Global Warming

The climate system 89

A simple model of the El Ni ̃no

El Ni ̃no events are good examples of the strong

coupling which occurs between the circulations of

ocean and atmosphere. The stress exerted by atmo-

spheric circulation – the wind – on the ocean surface

is a main driver for the ocean circulation. Also, as

we have seen, the heat input to the atmosphere from

the ocean, especially that arising from evaporation,

has a big influence on the atmospheric circulation.

A simple model of an El Ni ̃no event that shows

the effectof different kinds of wavemotions that

can propagate within the ocean is illustrated in Fig-

ure 5.12. In this model a wave in the ocean, known

as a Rossby wave, propagates westwards from a

warm anomaly in ocean surface temperature near

the equator. When it reaches the ocean’s western

boundary it is reflected as adifferent sortof wave,

known as a Kelvin wave, which travels eastward.

This Kelvin wave cancels and reverses the sign of

the original warm anomaly, so triggering a cold

event. The time taken for this half-cycle of the whole

El Ni ̃no process is determined by the speed with

which the waves propagate in the ocean; it takes

about two years. It is essentially driven by ocean dy-

namics, the associated atmospheric changes being

determined by the patterns of ocean surface temper-

ature (and in turn reinforcing those patterns) which

resultfrom the ocean dynamics. Expressedin terms

of this simple model, some of the characteristics of

the El Ni ̃no process appear to be essentially pre-

dictable.

Figure 5.12Schematic to illustrate El Nino oscillation ̃

over a period involves the averages of appropriate components of the
weather (for example, temperature and rainfall) over that period together
with the statistical variations of those components. In considering the
effect of human activities such as the burning of fossil fuels, changes in
climate over periods of decades up to a century or two ahead must be
predicted.