Shoes
Would you believe that tennis shoes, rubber duckies, and doll heads have all helped in the tracking of ocean currents? All of these things have fallen into the ocean as the result of spills from cargo ships during storms at sea.
Eighty thousand tennis shoes were washed overboard near the Alaskan Peninsula in 1990. Beachcombers from Alaska to northern California collected hundreds of them. Over the next few years, shoes washed up in Hawaii, the Philippines, and Japan.
Yellow duckies and green frogs ended up on beaches from Alaska to Oregon from a spill of plastic bathtub toys in the north Pacific Ocean in 1992.
What does this have to do with science? Some oceanographers have used the floating toys to plot ocean surface currents. Studies have shown that about 1 percent of a spilled cargo eventually washes ashore. The oceanographers have kept records of where the toys ended up over time to record the paths of currents.
The toys also provide information about where floating debris will go after spills. Thus, they may help scientists contain other — possibly dangerous — spills in the future.