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circulation features, such as cells of low and high
pressure at the surface or long waves aloft, play a
major role in redistributing momentum and energy.
Laboratory experiments with rotating 'dishpans' of
water to simulate the atmosphere, and computer studies
using numerical models of the atmosphere's behaviour,
demonstrate that a Hadley circulation cannot provide
an adequate mechanism for transporting heat polewards.
In consequence, the meridional temperature gradient
increases and eventually the flow becomes unstable
in the Hadley mode, breaking down into a number of
cyclonic and anticyclonic eddies. This phenomenon
is referred to as baroclinic instability . In energy terms,
the potential energy in the zonal flow is converted
into potential and kinetic energy of eddies. It is also
now known that the kinetic energy of the zonal flow is
derived from the eddies, the reverse of the classical
picture, which viewed the disturbances within the global
wind belts as superimposed detail. The significance
of atmospheric disturbances and the variations of
the circulation are becoming increasingly evident. The
mechanisms of the circulation are, however, greatly
complicated by numerous interactions and feedback
processes , particularly those involving the oceanic
circulation discussed below.
b North Atlantic Oscillation
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
airmass 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).
The relative strength of the Icelandic low and Azores
high was first observed to fluctuate on annual to decadal
scales by Sir Gilbert Walker in the 1920s. Fifty years
later, van Loon and Rogers discussed the related west-
east 'seesaw' in winter temperatures between western
Europe and western Greenland associated with the
north-south change in pressure gradient over the North
Atlantic. The North Atlantic Oscillation (NAO) is a
north-south oscillation in the pressure field between the
Icelandic low (65°N) and the Azores high (40°N). The
relationship between the positive and negative modes
of the NAO noted by Walker, and the associated tem-
perature and other anomaly patterns, are shown in Plate
E. When the two pressure cells are well developed as in
January 1984, the zonal westerlies are strong. Western
Europe has a mild winter, while the intense Icelandic
low gives strong northerly flow in Baffin Bay, low tem-
peratures in western Greenland and extensive sea ice
in the Labrador Sea. In the negative phase the cells
are weak, as in January 1970, and opposite anomalies
are formed. In extreme cases, pressure can be higher
to break down. This tendency is certainly increased
in the northern hemisphere by the arrangement of the
continents and oceans.
Detailed studies are now beginning to show that the
irregular index fluctuations, together with secondary
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