Geoscience Reference
In-Depth Information
(A)
(A)
L
880
880
880
L
(B)
(B)
70
70
60
60
50
50
40
40
30
30
20
-4
20
-4
-2
0
2
4 6
Wind speed (m s
-1
)
8
10
12
14
16
-2
0
2
4 6
Wind speed (m s
-1
)
8
10
12
14
16
Figure 7.24
Above: Mean 700mb contours (in
tens of feet) for December 1957, showing a fast,
westerly, small-amplitude flow typical of a high
zonal index. Below: Mean 700mb zonal wind speed
profiles (m s
-1
) in the Western Hemisphere for
December 1957, compared with those of a normal
December. The westerly winds were stronger than
normal and displaced to the north.
Source: After Dunn (1957).
Figure 7.25
Above: Mean 700mb contours (in
tens of feet) for February 1958. Below: Mean
700mb zonal wind speed profiles (m s
-1
) in the
Western Hemisphere for February 1958,
compared with those of a normal February. The
westerly winds were stronger than normal at low
latitudes, with a peak at about 33°N.
Source: After Klein (1958).
A relatively low index may also occur if the
westerlies are well south of their usual latitudes
and, paradoxically, such expansion of the zonal
circulation pattern is associated with strong
westerlies in lower latitudes than usual.
Figures
7.24
and
7.25
illustrate the mean 700mb contour
patterns and zonal wind speed profiles for two
contrasting months. In December 1957 the west-
erlies were stronger than normal north of 40
averaged around the hemisphere. They are rather
more noticeable in the winter months, when the
general circulation is strongest. The nature of the
changes is illustrated schematically in
Figure 7.23
.
The mid-latitude westerlies develop waves, and
the troughs and ridges become accentuated,
ultimately splitting up into a cellular pattern with
pronounced meridional flow at certain longitudes.
The strength of the westerlies between 35
N,
and the troughs and ridges were weakly devel-
oped, whereas in February 1958 there was a low
zonal index and an expanded circumpolar vortex,
giving rise to strong low-latitude westerlies. The
°
and
55°N is termed the
zonal index
; strong zonal
westerlies are representative of a high index, and
marked cellular patterns occur with a low index.
°