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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 (A) and
summer (B) seasons, 1970 to 1999.
Source : NCEP/NCAR Reanalysis Data from
the NOAA-CIRES Climate Diagnostics
Center.
A
B
tendency for the earth's atmosphere to move, with the
earth, around the axis of rotation. Angular momentum
is proportional to the rate of spin (that is, the angular
velocity) and the square of the distance of the air parcel
from the axis of rotation. With a uniformly rotating earth
and atmosphere, the total angular momentum must
remain constant (in other words, there is a conservation
of angular momentum ). If, therefore, a large mass of air
changes its position on the earth's surface such that its
distance from the axis of rotation also changes, then
its angular velocity must change in a manner so as to
allow the angular momentum to remain constant.
Naturally, absolute angular momentum is high at the
equator (see Note 3) and decreases with latitude to
become zero at the poles (that is, the axis of rotation),
so air moving poleward tends to acquire progressively
higher eastward velocities. For example, air travelling
from 42° to 46° latitude and conserving its angular
momentum would increase its speed relative to the
earth's surface by 29 m s -1 . This is the same principle
that causes an ice skater to spin faster when the arms
are progressively drawn into the body. In practice, the
increase of airmass velocity is countered or masked by
the other forces affecting air movement (particularly
friction), but there is no doubt that many of the important
features of the general atmospheric circulation result
from this poleward transfer of angular momentum.
The necessity for a poleward momentum transport
is readily appreciated in terms of the maintenance of
the mid-latitude westerlies (Figure 7.17). These winds
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