Currents and Tides Help (page 3)
Ocean waters are constantly on the move. Currents flow in complex patterns affected by wind, salinity and temperature, bottom topography, and the Earth’s rotation.
Determined by forces such as wind, tides, and gravity, currents keep the oceans stirred up. Currents move water thousands of miles. There are many different currents, but the seven main currents include the Antarctic Circumpolar Current (also called the West Wind Drift ), East Wind Drift , North and South Equatorial currents , Peru Current , Kuroshio Current , and Gulf Stream . These currents move in large rotating circles called gyres .
Warm surface currents move from the equator to the higher latitudes, driven by atmospheric winds and the Earth’s rotation. Cold surface currents start out from polar and temperate latitudes, then move south toward the equator. Like the warm surface currents, they are driven mainly by atmospheric forces. Gyres form when the main ocean currents intersect. Water flows in a circular path: clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
Large ocean currents are driven by atmospheric winds that whip the planet all year round. The winds at the equator are called the northeast and southeast trade winds. At the mid-latitudes, the winds are called the westerlies, and at the highest latitudes, the winds are called the polar easterlies. These winds blow in the same direction all year.
The Antarctic Circumpolar Current and the Kuroshio Current are two of the largest currents. The Antarctic Circumpolar Current circles eastward around Antarctica, while the Kuroshio Current, located off Japan’s coast, travels up to 120km a day at a speed of nearly 5km/hr. A global circular current takes place when deep water is created in the North Atlantic, sinks, moves south, roams around Antarctica, and then heads northward to the Indian, Pacific, and Atlantic basins. Oceanographers estimate that it takes roughly a thousand years for water from the North Atlantic to wind its way into the North Pacific.
The California current is an eastern boundary current. It is wide, leisurely, cool, and shallow. Eastern boundary currents are frequently linked with upwelling.
The Somali current , off Africa’s eastern coast, is a current with a twist. It does the impossible, it reverses direction twice a year. From May through September, the Somali current runs northward. Then from November through March it runs southward. April and October must serve as the turnaround months, like the round house for trains. As the Somali current flows northward, upwelling takes place and brings nutrients to marine life. However, when the current turns southward again, the banquet table is put away and everybody goes back to work.
Most people have heard of the Gulf Stream. It has a strong effect on the East Coast of the United States. The Gulf Stream surface current is a strong, western boundary current. It is warm, deep, swift, and fairly salty, separating open ocean water from coastal water. Western currents move commonly from tropical to temperate latitudes. The Gulf Stream commonly travels at a speed of nearly 4 knots.
The Gulf Stream is part of a greater current system that includes the North Atlantic Current , Canary Current , and the North Equatorial Current . From the Yucatan Peninsula in Mexico, the Gulf Stream moves north through the Straits of Florida and up the east coast of Florida. When it gets to around North Carolina of the United States, it starts to slip off into the North Atlantic toward the Grand Banks near Newfoundland.
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.
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.
We learned that water is divided into layers depending on temperature, salinity, pressure, and density. Currents and tides move seawater around changing the density. These movements change the depth of the thermocline layer. Underwater vehicles have to adjust their buoyancy to maintain a stationary depth.
When the Moon, Earth, and Sun line up, a happening called syzygy , shorelines experience the greatest change in high- and low-tide water levels. These spring tides take place twice a month, during the full and new Moon.
When the Moon is at perigee , or its closest distance to the Earth in its orbit, the tides are extremely high or low.
Then when the Sun and Moon form a 90° angle, like at the time of a half moon, their gravitational forces fight each other and there is a smaller change between high and low tides. These are known as neap tides .
There are other things that play into the equation. For example, when the Moon goes around the Earth, the planet’s tilt, ocean depth, and ocean topography all affect tides. This is why not all coasts have two high and low tides a day.
Semidiurnal tides happen twice a day. The Atlantic Ocean has semidiurnal tides. It has two high tides and two low tides in one day.
Diurnal tides take place once a day. The Gulf of Mexico has only one high tide and one low tide in a 25-hr period.
Then there are the exceptions to the rule, some seas and ocean sections, including parts of the Pacific Basin, have mixed tides . They don’t follow any set pattern, but many have one low tide followed by two high tides.
The Bay of Fundy , between New Brunswick and Nova Scotia holds the record for the highest tides in the world. The change in water level between high tide and low tide can be over 16 m; more than half the length of a football field.
Practice problems of this concept can be found at: Oceans Practice Test
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