Earth Sciences / 59with mild high-pressure areas in high latitudes and cold low-pressure areas in lower latitudes blocking
the easterly movement.
The unsettled nature of the climate at latitudes around 50–60° is due to the proximity of the
polar front and the vagaries of the jet stream associated with it: surface conditions are being
determined by events at a height of some 10 km. As the front moves north and south, places in
about the latitude of Britain may be exposed alternately to polar and tropical air masses and to
pressure systems that move rapidly or remain stationary for weeks at a time. Should the global
climate change, becoming generally warmer or cooler, there could be a more permanent shift in
the location of the polar front, with quite profound implications for places in this meteorologically
critical region.
Weather comprises more than temperature, of course, and temperature is dependent on more than the
simple convective transfer of heat. Both air temperature and surface weather involve the evaporation,
condensation, and transport of water by large masses of air that have acquired distinct characteristics
over the oceans or continental interiors then moved into different areas, and on reactions between
adjacent air masses with different properties.
The convection-cell model provides only a general, rather crude account of the general
circulation of the atmosphere, by which heat is transported from low to high latitudes. It takes
no account of the transport of heat by the oceans, for example. This is of major importance
and, as the El Niño and NADW-Dryas episodes demonstrate (see the next section), apparently
minor disturbances can produce dramatically different situations. The atmospheric system is
nothing if not dynamic!
16. Oceans, gyres, currents
In the early 1990s, southern Africa experienced its worst drought this century. Nearly 100 million
people went short of food. This was an extreme example of a change that brings abnormal
weather to many parts of the world every few years, associated with a rise in the sea-surface
temperature off the north-western coast of South America. That an apparently minor warming
of the sea, of not much more than 3°C, can have so profound an effect half-way round the world
demonstrates the degree to which our climate is influenced by the oceans. The link between the
temperature change and climate is now so well established that the surface temperature in the
eastern Pacific just south of the equator can be used to predict maize yields in Zimbabwe up to
a year ahead, with an even more advanced early warning of what to expect from the ability
scientists now have to predict the temperature changes themselves up to a further year ahead
(CANE ET AL., 1994).
The circumstances which have such drastic consequences in Africa are called ‘El Niño’ or, to give
the phenomenon its full title, an ‘El Niño-Southern Oscillation event’, or ENSO. Figure 2.20 shows
the parts of the world most seriously affected, but minor effects are felt even further afield.
Northwestern Europe, for example, often has a cool, wet summer following an ENSO. In the areas
most seriously affected, temperatures deviate by up to 0.5°C from normal, but deviations of about
0.2°C occur throughout the northern hemisphere. As well as droughts in various parts of the world,
a recent ENSO event was blamed for unusually warm weather in Alaska, a remarkably warm winter
in the eastern United States, a 100-mm rise in sea level and severe beach erosion in California, the
death of coral reefs in the Pacific, and a variety of diseases ranging from bubonic plague to encephalitis
in the United States!