Geoscience Reference
In-Depth Information
Figure 11.20
Average monthly rainfall
(mm) at six stations in the Indian region.
The annual total is given after the station
name.
Source
: Based on 'CLIMAT' normals of the
World Meteorological Organization for
1931 to 1960.
It must nevertheless be emphasized that it is still
uncertain how far these changes are caused by events in
the upper air or indeed whether the onset of the monsoon
initiates a readjustment in the upper-air circulation. The
presence of the Tibetan Plateau is certainly of impor-
tance even if there is no significant barrier effect on the
upper airflow. The plateau surface is strongly heated
in spring and early summer (R
n
is about 180 W m
-2
in
May) and nearly all of this is transferred via sensible
heat to the atmosphere. This results in the formation of
a shallow heat low on the plateau, overlain, at about 450
mb, by a warm anticyclone (see Figure 7.1C). The
plateau atmospheric boundary layer now extends over
an area about twice the size of the plateau surface itself.
Easterly airflow on the southern side of the upper anti-
cyclone undoubtedly assists in the northward shift of
the subtropical westerly jet stream. At the same time,
the pre-monsoonal convective activity over the south-
eastern rim of the plateau provides a further heat source,
by latent heat release, for the upper anticyclone. The
seasonal wind reversals over and around the Tibetan
Plateau have led Chinese meteorologists to distinguish
a 'Plateau Monsoon' system, distinct from that over
India.
4 Summer
By mid-July, monsoon air covers most of South and
Southeast Asia (see Figure 11.23), and in India the
equatorial trough is located at about 25 °N. North of
the Tibetan Plateau there is a rather weak upper westerly
current with a (subtropical) high-pressure cell over the
