Geoscience Reference
In-Depth Information
Figure 11.15
The annual distribution of tropical rainfall. The shaded areas refer to periods during which more than 75 per cent of
the mean annual rainfall occurs. Areas with less than 250 mm y
-1
(10 in y
-1
) are classed as deserts, and the unshaded areas are those
needing at least seven months to accumulate 75 per cent of the annual rainfall and are thus considered to exhibit no seasonal maximum.
Source
: After Ramage (1971), copyright © Academic Press. Reproduced by permission.
Figure 11.16
Schematic representation of the meridional
circulation over India at 90°E at five characteristic periods of the
year: winter monsoon (December to February); approach of the
monsoon season (April); the active summer monsoon (June to
August); a break in the summer monsoon (June to August); and
the retreat of the summer monsoon (September). Easterly (J
E
)
and westerly (J
W
) jet streams are shown at sizes depending on
their strength; the arrows mark the positions of the overhead sun;
and zones of maximum precipitation are indicated.
Source
: After Webster (1987a). Copyright © 1987. Reproduced by
permission of John Wiley & Sons, Inc.
of the interaction of planetary and regional factors,
both at the surface and in the upper troposphere. It is
convenient to look at each season in turn; Figure 11.16
shows the generalized meridional circulation at 90°E
over India and the Indian Ocean in winter (December to
February), spring (April) and autumn (September), and
those associated with active and break periods during
the June to August summer monsoon (p. 287).
1 Winter
Near the surface, this is the season of the out-blowing
'winter monsoon', but aloft westerly airflow dominates.
This reflects the hemispheric pressure distribution. A
shallow layer of cold high-pressure air is centred over
the continental interior, but this has disappeared even at
700 mb (see Figure 7.4) where there is a trough over
East Asia and zonal circulation over the continent. The
upper westerlies split into two currents to the north and
south of the high Tibetan (Qinghai-Xizang) Plateau
