Global Warming

Models for climateprediction 97

Figure 5.17Component

elements and parameters

of a coupled

atmosphere–ocean model

including the exchanges at

the atmosphere–ocean

interface.

ocean which responds to the seasonal heating and cooling at the Earth’s
surface. In such models, adjustments had to be made to allow for the
transport of heat by ocean currents. When running the model with a per-
turbation such as increased carbon dioxide, it was not possible to make
allowance for any changes in that transport which might occur. Such
models therefore possess severe limitations.
For an adequate description of the influence of theocean it is neces-
sary to model the ocean circulation and its coupling to the atmospheric
circulation. Figure 5.17 shows the ingredients of such a model. For the
atmospheric part of the model, in order to accommodate long runs on
available computers, the size of the grid has to be substantially larger,
typically 300 km in the horizontal. Otherwise it is essentially the same
as the global model for weather forecasting described earlier. The for-
mulation of the dynamics and physics of the ocean part of the model
is similar to that of its atmospheric counterpart. The effects of water
vapour are of course peculiar to the atmosphere, but the salinity (the
salt content) of the oceans has to be included as a parameter together
with its considerable effects on the water density. Because dynamical
systems, e.g. largescale eddies in the oceans, are of smaller scale than
their atmospheric counterparts, the grid size of the ocean component is
typically about half that of the atmospheric component. On the other
hand, because changes in the ocean are slower, the time step for model
integration can be greater for the ocean component.
At the ocean–atmosphere interface there is exchange of heat, water
and momentum (exchange of momentum leads tofriction) between the
two fluids. The importance of water in the atmosphere and its influence on
the atmospheric circulation have already been shown. The distribution
of fresh water precipitated from the atmosphere as rain or snow also
has a large influence on the ocean’s circulation through its effect on the