Sunday, September 04, 2005

A Primer on Ocean Currents

by The Windjammer

Our oceans are in constant motion. All one need do to observe that is to stand on any ocean shore and watch for a few minutes. The flow of currents within the walls of ocean water is a bit more difficult to see.

The ocean currents are divided into two major groups, warm and cold. The currents move large volumes of water, but as large as they appear, they are a part of larger systems called gyres. A gyre is best described as a large rotating system with a roughly circular shape.

Generally speaking, the rotation and the flow of the currents in the northern hemisphere is in a clockwise direction. The flow in the southern hemisphere is in a counterclockwise direction. There are notable exceptions which are impossible for me to explain.

I hope no one thinks to ask, "But why does a particular current always flow in the same fairly well-defined path and in the same direction?" I don't know, although I have a strong theory. It is really more a belief than theory. I suspect that if you ask someone else, they will tell you that they don=t know for sure either. They will probably have more knowledge than is stored in this old gray head and may be able to give you a definitive answer.

It is accepted by most that the currents are the result of the effect of constant wind on water. There is also an effect caused by the rotation of the earth which can disrupt the wind effect. There is another effect which causes the currents to change direction of travel (not direction of flow) which is called the Coriolis effect. Please don't ask me to explain why the flows stay in somewhat rigid water boundaries. There is also the factor of pieces of land standing in the way. The currents stay some distance offshore and their direction of travel is changed by protrusions of land such as peninsulas into the ocean.

It should be understood that in the following discussion, the "starting point" mentioned is not actually the starting point of the current, but only a reference point to help you more easily locate the system being described.

Please understand, too, that the locations are approximate and that you should consult authoritative books on the subject for more accurate descriptions. This piece is intended only to whet your appetite for knowledge. It is not the bible on currents and hurricanes.

I will try to stick pretty much to the warm currents in the early part, but there will be some overlap. I will attempt to classify each at the beginning of its portion.

Most of us in the eastern United States are familiar with the Gulf Stream, a warm current. The Gulf Stream is a part of the larger system which has been named "The North Atlantic Gyre." It might help to view it as a giant, slow-moving whirlpool located in the Atlantic Ocean north of the equator and stretching from the east coast of North America to the west coast of Africa.

Let us assume that the starting point for the Gulf Stream is off the northwest coast of Africa. The current flows almost southwestward from that point across the Cape Verde Basin to just north of the equator, then bears westward to then swing northwest on or near the northeast edge of the Guiana Basin. The warmed waters then skirt the Caribbean Islands of Puerto Rico and Cuba and head for the east coast of Florida. When the current passes the coast of Florida, it can be flowing at six miles per hour and moving more than a billion cubic feet of water per second, making it one of the strongest ocean currents in the world. It is generally the same path which the tropical depressions follow to later become tropical storms or hurricanes when the storms pick up heat from the warm water.

The current then moves northeastward along the eastern coast of the United States. It begins to shift toward the Atlantic at some point near Cape Hatteras on the Outer Banks of North Carolina. It is comparatively narrow as it progresses along the southeast coast, but after it shifts eastward, can be as much as a hundred miles wide. The stream is often sharply defined by temperature with the western side of the stream along the east coast being defined by a "cold wall." Water temperatures within the Gulf Stream can be as much as twenty degrees or more warmer than in the adjacent water in less than one hundred feet west.

When it enters the North Atlantic, a part of it then flows northeastward as the North Atlantic Drift, affecting the weather of Nova Scotia and the British Isles.

The southern part flows eastward across the Atlantic to start the process all over again.

The North Atlantic Drift, a warm current, could be described as a northeast extension of the Gulf Stream. It flows northeast to a point west of the British Isles, then continues northeast to just north of those islands where it becomes cooler and changes its name to the Irminger Current, still classified as a warm current although it has cooled considerably, as it enters the waters of the Norwegian Sea, lending its warmth to such places as Iceland and Norway as it heads toward the Arctic Ocean.

The South (Atlantic) Equatorial Current, a warm current, begins in the eastern part of the South Atlantic and flows northwestward to pass the eastern coast of northern South America. It is actually a countercurrent since it follows a lower right to upper left direction. It flows along the Guiana Basin as it crosses the equator to enter the Venezuelan Basin and then to flow almost alongside the southern part of the Gulf Stream, but on the southern side of Puerto Rico and Cuba as it crosses the Caribbean. A part of this current enters the Gulf of Mexico.

There is another Atlantic current known as the Guinea Current, a warm current. It flows southward off the west coast of southern Africa. It is considered to be an offshoot of the current in the previous paragraph.

I will now jump across a big piece of the planet to discuss the Kuroshio (Current). The Kuroshio, a warm current, like the Gulf Stream in the Atlantic, is a part of a larger system which forms an even larger whirlpool in the Pacific. That system stretches from near the southeast coast of the Asian continent to just off the west coast of North America.

Again, assuming that the starting point is just off the west coast of Mexico, the Kuroshio travels westward in an almost straight path just north of the equator to a point east of the Philippine Islands where it turns sharply northeast to bring its warmth to Japan. It flows northeast and becomes the North Pacific Current, a warm current, which flows toward southern Alaska. The North Pacific Current can be described as the northern arm of the Kuroshio.

The California Current which is considered a cold water current and can be pictured as an extension of the North Pacific Current, carries the cooler waters left over from the Kuroshio and North Pacific Currents southward off the west coast of North America to Baja California and Mexico where it starts the trip all over again.

The Somali Current, a warm current, is located in the Indian Ocean. It starts just off the east coast of Africa north of Madagascar. The whirlpool formed by the current straddles the equator which may account for the fact that it flows for a part of the year in a clockwise direction and in the rest of the year in a counterclockwise direction.

During the Southwest Monsoon season which occurs in the winter months, if one could call them that, of June through August, the current flows north. During the rest of the year, it flows in the other direction. I do not know of another current which does that, although there is another which is sometimes interrupted.

The Agulhas Current, a warm current also in the Indian Ocean, is the strongest current in the southern hemisphere. It flows southward along the east coast of southern Africa.

The Humboldt Current, a cold current, is also known as the Peru Current. It starts off the west coast of Chile as a cold current and moves north along the coast of South America. It gains some warmth as it moves nearer the equator and heads westward where a part of it turns south toward Australia while a part of it goes north of Micronesia and turns sharply to the east when it becomes the Equatorial Countercurrent. That warm current flows almost directly east toward the northeast coast of South America. The Humboldt normally carries rich nutrients from the ocean floor, causing aquatic life to flourish. Periodically, weather conditions cause the Equatorial Countercurrent to strengthen and disrupt the flow of its Humboldt ancestor. The cessation in the flow of nutrients results in the destruction or starvation of fish and other life forms. This phenomenon has been named "El Nino" or AThe Child, more specifically "Boy Child." The name is a derivation from the Christ Child because the phenomenon takes place near the time of Christmas.

The name "El Nina" or "Girl Child" has been given to a lesser event which is similar.

The Antarctic Circumpolar Current is a cold current which travels around the continent of Antarctica. It is about as far south as you can get in any direction without setting your feet on dry (or soggy) land. It is the largest current and is said to be the result of the Westerlies, winds which push it along at about a half mile per hour. The waters which branch off as arms feed most of the southernmost currents.

The East Greenland Current is a cold current which brings water southward from the Arctic Ocean along the east coast of Greenland.

The Labrador Current is a cold current which brings water from Baffin Bay along the east coast of Quebec to where it intersects with the Gulf Stream. It has been called the "Iceberg Current" because it brings the bergs which are a hazard to ship traffic in the North Atlantic.

The Benguela Current is a cold current in the South Atlantic which is one of the arms of the Antarctica Circumpolar Current. The Cape of Good Hope and the west coast of Africa interrupts to some degree the flow and diverts some of the water of the Circumpolar Current north and west to become the Benguela Current which then feeds the (South) Atlantic Equatorial Current.
The Oyashio (Current), a cold current, flows southward from the Bering Sea which is the southern part of the Arctic Ocean through the Bering Strait and into the Pacific.

The California Current is a cold current which is a part of the same gyre as the Kuroshio and North Pacific Currents. It is this current which carries the cooled waters of those currents back toward the starting point of the Kuroshio Current off the west coast of Mexico.

I hope that this short piece on the currents flowing through our oceans has instilled in you a desire to learn more.

I have tried to depict the currents and their location, direction of flow, direction of travel and other pertinent information correctly.

If you find any errors in any of my presentation, please let me know.

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Anonymous said...

Couple of things, but I thought that was well written.

I don't think wind blowing on the ocean really effects the current at all. It's actually the ocean current and temperature which in turn effect the power of the wind (hurricanes for instance, take energy from the ocean- the warmer the ocean, the more energy available for the wind).

Another factor on ocean currents is salinity. Saltier water drops, while fresher water rises. As snow and ice melt as a result of global warming, more and more fresh water makes it to the ocean. If enough fresh water gets ijnto one of these currents, it can drastically chenge the current, and hence the weather patterns affected by it.

Anonymous said...

Thanks for the explanations of the many ocean currents. It brings back fond memories of my Descriptive Oceanography class at the US Navy Postgraduate School in 1965. I made use of much of that knowledge while out chasing submarines. Actually I was on an Admiral's staff riding an aircraft carrier and others got to do the chasing. My happy group of sailors would prepare daily sonar forecasts (probable detection ranges for the various ship and aircraft equipments) based primarily on the temperature of the water in our area of operations. It would get real complicated when operating off the East Coast with the Gulf Stream. At times a loop of the Gulf Stream would actually get cut off and be left spinning and slowly being mixed with the surrounding waters. That could really cause problems with our forecasts. On the other hand for those floating under the water surface the temperature variation could be a definite benefit to avoid detection.
Time passes, the earth turns, the winds blow, and the ocean currents continue on their way, as do we all.
Thanks again.

Rander, USN, Ret. (Commander)