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Oceans
Energy reaches the surface of the earth in larger amounts in the tropical
and subtropical regions (see Figure 4).
This leads to a net flow of energy in the atmosphere
and ocean away from the tropical regions and toward the poles. Ocean water is
heated in tropical regions and moves toward the polar regions in currents such
as the Gulf Stream off the east coast of the U.S, transporting large amounts of
heat from low latitudes to high latitudes. If it weren't for the Gulf Stream,
Scandinavia would be about 10°C colder than it is right now. In
the atmosphere also, energy moves poleward by means of the global circulation
cycle. A subtle way of transforming energy from low latitudes to high latitudes
is through latent heat: water that evaporates from the warm tropical oceans is
transported as water vapor to higher latitudes. This vapor condenses into liquid
and gives up the amount of energy that was used for evaporation in the tropics.
Polar regions lose more energy to space than they receive from the sun, so the
difference is made up by this transport of energy from low latitudes to high
latitudes.
Oceans cover 71% of the earth's surface and land only 29%, with most (90%) of the land being in the Northern Hemisphere. It is notable that in the Southern Hemisphere polar region, the Antarctic continent forms a nearly circular land mass centered on the pole, while in the Northern Hemisphere, the polar region has no comparable land mass.
A graph of average January air temperature near the surface of the earth (see Figure 5) shows lines of constant temperature, called isotherms, generally follow the lines of constant latitude running east-west, particularly in the Southern Hemisphere. Over continental areas, these lines are displaced southward in both hemispheres. At this time, the Southern Hemisphere is having its summer, and the land is warmer than the ocean at the same latitude because the land absorbs more energy than the water. This agrees with our experience that you can cool off during a warm summer day by going to the ocean. In the Northern Hemisphere, of course, January corresponds to winter, and the southward displacement of the isotherms means that the land is colder than the water at the same latitude.
Average July temperatures (see Figure 6) show isotherms being displace northward in both hemispheres in response to the land being warmer than the water at the same latitude in the Northern Hemisphere and cooler than the water in the Southern Hemisphere.
