88 Modelling theclimate
mid latitudes are associated with these El Ni ̃no events (see Figure 1.4).
A good test of the atmospheric models described above is to run them
with an El Ni ̃no sequence of sea surface temperatures and see whether
they are able to simulate these climate anomalies. This has now been
done with a number of different atmospheric models; they have shown
considerable skill in the simulation of many of the observed anomalies,
especially those in the tropics and sub-tropics.^6
Because of the large heat capacity of the oceans, anomalies of ocean
surface temperature tend to persist for some months. The possibility
therefore exists, for regions where there is a strong correlation between
weather and patterns of ocean surface temperature, of making forecasts
of climate (or average weather) some weeks or months in advance. Such
seasonal forecasts have been attempted especially for regions with low
rainfall; forinstance, for north east Brazil and for the Sahel region of
sub-Saharan Africa, a region where human survival is very dependent
on the marginal rainfall (see box below).
The ability to make accurate seasonal forecasts significantly in ad-
vance depends critically on being able to forecast changes in ocean
surface temperature. To do that requires understanding of, and the abil-
ity to model, the ocean circulation and the way it is coupled to the
atmospheric circulation. Because the largest changes in ocean surface
temperature occur in the tropics and because there are reasons to suppose
that the ocean may be more predictable in the tropics than elsewhere,
most emphasis on the prediction of ocean surface temperature has been
placed in tropical regions, in particular on the prediction of the El Ni ̃no
events themselves (see box below).
Later on in this chapter the coupling of atmospheric models and
ocean models is described. For the moment it will suffice to say that, using
coupled models together with detailed observations of both atmosphere
and ocean in the Pacific region, significant skill in the prediction of
El Ni ̃no events up to a year in advance has been achieved^7 (see also
Chapter 7).The climate system
So far the forecasting of detailed weather over a few days and of average
weather for a month or so, up to perhaps a season ahead, has been de-
scribed in order to introduce the science and technology of modelling,
and also because some of the scientific confidence in the more elabo-
rate climate models arises from their ability to describe and forecast the
processes involved in day-to-day weather.
Climate is concerned with substantially longer periods of time, from
a few years to perhaps a decade or longer. A description of the climate