361
The temperature gradient over land is now greater than TE , an unexpected result, which is understood as follows. With no ocean at all,
the zonal mean temperature gradient must be TE so that locally the
meridional atmospheric transport matches the radiative budget. Assume now that TL were equal to TE , despite an ocean with temperature
gradient T < TE . The zonal mean temperature gradient would then
be less than TE , and hence meridional heat transport over land too
low to achieve energy balance. Consequently, the land temperature
gradient must be larger than TE , without, however, raising the zonal
mean temperature gradient to TE (Fig. 7b). Fig. 7a illustrates that
with no zonal transport, greater E leads to more extreme gradients
over land. For finite fL, this effect is counteracted by the moderating
influence the ocean has on land temperatures, which is stronger for
greater E.
Equation (63) for the effective area ratio gives in this limit
EL = {B + 2X(1- E)}/B,
(66)
reflecting that land temperatures and hence the meridional heat transport respond only weakly to a change in the temperature gradient in
a small ocean basin. Anomalies are damped away only slightly more
rapidly than on the radiative timescale.
3. Ocean-Covered Planet, E --+ 1:
Now we have TL --+ T, EL --+ E and land plays no role. Fig. 7a indicates
that if fL goes to zero also, some care must be taken. Since the physical
picture is clear, we will ignore the question of properly defining the
simultaneous limit.
4. Very Small Ocean, E « 1:
We obtain for the effective ocean area ratio from eq. (63)
(67)