The amount of energy per unit area and per unit time arriving at or passing through a certain surface is called energy flux density. In the context of climate, we mostly discuss energy flux density with respect to a horizontal surface, i.e. a surface parallel to the surface of the earth. The curve `a' in Fig. 4.1-2 shows energy flux density of solar radiation incoming to `the top of the atmosphere' at each latitude, averaged over all seasons and all longitudes. The curve `b' shows the part of it that is absorbed by the earth (as a whole, consisting of the atmosphere including clouds, the ocean and the land). Averaged over the earth, 30 % of incoming solar radiation is reflected and 70 % is absorbed. The curve `c' shows energy flux density of terrestrial radiation that the earth emits to space. Both `b' and `c' show larger values in lower latitudes and smaller values in higher latitudes, but the disparity among latitudes is much larger in `b'. Consequently, absorbed solar radiation is larger than emitted terrestrial radiation in low latitudes, and the opposite in high latitudes. The difference between `b' and `c' is shown as `d'. The average of `d' with weights proportional to surface area of latitude bands is almost zero.
Though the climate has some interannual variability, the annual mean state of energy can roughly be regarded as a steady state at each latitudes. Therefore, energy is transported to higher latitude by the fluid motion in the atmosphere and in the ocean to compensate for the surplus and deficiency shown in `d'. Fig. 4.1-3 shows the amount of meridional (north-south) transport of energy (positive values mean northward transport). The curve `a' is for the total, `b' is for the atmosphere, and `c' is for the ocean. The contributions of the two components to the energy transport have similar magnitudes.
Fig. 4.1-4 shows daily mean energy flux density of incoming solar radiation at `the top of the atmosphere' like a two-dimensional contour map, taking season as the horizontal axis and latitude as the vertical axis. The values in the polar regions in winter is zero because solar radiation does not reach there. In the season around the summer solstice, the amount of daily mean energy flux density is larger in high latitudes than in equatorial regions, due to longer duration of daytime, despite of not so high solar elevation (the angle of the sun above the horizon). We may naturally imagine, from this fact combined with the discussion of convection (see below), such vertical and meridional (=north-south) circulation as being upward in the summer hemisphere, downward in the winter hemisphere, which reverses the direction in the opposite seasons. The circulation in the middle atmosphere (stratosphere and mesosphere) is actually like this. But the vertical and meridional circulation of the lower atmosphere (troposphere) as we directly know is basically the circulation between the low latitudes (equatorial region) and the high latitudes (polar region), though the strength and the position somewhat vary according to seasons. The main cause of this structure is due to the ocean which diminishes the difference of temperature between summer and winter by storing surplus energy in summer and releasing it in winter.