Environmental Engineering Reference
In-Depth Information
Figure 2
Time series of the Southern Oscillation Index from 1950 to
1999. Positive values indicate El Niño conditions and negative
values indicate La Niña.
Increasingly climatologists have begun to realize that anomalies in some parts of the
globe can exert an influence on other parts. These effects have been termed
atmospheric
teleconnections
.
In 1999 the reverse state developed (termed La Niña), with colder than average sea
surface temperatures in the eastern Pacific. This produced further anomalies of
temperature and precipitation but in different areas and of different magnitudes. Much of
Australia had above-average precipitation, with water partially filling Lake Eyre, and
much of southern Africa was wet, including the severe floods in Mozambique referred to
earlier.
THE INDEX CYCLE
It has been argued that the Rossby waves do alter their amplitude and wavelength in a
roughly cyclical manner over a period of between three and eight weeks. If we follow the
pattern of waves over a period of weeks the waves often undergo change, as shown in
Figure 6.18. Initially there may be little meridional element in the flow, with a strong
westerly circulation - termed a high index flow. Gradually the north-south element
becomes more dominant until eventually the flow breaks down into a series of cut-off
troughs and ridges like the meandering pattern of a river. This is termed a low index flow,
in which north-south exchanges are strong. Low index flow favours the formation of
blocking highs and cut-off lows (Chapter 7), which have a marked effect on the weather
in some parts of the westerlies of the northern hemisphere.
KEY POINTS
1
Movement of air in the atmosphere is determined by the pressure gradient force and
modified by the Coriolis force and, near the ground, by friction.
2
Because of Earth's size and rate of rotation, and the energy imbalance caused by
astronomic factors, the large-scale circulation of the globe splits into a series of
systems with easterlies in the tropics flowing towards the equatorial trough and
