The Earth's Climate Machine
Wind and Ocean Currents: “Heat Movers”
Both air currents and ocean currents move heat. Energy is stored as "latent heat" in the atmosphere. In the surface waters of the ocean it is stored as "sensible heat." Atmospheric and oceanic circulation share the task of heat redistribution on a roughly fifty-fifty basis. Even though transfer of heat by moving water in ocean currents is much less efficient than transfer by moving vapor in wind, the masses involved are much greater in the ocean currents than in the air currents.
On the whole, heat has to be moved from the tropics (where the Sun is close to being overhead at noon) to the higher latitudes (where the Sun is 40 degrees off the vertical or lower, on average). These regions, beyond the 40th degree of latitude on either side of the equator, have a heat deficit, while the tropical belt has excess heat. "Heat deficit" refers to the fact that from these regions more heat is radiated back to space than is received from the Sun, while for the regions with a "heat excess" the reverse is the case. The climate machine acts as a heat-distributing device; It involves the energy received from the Sun (see "solar constant"), the energy reflected back to space (see "albedo") and the energy retained in the atmosphere through the trapping of infrared radiation (see "greenhouse effect"). If there were no energy transfer the poles would be 25 degrees Celsius cooler, and the equator 14 degrees Celsius warmer!
“Sensible” Versus ”Latent” Heat
As mentioned, the redistribution of energy across the Earth's surface is accomplished by "sensible heat flux" and as "latent heat flux." Sensible heat is the energy associated with the temperature of a body; it is greater in a warm body than a cold one. Warm water that is heated in the tropics and cooled in high latitudes brings sensible heat poleward. Warm winds carry sensible heat from one place to another. When picking up water along the way (for example from the sea surface), the winds can change some of the sensible heat into latent heat. Latent heat is the energy associated with changing the "phase" of a substance, that is, changing the state from gas to fluid or from fluid to solid, or the reverse. The enormous amounts of latent heat involved in evaporation and precipitation of water makes the hydrologic cycle a central player in the operation of the climate machine.
Diagram illustrating latent heat flux
. Latent heat flux
is the global movement of latent heat energy through circulations of air and water. Here we illustrate how atmospheric circulation moves latent heat energy horizontally to cooler locations where it is condensed as rain or is deposited as snow releasing the heat energy stored within it.
Ocean Circulation and Heat Distribution
Because air currents and ocean currents work together to redistribute heat, the general circulation of the atmosphere is closely related to the general circulation of the ocean. Winds blowing over the sea surface produce ocean currents. Winds also evaporate water, which precipitates elsewhere as rain. A large amount of heat is transferred to the atmosphere in the process of condensation, because of the latent heat of vaporization. Evaporation and precipitation also affect the patterns of water density on the surface of the ocean, which are inseparably tied to ocean circulation. Quite generally, the ocean surface is a source for heat in the atmosphere. The sea surface tends to stay at the same temperature for a long time, because of the high specific heat off water and the thickness of the mixed layer. Thus, the ocean surface stabilizes temperature gradients that, in turn, are responsible for much of the atmospheric flow patterns. Without this stabilizing influence of the ocean, the weather presumably would be much more fickle.
Whether the ocean invariably stabilizes the climate, though, must be doubted. Sea ice expands upon cooling surface waters below the freezing point — and this reflects more sunlight. A warming ocean gives off carbon dioxide, favoring further warming. Both of these mechanisms are "positive feedback" within the climate machine. Positive feedback (a change of the system which favors further change in the same direction) destabilizes conditions and can lead to "run-away" situations. The most difficult task in understanding the workings of the climate machine is to get the positive and negative feedback mechanisms right. You can judge this for yourself - try to figure out the figure below.