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fully known, although high-level divergence in the
waves of the subtropical westerly jet stream appears to
be essential. The early onset of summer rains in Bengal,
Bangladesh, Assam and Burma (e.g. Chittagong, Figure
11.20) is favoured by an orographically produced trough
in the upper westerlies, which is located at about 85 to
90°E in May. Low-level convergence of maritime air
from the Bay of Bengal, combined with the upper-level
divergence ahead of the 300 mb trough, generates
thunder squalls. Tropical disturbances in the Bay of
Bengal are another source of these early rains. Rain also
falls during this season over Sri Lanka and southern
India (e.g. Minicoy, Figure 11.20) in response to the
northward movement of the equatorial trough.
3 Early summer
Generally, during the last week in May the southern
branch of the high-level jet begins to break down,
becoming intermittent and then gradually shifting
northward over the Tibetan Plateau. At 500 mb and
below, however, the plateau exerts a blocking effect on
the flow and the jet axis there jumps from the south to
the north side of the plateau from May to June. Over
India, the equatorial trough pushes northward with each
weakening of the upper westerlies south of Tibet, but the
final burst of the monsoon, with the arrival of the humid,
low-level southwesterlies, is not accomplished until the
upper-air circulation has switched to its summer pattern
(see Figures 11.19 and 11.23). Increased continental
convection overcomes the spring subsidence and the
return upper-level flow to the south is deflected by
the Coriolis force to produce a strengthening easterly
jet located at about 10 to 15 °N and a westerly jet to
the south of the equator (see Figure 11.16). One theory
suggests that this takes place in June as the col between
the subtropical anticyclone cells of the western Pacific
and the Arabian Sea at the 300-mb level is displaced
northwestward from a position about 15 °N, 95 °E in
May towards central India. The northwestward move-
ment of the monsoon (see Figure 11.24) is apparently
related to the extension over India of the upper tropos-
pheric easterlies.
The recognition of the upper airflow has widespread
effects in southern Asia. It is directly linked with the
Maiyu rains of China (which reach a peak about 10 to
15 June), the onset of the southwest Indian monsoon
and the northerly retreat of the upper westerlies over the
whole of the Middle East.
Figure 11.19 Mean 200-mb streamlines and isotachs in knots
over Southeast Asia for January and July, based on aircraft reports
and sounding data.
Source : From Sadler (1975b) courtesy Dr J. C. Sadler, University of
Hawaii.
along the convective ITCZ trough centred over the
equator and following the overhead sun northward over
the warm Indian Ocean (see Figure 11.16). The weather
over northern India becomes hot, dry and squally in
response to the greater solar radiation heating. Mean
temperatures in Delhi rise from 23°C in March to 33°C
in May. The thermal low-pressure cell (see Chapter
9H.2) now reaches its maximum intensity, but although
onshore coastal winds develop, the onset of the mon-
soon is still a month away and other mechanisms cause
only limited precipitation. Some precipitation occurs in
the north with 'westerly disturbances', particularly
towards the Ganges Delta, where the low-level inflow
of warm, humid air is overrun by dry, potentially cold
air, triggering squall lines known as nor'westers. In
the northwest, where less moisture is available, the
convection generates violent squalls and dust storms
termed andhis . The mechanism of these storms is not
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