Geoscience Reference
In-Depth Information
20
Stratospheric
tropical air
au
18
Upper
troposheric
tropical air
J
E
16
Stratospheric
middle latitude
air
14
Secondary
tropical tropopause
12
o
J
S
Stratospheric
polar air
10
J
P
p
Stratospheric
arctic air
Stratospheric
middle latitude
air
Lower
tropospheric
tropical Air
8
J
A
6
4
Subtrop.
surface
high
2
Polar
easterlies
Trade Wind
belt
Polar
westerlies
0
90°N
80°
70°
60°
50°
40°
30°
20°
10°
0°
10°S
Figure 7.8
The meridional structure of the tropopause and the primary frontal zones. The 40m s
-1
isotach (dashed)
encloses the arctic (J
A
), polar (J
P
) and subtropical (J
S
) jet streams. The tropical easterly (J
E
) jet stream is also shown.
Occasionally, the arctic and polar or the polar and subtropical fronts and jet streams may merge to form single
systems in which about 50 percent of the pole-to-equator mid-tropospheric pressure gradient is concentrated into a
singe frontal zone approximately 200km wide. The tropical easterly jet stream may be accompanied by a lower
easterly jet at about 5km elevation.
Source: Shapiro et al. (1987). From Monthly Weather Review115, p. 450 by permission of the American Meteorological Society.
summer but are thermally intensified over them in
winter. The principal subtropical high pressure
cells are located: (1) over the Bermuda-Azores
ocean region (at 500mb the center of this cell lies
over the east Caribbean); (2) over the south and
southwest United States (the Great Basin or
Sonoran cell) - this continental cell is seasonal,
being replaced by a thermal surface low in summer;
(3) over the east and north Pacific - a large and
powerful cell (sometimes dividing into two,
especially during the summer); and (4) over the
Sahara - this, like other continental source areas,
is seasonally variable in both intensity and extent,
being most prominent in winter. In the Southern
Hemisphere, the subtropical anticyclones are
oceanic, except over southern Australia in winter.
4 Surface pressure conditions
The most permanent features of the mean sea-level
pressure maps are the oceanic subtropical high
pressure cells (
Figures 7.9
and
7.10
). These anti-
cyclones are located at about 30
latitude,
suggestively situated below the mean Subtropical
Jet Stream. They move a few degrees equatorward
in winter and poleward in summer in response to
the seasonal expansion and contraction of the two
circumpolar vortices. The anticyclones located
in the eastern sectors of the subtropical North
Atlantic and North Pacific are shallow, unlike
those in the western parts. In the Northern
Hemisphere, the subtropical ridges of high
pressure weaken over the heated continents in
°
