Environmental Engineering Reference
In-Depth Information
have come to dominate mid latitudes, and have
even replaced the simple thermal cell of polar
latitudes (Barry and Chorley 1992).
The flow pattern adopted by the Rossby waves
is quite variable, and that variability makes a
major contribution to energy transfer. When the
westerlies follow a latitudinal path, from west
to east they are said to be zonal, and the strength
of the latitudinal flow is indicated by the zonal
index. If the westerly flow is strong, the zonal
index is high. In contrast, as the amplitude of
the Rossby waves increases, the flow becomes
less zonal and more meridional (i.e. it follows a
north-south or longitudinal path). The zonal
index is then said to be low. Changes in the wave
patterns occur as indicated in Figure 2.11 and
the entire sequence from high-zonal index,
through low-zonal index and back to high is
called the index cycle. It has an important role in
the atmospheric energy exchange process. As the
amplitude of the Rossby waves increases, and
the westerlies loop southwards, they carry cold
air into lower latitudes. Conversely, as they loop
northwards they introduce warmer air into higher
latitudes. These loops are often short-circuited,
leaving pools of abnormally cool air in lower
latitudes and abnormally warm air in high
latitudes. The net result is significant latitudinal
energy transfer. Such developments are not
completely random, but neither are they
predictable. The cycle occurs over a period of 3
to 8 weeks, and is repeated with some frequency,
yet it lacks the regularity necessary for
forecasting.
The jet streams
The modification of the three-cell model in the
1940s and 1950s was made possible in large
part by improved knowledge of conditions in
the upper atmosphere. The upper atmospheric
circulation is quite complex in detail, but in
general terms it is characterized by an easterly
flow in the tropics and a westerly flow
associated with the Rossby waves in mid to high
latitudes. Within these broad airflows, there are
relatively narrow bands of rapidly moving air
called jet streams, in which wind speeds average
125-130 km per hour in places. The jet streams
are usually located at the tropopause, and are
associated with zones in which steep
temperature gradients exist, and where, in
consequence, the pressure gradient is also steep.
The most persistent jets are found in the sub-
Figure 2.12
Schematic diagram of the vertical circulation of the atmosphere and the location of the major jet
streams in the northern hemisphere
