Geoscience Reference
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ENSO events) is the most striking (see
Chapter11G.1):
(A)
1 High phase ( Figure 7.22A ). This features four
major zonal cells involving rising low pressure
limbs and accentuated precipitation over
Amazonia, central Africa and Indonesia/India;
and subsiding high pressure limbs and
decreased precipitation over the eastern
Pacific, South Atlantic and western Indian
Ocean. During this phase, low-level easterlies
strengthen over the Pacific and subtropical
westerly jet streams in both hemispheres
weaken, as does the Pacific Hadley cell.
2 Low phase ( Figure 7.22B ). This phase has five
major zonal cells involving rising low pressure
limbs and accentuated precipitation over the
South Atlantic, the western Indian Ocean, the
western Pacific and the eastern Pacific; and
subsiding high pressure limbs and decreased
precipitation over Amazonia, central Africa,
Indonesia/India and the central Pacific. During
this phase, low-level westerlies and high-level
easterlies dominate over the Pacific, and sub-
tropical westerly jet streams in both hemi-
spheres intensify, as does the Pacific Hadley
cell.
(B)
(C)
(D)
Figure 7.23 The index cycle. A schematic
illustration of the development of cellular patterns
in the upper westerlies, usually occupying three
to eight weeks and being especially active in
February and March in the Northern Hemisphere.
Statistical studies indicate no regular periodicity in
this sequence. A: High zonal index. The jet stream
and the westerlies lie north of their mean position.
The westerlies are strong, pressure systems have
a dominantly east-west orientation, and there is
little north-south air-mass exchange. B and C: The
jet expands and increases in velocity, undulating
with increasingly larger oscillations. D: low zonal
index. The latter is associated with a complete
breakup and cellular fragmentation of the zonal
westerlies, formation of stationary deep occluding
cold depressions in lower mid-latitudes and deep
warm blocking anticyclones at higher latitudes.
This fragmentation commonly begins in the east
and extends westward at a rate of about 60° of
longitude per week.
Source: After Namias; from Haltiner and Martin (1957).
2 Variations in the circulation of
the Northern Hemisphere
The pressure and contour patterns during certain
periods of the year may be radically different
from those indicated by the mean maps (see
Figure 7.4 ). Several kinds of variability are of
special importance. On the largest scale are
changes in the strength of the hemispheric zonal
westerly circulation over a period of weeks.
Important variations on more regional scales
include oscillations in pressure over the North
Atlantic and North Pacific.
Zonal index variations
Variations of three to eight weeks' duration are
observed in the strength of the zonal westerlies,
 
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