Annually, the net surface heat balance (the net effect of all the processes shown in Figure 1.01) is such that the tropics receive more heat from the Sun than they lose from longwave radiation, conduction, or latent heat. The poles, on the other hand, lose more heat annually than they receive from the Sun. If this set of circumstances remained unchanged by other processes, what would eventually happen at the tropics? At the poles? What is keeping the tropics from continually getting warmer and the poles cooler? The excess heat in the tropics must be transported to the poles by the atmosphere and ocean. But how? In this section, we will examine the transport of heat by ocean currents.

Figure 1.05 illustrates the movement of water by ocean currents. In general, warm water is transported poleward along the western boundary of each ocean basin from about 25° to 45° latitude in both hemispheres (depicted with red arrows in Figure 1.05), and cool water is transported equatorward along the eastern boundary of each ocean basin at these latitudes (depicted with blue arrows in Figure 1.05).

The net north-south circulation of water, then, acts to move warmer water from the tropics toward the poles where it is cooled, and cooler water from the poles toward the tropics where it is warmed. In this way, ocean circulation helps the global heat balance by removing excess heat from the tropics and adding heat to the polar regions. This north-south circulation of heat is seen in the global SST distribution as a northward displacement of isotherms on the western sides of ocean basins and a southward displacement of isotherms along the eastern sides (study Figure 1.04 once again). Figure 1.06 is an image of SST for May 26, 1996, which shows this northward displacement of isotherms associated with the Gulf Stream. Orange and red in the figure represent warm temperatures (25°C - 30°C) while green and blue represent cool temperatures (15°C - 20°C). The Gulf Stream is responsible for carrying tremendous amounts of heat from south to north, and is shown in the next image as a well-defined, red “river” of water flowing northward along the western boundary of the North Atlantic Ocean.