Global Warming

86 Modelling theclimate

future? Although we cannot expect to forecast the weather in detail, is

there any possibility of predicting the average weather, say, a few months

ahead? As this section shows, it is possible for some parts of the world,

because of the influence of the distribution of ocean surface temperatures

on the atmosphere’s behaviour. For seasonal forecasting it is no longer

the initial state of the atmosphere about which detailed knowledge is

required. Rather, we need to know the conditions at the surface and how

they might be changing.

In the tropics, the atmosphere is particularly sensitive to sea surface

temperature. This is not surprising because the largest contribution to the

heat input to the atmosphere is due to evaporation of water vapour from

the ocean surface and its subsequent condensation in the atmosphere,

releasing its latent heat. Because the saturation water vapour pressure

increases rapidly with temperature – at higher temperatures more water

can be evaporated beforethe atmosphere is saturated – evaporation from

the surface and hence the heat input to the atmosphere is particularly

large in the tropics.

It is during El Ni ̃no events in the east tropical Pacific (see Figure 5.9)

that the largest variations are found in ocean temperature. Anomalies in

the circulation and rainfall in all tropical regions and to a lesser extent at

Figure 5.9Changes in sea
surface temperature
1871–2002, relative to the
1961–90 average, for the
eastern tropical Pacific off
Peru.