Thermodynamics and the Flow of Heat through the Climate System - Climate Dynamics - page 100

of heat to the surface is the longwave back radiation from the atmosphere
(~333 W/m 2 ). The surface and the lower atmosphere are tightly coupled by this
vigorous exchange of longwave radiation.
The net longwave cooling of the surface is 63 W/m 2 . The latent heat flux
from the surface is about 80 W/m 2 , while the sensible heat flux is estimated at
18 W/m 2 . The Bowen ratio , defined as the ratio of the sensible and latent heat
fluxes, is about 0.23 in the global mean, but the relative values of the turbulent
heat fluxes vary widely from place to place.
In the estimated global heat budget shown in Figure 5.4, the net shortwave
heating of the surface (161 W/m 2 ) is balanced by the net cooling of the surface
(also 161 W/m 2 ). The approximated fluxes at the top of the atmosphere balance
as well. This heat budget represents a system in balance, with no temperature
trend. Changes in the atmosphere's composition perturb this balance. Increases
in atmospheric CO 2 and other greenhouse gases increase the longwave back
radiation and change the balance at the surface. If snow and ice coverage is
reduced as a consequence, the solar radiation absorbed will increase, and there
will be additional repercussions throughout the system. In chapter 11, a simple
climate model based on the surface heat balance is used to estimate tempera-
ture changes due to increasing greenhouse gases.
OBSERVED DISTRIBUTION: TOP OF THE ATMOSPHERE
The heat balance at the top of the atmosphere has only two components, namely,
the shortwave absorbed by the system and the OLR (Eq. 5.8). If the climate sys-
tem is in an equilibrium state, these components must balance in the global and
climatological mean, but they need not balance locally and, in fact, they do not.
The annual mean distribution of solar radiation incident at the top of the
atmosphere ( Fig. 5.5a) is zonally uniform and symmetric about the equator. 1
It decreases by a factor of about 2 from the equator to 70° latitude due to the
curvature of the earth.
The solar radiation absorbed by the earth system ( Fig. 5.5b) has east-west
variations in addition to meridional structure. It varies due to variations in the
planetary albedo (shown in Fig. 5.6 ) that are associated primarily with cloud
and surface albedo distributions. Note that while the incident solar radiation
varies by a factor of about 2 between the equator and high latitudes, the solar
radiation absorbed changes by a factor of 3 or 4. The reason for this difference
is the latitudinal dependence of the planetary albedo.
Two factors cause the strong increase in planetary albedo with latitude. One
is that surface albedos are larger at high latitudes due to the presence of snow
and ice on the surface (see Table 5.1) . Another important factor is the depen-
dence of the albedo of water on the solar zenith angle. When the solar zenith
angle is large, the albedo of water is much larger than the nominal value of
1 This figure, and the figures of heating components that follow, are from the ERA-Interim
reanalysis climatology for 1979-2009. This reanalysis is provided by the European Center for
Medium Range Weather Prediction. The full reference is given in chapter 2 .