Geoscience Reference
In-Depth Information
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 percent of winds from between
southwest and northwest, and the comparable
figure of 75 percent for Macquarie Island (54
(A)
Winter
8
7
Southern
Hemisphere
Southern
Hemisphere
6
5
4
WEST
WEST
3
°
S,
2
Northern
Hemisphere
Northern
Hemisphere
1
159
E) shows that this predominance is wide-
spread over the southern oceans. However, the
apparent zonality of the southern circumpolar
vortex (see
Figure 7.10
) conceals considerable
synoptic variability of wind velocity.
°
0
-1
EAST
EAST
-2
-3
-4
-5
-6
-
90°
80°
70°
60°
50°
Latitude
40°
30°
20°
10°
EQ
(B)
Summer
4 The polar easterlies
This term is applied to winds that occur between
polar high pressure and subpolar low pressure.
The polar 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
situation in high latitudes of the Southern
Hemisphere is complicated by the presence of
Antarctica, but anticyclones 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
1902-1903 the expedition ship
Gauss
, at 66
10
8
Southern
Hemisphere
Southern
Hemisphere
6
WEST
WEST
4
2
Northern
Hemisphere
Northern
Hemisphere
0
-2
EAST
EAST
-4
-6
90°
80°
70°
60°
50°
Latitude
40°
30°
20°
10°
EQ
Figure 7.15
Profiles of the average west wind
component (m s
-1
) at sea level in the Northern
and Southern Hemispheres during their respective
winter (above) and summer (below) seasons,
1970-1999.
Source: NCEP/NCAR Reanalysis Data from the NOAA-CIRES
Climate Diagnostics Center.
S,
90°E, observed winds between northeast and
southeast for 70 percent 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.
°
primary driving forces, the imbalance of radiation
between lower and higher latitudes (see
Figure
2.26
), 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 gradients in 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
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
