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Currents and Tides Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 4, 2011

Coastal Current

Over half of the world’s population, roughly 2.7 billion, lives within 100km of a coastline. Some countries, like Australia, have all-encompassing coastlines. Canada has the longest coastline of any country, at 90,889km or around 15% of the world’s 599,538km of coastlines.

Most currents found along a coast are more limited. When waves hit the beach at an angle, it is called a Longshore current . The wave front smacks the shallow water first and then slows down. The rest of the wave folds as it comes on shore forming a current that parallels the shoreline. Larger waves hitting the beach at greater angles cause stronger Longshore currents. Longshore current action can also cause sandbars to form.

Rip currents or rip tides are a dangerous side effect of Longshore currents. Rip tides happen when Longshore currents, moving parallel to the beach, react seaward because of an obstacle in the bottom.

Rip tides or rip currents are caused by a combination of Longshore currents and underwater features that react with the current.

Sandbar cuts frequently provide good spots for rip tides to occur. Swimmers should always be aware of rip tide conditions, especially on vacation or in an unfamiliar area. Rip tide warning signs are usually present, but asking about local currents is smart. Swimmers can be pulled out to sea quickly in a reacting rip current.

If caught in one of these currents, stay calm and swim parallel to the beach. When you are past the turbulent current action, you can turn and swim to shore without a problem.

Upwelling

Offshore winds, blowing out from the land push water away from the shore. When this happens, deep, cold water rises to replace the water that has been blown out to sea. A vertical current, called upwelling , is then created. This forms a circular flow from the ocean bottom that brings different nutrients to the surface. Marine life increases in these nutrient-rich waters.

These nutrients come from the remains of dead organisms and fecal matter that sinks to the ocean bottom. As this material decays, nutrients are freed. They stay where they fall on the ocean’s floor until an upwelling blows them back up to the surface. Large plankton increases, called ocean blooms , often take place after a coastal upwelling because of the nutrients that it distributes.

Coastal upwelling takes place along the western sides of continents in the Atlantic, Indian, and Pacific. Upwelling plays an important part in the ocean ecosystem. It supports about half of the world’s natural fisheries (hatching areas), although these cold waters account for only 10% of the ocean’s total surface area.

What goes up must come down. Downwelling takes place from the opposite angle. Onshore winds, winds blowing toward the beach push water toward the land. This wind action drives the shore surface water down and outward from the beach.

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