362
The value of eL is now very sensitive to f..L (see Fig. 7c); if the zonal
heat transport efficiency is roughly equal to or greater than the meridional and radiative ones, the second term in the denominator of eq.
(67) dominates, and eL is proportional to e and very small as well.
Consequently, the Newtonian damping is very strong. If, however,
f..L/e is much smaller than B, eL is equal to (2X + B)/ B, and the
Newtonian damping coefficient in eq. (62) is just B, which means
very weak damping. Physically, these two limiting cases can be understood as follows. If zonal mixing is very efficient, even a small
ocean basin dominates the zonal mean temperatures, and a change in
ocean temperature gradient leads to a proportional change in meridional heat transport and longwave radiation. These will not balance
over land, and the residual must go into or come out of the ocean.
But land has a much greater area, and so the change in surface heat
flux is amplified by c. If, on the other hand, zonal mixing is extremely
weak and the ocean basin small, a change in ocean temperature has
no effect on either meridional heat transport or land temperature, so
the only response is changed local longwave radiation.
The ocean area ratio causes two competing effects on the efficiency of
the Newtonian damping. A smaller ocean experiences stronger surface
heat flux variations due to imbalances in the heat budget over land than a
larger ocean, tantamount to stronger Newtonian damping. Simultaneously,
a smaller ocean has less influence on the zonal mean temperature gradient
and hence on the atmospheric heat budget, meaning weaker Newtonian
damping. Which effect dominates depends on the efficiency of zonal heat
transport; Fig. 7c shows that for roughly isotropic atmospheric transport
efficiency, the effective area ratio is quite insensitive to the actual area ratio
over a wide parameter range.
The effect of finite zonal homogenization on the moisture transport and
the associated feedback #4 is more difficult to answer with this simple
model because the ocean-to-catchment area ratio ew enters as another
free parameter (see Section 2.3). A small ocean basin influences the zonal
mean temperature gradient and hence the meridional moisture transport
only weakly. If the ocean receives no river runoff (ew = 1), this means
that feedback #4 is weak as well. If, however, river runoff is substantial
(ew = e, in the extreme case), the same intensification of surface freshwater
flux occurs as for surface heat flux, and even a small change in atmospheric
The value of eL is now very sensitive to f..L (see Fig. 7c); if the zonal
heat transport efficiency is roughly equal to or greater than the meridional and radiative ones, the second term in the denominator of eq.
(67) dominates, and eL is proportional to e and very small as well.
Consequently, the Newtonian damping is very strong. If, however,
f..L/e is much smaller than B, eL is equal to (2X + B)/ B, and the
Newtonian damping coefficient in eq. (62) is just B, which means
very weak damping. Physically, these two limiting cases can be understood as follows. If zonal mixing is very efficient, even a small
ocean basin dominates the zonal mean temperatures, and a change in
ocean temperature gradient leads to a proportional change in meridional heat transport and longwave radiation. These will not balance
over land, and the residual must go into or come out of the ocean.
But land has a much greater area, and so the change in surface heat
flux is amplified by c. If, on the other hand, zonal mixing is extremely
weak and the ocean basin small, a change in ocean temperature has
no effect on either meridional heat transport or land temperature, so
the only response is changed local longwave radiation.
The ocean area ratio causes two competing effects on the efficiency of
the Newtonian damping. A smaller ocean experiences stronger surface
heat flux variations due to imbalances in the heat budget over land than a
larger ocean, tantamount to stronger Newtonian damping. Simultaneously,
a smaller ocean has less influence on the zonal mean temperature gradient
and hence on the atmospheric heat budget, meaning weaker Newtonian
damping. Which effect dominates depends on the efficiency of zonal heat
transport; Fig. 7c shows that for roughly isotropic atmospheric transport
efficiency, the effective area ratio is quite insensitive to the actual area ratio
over a wide parameter range.
The effect of finite zonal homogenization on the moisture transport and
the associated feedback #4 is more difficult to answer with this simple
model because the ocean-to-catchment area ratio ew enters as another
free parameter (see Section 2.3). A small ocean basin influences the zonal
mean temperature gradient and hence the meridional moisture transport
only weakly. If the ocean receives no river runoff (ew = 1), this means
that feedback #4 is weak as well. If, however, river runoff is substantial
(ew = e, in the extreme case), the same intensification of surface freshwater
flux occurs as for surface heat flux, and even a small change in atmospheric