THE
GREAT CONVEYOR BELT
The part of the water world’s "Great Conveyor Belt" that
we know best is the Gulf Stream. Flowing northward along the North American
coast, and then toward Europe, it is the main factor in maintaining those
regions as habitable. It is, however, just a portion of the Great Conveyor
Belt that sends vast currents of water in important directions on the
globe. Moreover, it is intimately involved with global warming, and hence
is of over-riding interest to all. We U.S. residents look at it from the
standpoint of the Americas, but remember that Europe and indeed the rest
of the world are involved.
Imagine a current of water originating in the Atlantic somewhere south of North America. As we shall see later, it is less salty and less dense than the rest of the ocean surrounding it. It flows generally northward just off the U.S. coast, and having originated in warm southern waters, it is still a warming device for us. It keeps us from enduring another Ice Age. Since it started in the Pacific, it is less salty than the water surrounding it (the Pacific is less salty than the Atlantic). Light and comparatively sweet, it forms a very distinct surface layer,
That's the beginning of the story that most youngsters learn in school.
But as the warm surface water flows on, it evaporates, increasing the salt concentration and losing heat. Moreover, the northerly winds off Greenland and the rest of the northeastern coast of North America ensure that the loss of heat is at a maximum. Now we have a stream of salty rather cool water that sinks in the sea while still maintaining its northward flow. At this point Coriolis forces come into play, bending the course of the stream more to the east; that explains how it warms Europe; remember it has not lost all of its heat. (Risk of a slight digression: "Coriolis" forces are the result of the movement of water from the "lower" latitudes near the Equator toward the "higher" latitudes near the Pole. Because of the Earth's rotation, the Gulf Stream water is flowing faster eastward than the surrounding seas with the result that there is a net curvature of the stream towards Europe). This explains why Europe is temperate instead of suffering an Ice Age. The coasts of Europe and West Africa turn the current still more, so that it flows southward toward the Great Southern Ocean - the name given to the vast body of water comprising the Atlantic, the Pacific, and the Indian 0ceans in the Southern Hemisphere. The Cape of Good Hope is a marker for their confluence. From there the water flows eastward past Cape Horn, and finally turns northerly, completing the "belt"-like course of the flow.
(Q: what drives the water up the South American coast to start the Gulf Stream farther north).
The answer comes from a rather idealized picture of the "normal" movement of the Great Conveyor Belt of which the Gulf Stream is merely a part. We know it best because it flows along the east coast of the United States. And there's a bit more to it also, that we'll come to in a few minutes.
What happens when, due to global warming, the ice in the area of the North Pole melts? It forms a stream of fresh cold water that flows southward along the coast of Greenland. As a result, it cools the Gulf Stream - fortunately for Northern Europe, it is not so cold as to cause the onset of an ice age.
This brings us to another question. There have been other ice ages all throughout history and before history began. Is another due, perhaps owing to some cause allied with the cooling of the Gulf Stream?
Here improved technology has come to our rescue. By using modern drilling equipment on the deep and ancient layer of ice covering Greenland, investigators have found that some transitions from warm to cold and back again have been phenomenally rapid. Not years and months were required, but days! This argues that if the same kind of thing happens again due to whatever changes occur in the Great Conveyor Belt, they could come upon us so quickly that it might be impossible to prepare for the onslaught of the impending ice age.
Q: So again, what drives the current northward along the coast of South America?
Having been diluted with fresher, colder water from the Arctic and its rivers, it's still a bit less dense than the ocean surrounding it, so it's closer to the surface, and wind and Coriolis forces combine to push it against the coast of South America, which then directs it into a northerly course.
Q: If the winds from Greenland are cold, and cool the water, how is it that the Gulf Stream warms the coast of Europe?
It turns eastward in the comparatively narrow part of the Atlantic between (say) Greenland and Europe's west coast. The driving force may be (this is pure speculation) the result of the south going current of cold water from the Arctic and the Siberian rivers, and the north-flowing stream of warmer water mainly at lesser depths. Having warmed western Europe - thus giving up most of its heat - it probably continues in somewhat more diffuse format southward until it meets the east - going currents of the Great Southern Ocean, and so continues to circulate toward South America and into the Atlantic for another cycle.
The overall pattern of the Great Conveyor Belt's travels is fairly well established. The most interesting question is probably: How much notice will it give if it stops warming us up - and will we have to migrate southward? Wish we knew!
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