5.6 Geographic Distribution of Terms in the Heat Budget
Various groups have used ship and satellite data in numerical weather models to calculate globally averaged values of the terms for Earth's heat budget. The values give an overall view of the importance of the various terms (Figure 5.6). Notice that insolation balances infrared radiation at the top of the atmosphere. At the surface, latent heat flux and net infrared radiation tend to balance insolation, and sensible heat flux is small.
Note that only 20% of insolation reaching Earth is absorbed directly by the atmosphere while 49% is absorbed by the ocean and land. What then warms the atmosphere and drives the atmospheric circulation shown in Figure 4.3? The answer is rain and infrared radiation from the ocean absorbed by the moist tropical atmosphere. Here's what happens. Sunlight warms the tropical oceans which must evaporate water to keep from warming up. The ocean also radiates heat to the atmosphere, but the net radiation term is smaller than the evaporative term. Trade-winds carry the heat in the form of water vapor to the tropical convergence zone where it falls as rain. Rain releases the latent heat evaporated from the sea, and it heats the air in cumulus rain clouds by as much as 500 W/m2 averaged over a year (See Figure 14.1).
At first it may seem strange that rain heats the air. After all, we are familiar with summertime thunderstorms cooling the air at ground level. The cool air from thunderstorms is due to downdrafts. Higher in the cumulus cloud, heat released by rain warms the mid-levels of the atmosphere causing air to rise rapidly in the storm. Thunderstorms are large heat engines converting the energy of latent heat into kinetic energy of winds.
The zonal average of the oceanic heat-budget terms (Figure 5.7) shows that insolation is greatest in the tropics, that evaporation balances insolation, and that sensible heat flux is small. Zonal average is an average along lines of constant latitude. Note that the terms in Figure 5.7 don't sum to zero. The areal-weighted integral of the curve for total heat flux is not zero. Because the net heat flux into the oceans averaged over several years must be less than a few watts per square meter, the non-zero value must be due to errors in the various terms in the heat budget.
Errors in the heat budget terms can be reduced by using additional information. For example, we know roughly how much heat and fresh water are transported by the oceans and atmosphere, and the known values for the transports can be used to constrain the calculations of net heat fluxes (Figure 5.7). The constrained fluxes show that the heat gained by the ocean in the tropics is balanced by heat lost by the ocean at high latitudes.
Maps of the regional distribution of fluxes give clues to the processes producing the fluxes. Clouds regulate the amount of sunlight reaching the sea surface (Figure 5.8 top), and solar heating is everywhere positive. The net infrared heat flux (Figure 5.8 bottom) is largest in regions with the least clouds, such as the center of the oceans and the eastern central Pacific. The net infrared flux is everywhere negative. Latent heat fluxes (Figure 5.9) are dominated by evaportion in the trade-wind regions and the offshore flow of cold air masses behind cold fronts in winter offshore of Japan and North America. Sensible heat fluxes (Figure 5.10 top) are dominated by cold air blowing o continents. The net heating gain (Figure 5.10 bottom) is largest in equatorial regions and net heat loss is largest downwind on Asia and North America.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, email@example.com
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Updated on September 8, 2008