Geoscience Reference
In-Depth Information
Figure 7.8
The meridional structure of the tropopause and the primary frontal zones. The 40 m 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 per cent of the pole-to-equator
mid-tropospheric pressure gradient is concentrated into a singe frontal zone approximately 200 km wide. The tropical easterly jet stream
may be accompanied by a lower easterly jet at about 5 km elevation. (see chapter 11C, D).
Source
: Shapiro
et al
. (1987)
From Monthly Weather Review
115, p. 450, by permission of the American Meteorological Society.
Bermuda-Azores ocean region (at 500 mb the centre
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 both in
intensity and extent, being most prominent in winter. In
the southern hemisphere, the subtropical anticyclones
are oceanic, except over southern Australia in winter.
The latitude of the subtropical high-pressure belt
depends on the meridional temperature difference
between the equator and the pole and on the temperature
lapse rate (i.e. vertical stability). The greater the merid-
ional temperature difference the more equatorward is
the location of the subtropical high-pressure belt (Figure
7.11).
In low latitudes there is an equatorial trough of low
pressure, associated broadly with the zone of maximum
insolation and tending to migrate with it, especially
towards the heated continental interiors of the summer
hemisphere. Poleward of the subtropical anticyclones
lies a general zone of subpolar low pressure. In the
southern hemisphere, this sub-Antarctic trough is
virtually circumpolar (see Figure 7.10), whereas in the
northern hemisphere the major centres are near Iceland
and the Aleutians in winter and primarily over con-
tinental areas in summer. It is commonly stated that in
high latitudes there is a surface anticyclone due to the
cold polar air, but in the Arctic this is true only in spring
over the Canadian Arctic Archipelago. In winter, the
polar basin is affected by high- and low-pressure cells
with semi-permanent cold air anticyclones over Siberia
and, to a lesser extent, northwestern Canada. The
shallow Siberian high is in part a result of the exclusion
of tropical airmasses from the interior by the Tibetan
