Geoscience Reference
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scale zonal (i.e. east-west) components of tropical
airflow. These zonal circulations are driven by major
east-west pressure gradients that are set up by differ-
ences in vertical motion. On one hand, air rises over
heated continents and the warmer parts of the oceans
and, on the other, air subsides over cooler parts of the
oceans, over continental areas where deep high-pressure
systems have become established, and in association
with subtropical high-pressure cells. Sir Gilbert Walker
first identified these circulations in 1922 to 1923 through
his discovery of an inverse correlation between pres-
sure over the eastern Pacific Ocean and Indonesia. The
strength and phase of this so-called Southern Oscillation
is commonly measured by the pressure difference
between Tahiti (18°S, 150°W) and Darwin, Australia
(12°S, 130°E). The Southern Oscillation Index (SOI)
has two extreme phases (Figure 7.22):
central Africa, Indonesia/India and the central
Pacific. During this phase, low-level westerlies and
high-level easterlies dominate over the Pacific, and
subtropical westerly jet streams in both hemispheres
intensify, as does the Pacific Hadley cell.
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 indi-
cated by the mean maps (see Figures 7.3 and 7.4). Two
distinct kinds of variability are of especial importance.
One involves the zonal westerly circulation on a scale
of weeks and the other north-south oscillations in
pressure over the North Atlantic creating interannual
differences in climate.
positive when there is a strong high pressure in the
southeast Pacific and a low centred over Indonesia
with ascending air and convective precipitation;
negative (or low) when the area of low pressure and
convection is displaced eastward towards the Date
Line.
a Zonal index variations
Variations of three to eight weeks' duration are observed
in the strength of the zonal westerlies, 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° 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 (see Plate 15). A rela-
tively 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 700-mb
contour patterns and zonal wind speed profiles for two
contrasting months. In December 1957, the westerlies
were stronger than normal north of 40°N, and the
troughs and ridges were weakly developed, whereas
in February 1958 there was a low zonal index and an
expanded circumpolar vortex, giving rise to strong low-
latitude westerlies. The 700-mb pattern shows very
weak subtropical highs, deep meridional troughs and a
blocking anticyclone off Alaska (see Figure 7.25A). The
cause of these variations is still uncertain, although it
would appear that fast zonal flow is unstable and tends
Positive (negative) SOI implies strong easterly trade
winds (low-level equatorial westerlies) over the central-
western Pacific. These Walker circulations are subject
to fluctuations in which an oscillation (El Niño-
Southern Oscillation: ENSO) between high phases (i.e.
non-ENSO events) and low phases (i.e. ENSO events)
is the most striking (see Chapter 11G.1):
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,
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