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Global Circulation Pattern
Figure 5
gives a conceptual picture of the global circulation
pattern that links the major ocean basins of the planet. The large region
of open ocean in the Equatorial Pacific allows significant warming of water
as it drifts to the west and past the northern edge of Australia. This
warm surface water drifts on westward through the Indian Ocean and around
the Southern tip of Africa. From here it turns northward, crossing the Equator
(taking a more easterly direction than shown here), and creating the Gulf
Stream off eastern North America. The warm surface water continually cools
as it moves northward past Great Britain and into the Norwegian Sea. By
this time the water is sufficiently cold and dense that it sinks to lower
depths in the ocean. This creates the start of the global scale return
current at low depths that moves southward across the Equator, back around Africa, past the southern edge of Australia and back to the
central Pacific Ocean. A smaller branch of the return current splits off
after passing Africa and enters the Indian ocean where sufficient warming
augments the warm surface current coming from the central Pacific.
Flow in the lower depths of the ocean is shown in Figure 6. Of particular note in this graph are the regions of subsidence (downward motion) northeast of Iceland and in the south Atlantic near Antarctica, which remove water from the surface and deliver it to deeper levels in the ocean. A third subsidence region of lesser extent exists in the Labrador Sea between Laboratory and Greenland. To understand the reason for these subsidence regions we must examine the driving forces for vertical circulation in the ocean.
