Geoscience Reference
In-Depth Information
there is a weak indirect mid-latitude cell in much
reduced form (
Figure 7.20
). The relationship of
the jet streams to regions of steep meridional
temperature gradient has already been noted (see
Figure 7.7
). A complete explanation of the two
wind maxima and their role in the general
circulation is still lacking, but they undoubtedly
form an essential part of the story.
In the light of these theories, the origin of the
subtropical anticyclones that play such an
important role in the world's climates may be re-
examined. Their existence has been variously
ascribed to: (1) the piling up of poleward-moving
air as it is increasingly deflected eastward through
the earth's rotation and the conservation of
angular momentum; (2) the sinking of poleward
currents aloft by radiational cooling; (3) the
general necessity for high pressure near 30
leads to a vertical cell with descent over the eastern
Mediterranean, eastern Sahara Desert and the
Kyzylkum-Karakum desert. However, while the
ascending air originates in the tropical easterlies,
Rossby waves in the mid-latitude westerlies are
thought to be the source of the descending air and
this may provide a link with the first mechanism.
Neither of these arguments addresses the winter
subtropical anticyclones. Clearly, these features
await a definitive and comprehensive explanation.
It is probable that the high-level anticyclonic
cells that are evident on synoptic charts (these
tend to merge on mean maps) are related to anti-
cyclonic eddies that develop on the equatorward
side of jet streams. Theoretical and observational
studies show that, as a result of the latitudinal
variation of the Coriolis parameter, cyclones in the
westerlies tend to move poleward and anticyclonic
cells equatorward. Hence the subtropical anti-
cyclones are constantly regenerated. There is a
statistical relationship between the latitude of the
subtropical highs and the mean meridional
temperature gradient (see
Figure 7.11
); a stronger
gradient causes an equatorward shift of the high
pressure, and vice versa. This shift is evident on a
seasonal basis. The cellular pattern at the surface
clearly reflects the influence of heat sources.
The cells are stationary and elongated north-
south over the Northern Hemisphere oceans in
summer, when continental heating creates low
pressure and also the meridional temperature
gradient is weak. In winter, on the other hand,
the zonal flow is stronger in response to a greater
meridional temperature gradient, and continental
cooling produces east-west elongation of the
cells. Undoubtedly, surface and high-level factors
reinforce one another in some sectors and tend to
cancel out in others.
Just as Hadley circulations represent major
meridional (i.e., north-south) components of the
atmospheric circulation, so Walker circulations
represent the large-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 differences in
°
latitude separating approximately equal zones of
east and west winds; or to combinations of such
mechanisms. An adequate theory must account
not only for their permanence but also for their
cellular nature and the vertical inclination of the
axes. The preceding discussion shows that ideas
of a simplified Hadley cell and momentum
conservation are only partially correct. Moreover,
recent studies rather surprisingly show no rela-
tionship, on a seasonal basis, between the intensity
of the Hadley cell and that of the subtropical highs.
Descent occurs near 25°N in winter, whereas
North Africa and the Mediterranean are generally
driest in summer, when the vertical motion is
weak.
Two new ideas have recently been proposed
(
Figure 7.21
)
. One suggests that the low-level
subtropical highs in the North Pacific and North
Atlantic in summer are remote responses to
stationary planetary waves generated by heat
sources over Asia. In contrast to this view of
eastward downstream wave propagation, another
model proposes regional effects from the heating
over the summer monsoon regions of India, West
Africa and southwestern North America that act
upstream on the western and northern margins of
these heat sources. The Indian monsoon heating
