22. The Circulation of Earth’s Atmosphere and Ocean

The modern atmospheric circulation reflects the state of the planet with ice on both poles. Warm air rises in the equatorial region and sinks in the polar regions. But the rotation of the earth causes equatorward flowing air to appear to be turned to the right in the northern hemisphere and to the left in the southern hemisphere. The result is three cells per hemisphere. Those on either side of the equator are the Hadley cells, those in the polar regions are the Polar cells; these drive an intermediate pair of cells, the Ferrell cells. The winds drive the ocean currents; the water moves to the right of the wind in the northern hemisphere, and to the left in the southern hemisphere. The winds set up large anticyclonic gyres between the equator and a latitude of about 60 ̊N and S. These warm water gyres overlie cold water that sinks in the polar region. The waters between these two masses in which the temperature declines rapidly with depth are known as thermocline and intermediate waters. They sink into the ocean along the Subtropical and Polar frontal system that border the great gyres. The deep waters of the ocean form a ‘Great Conveyor.’ Cold saline waters sink in the Greenland–Iceland–Norwegian (GIN) Sea and overflow the Greenland–Scotland Ridge into the North Atlantic Basin. There they mix with warm saline Mediterranean outflow waters, and flow southward as North Atlantic deep water. Some of this wells up near the Weddell Sea, is chilled, and sinks again as Antarctic Bottom Water, flowing eastward to end up in the Pacific. Slow upwelling of deep waters occurs throughout the lower latitude ocean, but more rapid upwelling occurs along the eastern margins of the ocean basins where equatorward blowing winds drive the surface water offshore. Special disturbances of the general circulation occur periodically in the Pacific (El Niño - La Niña, The Southern Oscillation) and the Atlantic (North Atlantic Oscillation) Ocean circulation on the warmer Earth of the Cretaceous was very different from that of today. Atmospheric pressure systems at the poles reversed with the seasons, winds were inconstant, and the great ocean gyres were replaced by myriads of eddies.