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Jet stream
(a) Polar jet stream with small undulations
Jet stream
Co d a i i r
Cold air
r
W arm air
Polar
view
(b) Rossby waves develop, causing north-south
motion of large masses of warm and cold air
Jet stream
Cold air
d a i r
C C C
C
C
Warm air
W a r
m a
Polar
view
(c) Cells of cold air break off from the larger air mass,
forming isolated cyclones of cold air
Figure 6.21 Development of Rossby waves in the midlatitudes of the Northern Hemi-
sphere. The progressive development of jet stream undulations along the polar front ulti-
mately results in the pinching off of cold air pools and the reestablishment of zonal flow.
At any given location in the midlatitudes, the flow of the
polar jet stream essentially follows a smooth west-to-east path
for several weeks. This circulatory pattern is typically called
zonal flow. because cold arctic air is confined to a small zone
at very high latitudes. As the temperature contrast on either
side of the polar front increases, the midlatitude atmosphere
responds by forming an undulation in the jet stream, begin-
ning the Rossby wave. The core of this undulation becomes
a midlatitude cyclone rotating counterclockwise. With the
continued development of this Rossby wave, the westerly
winds no longer flow directly west to east, but have significant
northerly and southerly components as well due to the coun-
terclockwise circulation. In contrast to zonal flow, the vari-
able flow associated with a developing Rossby wave is called
meridional flow , because the air frequently flows parallel to
the meridians.
Zonal flow Jet stream pattern that is tightly confined to the
high latitudes and is thus circular to semicircular in polar view.
Meridional flow Jet stream pattern that develops when strong
Rossby waves exist and the polar front jet stream flows parallel
to the meridians in many places.
 
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