Geoscience Reference
In-Depth Information
continents assists the northward displacement of
the equatorial trough (see Figure 11.1). Over Africa, the
westerlies reach 2 to 3 km and over the Indian Ocean 5
to 6 km. In Asia, these winds are known as the 'Summer
Monsoon', but this is now recognized to be a complex
phenomenon, the cause of which is partly global and
partly regional in origin (see Chapter 11C). The equa-
torial westerlies are not simply trades of the opposite
hemisphere that recurve (due to the changed direction
of the Coriolis deflection) on crossing the equator. There
is
on average
a westerly component in the Indian Ocean
at 2 to 3°S in June and July and at 2 to 3°N in December
and January. Over the Pacific and Atlantic Oceans, the
ITCZ does not shift sufficiently far from the equator to
permit the development of this westerly wind belt.
high, as has already been pointed out, is by no means
a quasi-permanent feature of the Arctic circulation.
Easterly winds occur mainly on the poleward sides of
depressions over the North Atlantic and North Pacific
(Figure 7.12). If average wind directions are calculated
for entire high-latitude belts there is found to be little
sign of a coherent system of polar easterlies. The situa-
tion in high latitudes of the southern hemisphere is
complicated by the presence of Antarctica, but anti-
cyclones appear to be frequent over the high plateau
of eastern Antarctica, and easterly winds prevail over
the Indian Ocean sector of the Antarctic coastline.
For example, in 1902 to 1903 the expedition ship
Gauss
, at 66°S, 90°E, observed winds between north-
east and south-east for 70 per cent of the time, and at
many coastal stations the constancy of easterlies
may be compared with that of the trades. However,
westerly components predominate over the seas off west
Antarctica.
3 The mid-latitude (Ferrel) westerlies
These are the winds of the mid-latitudes emanating from
the poleward sides of the subtropical high-pressure cell
(see Figure 7.12). They are far more variable than the
trades in both direction and intensity, for in these regions
the path of air movement is frequently affected by cells
of low and high pressure, which travel generally east-
ward within the basic flow. In addition, in the northern
hemisphere the preponderance of land areas with their
irregular relief and changing seasonal pressure patterns
tend to obscure the generally westerly airflow. The Isles
of Scilly, off southwest England, lying in the south-
westerlies, record 46 per cent of winds from between
south-west and north-west, but fully 29 per cent from the
opposite sector, between north-east and south-east.
The westerlies of the southern hemisphere are
stronger and more constant in direction than those of
the northern hemisphere because the broad expanses
of ocean rule out the development of stationary pressure
systems (Figure 7.15). Kerguelen Island (49°S, 70°E)
has an annual frequency of 81 per cent of winds from
between south-west and north-west, and the comparable
figure of 75 per cent for Macquarie Island (54°S, 159°E)
shows that this predominance is widespread over the
southern oceans. However, the apparent zonality of
the southern circumpolar vortex (see Figure 7.10) con-
ceals considerable synoptic variability of wind velocity.
C THE GENERAL CIRCULATION
We next consider the mechanisms maintaining the
general circulation
of the atmosphere - the large-scale
patterns of wind and pressure that persist throughout the
year or recur seasonally. Reference has already been
made to one of the primary driving forces, the imbalance
of radiation between lower and higher latitudes (see
Figure 3.25), but it is also important to appreciate the
significance of energy transfers in the atmosphere.
Energy is continually undergoing changes of form, as
shown schematically in Figure 7.16. Unequal heating
of the earth and its atmosphere by solar radiation
generates potential energy, some of which is converted
into kinetic energy by the rising of warm air and the
sinking of cold air. Ultimately, the kinetic energy
of atmospheric motion on all scales is dissipated by
friction and small-scale turbulent eddies (i.e. internal
viscosity). In order to maintain the general circulation,
the rate of generation of kinetic energy must obviously
balance its rate of dissipation. These rates are estimated
to be about 2 W m
-2
, which amounts to only 1 per cent
of the average global solar radiation absorbed at the
surface and in the atmosphere. In other words, the
atmosphere is a highly inefficient heat engine (see
Chapter 3E).
A second controlling factor is the angular
momentum of the earth and its atmosphere. This is the
4 The polar easterlies
This term is applied to winds that occur between
polar high pressure and subpolar low pressure. The polar
