Poleward Energy Transport by the General Circulation

Atmospheric motions are fundamentally caused by an excess of energy near the equator and an energy deficit near the poles. This gradient in energy is reflected in temperature and moisture (warm and moist tropics, cold and dry polar regions). The atmospheric circulation exists physically as a mechanism to limit these gradients in spite of continual forcing. Figure 8.2 shows the zonally averaged northward energy transport for the globe. The maximum energy transport poleward in both hemispheres by the atmosphere occurs in midlatitudes at about 40°. This reflects the mixing action of midlatitude cyclonic (frontal) systems discussed in the previous section. Ocean transport of energy is of smaller magnitude with maxima in the sub-tropics.161 A basic property of the overall atmospheric flow is that it is “chaotic” and therefore unpredictable beyond a week or two.171

Poleward energy transport by the general circulation by atmosphere, ocean, and the sum of the two. Source

FIGURE 8.2 Poleward energy transport by the general circulation by atmosphere, ocean, and the sum of the two. Source: Modified from Trenberth & CaronJ61

Role of the Hydrological Cycle in the General Circulation

The fundamental role of the water molecule in the general circulation cannot be overstated. While H20 in all its phases is universally recognized as fundamental for the existence of life in general and human activities such as agriculture in particular, from the perspective of the atmospheric circulation water substance plays a central role in major energy transport mechanisms. This is expressed by the latent heating due to phase changes. Energy is needed to evaporate water from a wet land surface or ocean, and this energy is later released in the upper atmosphere if enough cooling occurs to cause condensation, resulting in a net upward energy transport. The radiative properties of the atmosphere dictate that the atmosphere is mostly heated from below with energy transfer from the Earth’s surface to the lower atmosphere. Approximately three quarters of the net radiation arriving at the Earth’s surface (Figure 8.3) is transferred back to the atmosphere as latent heat (energy transfer due to the phase changes of H,0 that would dominant over ocean areas) as opposed to sensible heat (direct heating of the lower atmosphere) and infrared radiation.

Figure 8.4 shows the partitioning of atmospheric heating by process, altitude, and latitude and indicates that latent heating (third panel from top) is the major process supplying energy to the atmosphere. Compared to sensible heating (fourth panel from top and labeled boundary layer heating), latent heating is of considerably larger magnitude and is realized through a much greater depth of the atmosphere. Net radiative heating indicates mostly a cooling effect due to radiative losses to space. The top panel, total diabatic heating, represents the sum of the other three panels.

The role of the hydrological cycle in the general circulation of the future is of immense interest because three of the largest positive feedback mechanisms (processes that add to the magnitude of the original forcing) to warming due to increased C02 involve the H,0 molecule, one feedback for each of its three phases. The water vapor feedback (increased atmospheric water vapor content due to increased temperature and therefore increased saturation point) is responsible for the largest part of the simulated global warming signal.110111 Increased cloudiness due to the increased water vapor is also thought to amplify global warming, though the precise magnitude remains poorly known because the net response is a function of the exact composition and altitude of changing clouds. The ice albedo feedback, where melting ice leads to decreased reflection of solar energy and therefore increased energy absorption, is also recognized as a positive feedback to global warming.114

Annual average surface energy balance over the lobe as a whole indication the proportion of net radiation

FIGURE 8.3 Annual average surface energy balance over the lobe as a whole indication the proportion of net radiation (R) returned to the atmosphere as sensible heating (SH), latent heating (LH) by latitude. ДFco represents energy transferred horizontally out of the land ocean below surface column.

Source: From Hartmann.1®1

Comparison by altitude and latitude of

FIGURE 8.4 Comparison by altitude and latitude of: (a) total diabatic heating due to radiation + sensible + latent heating, (b) net radiative heating, (c) latent heat, and (d) sensible heat.

Source: From Peixoto 8t Oort.19'

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