Geoscience Reference
In-Depth Information
Figure 11.31
The mean monthly
positions (A) and the mean July velocity
(m s
-1
) (B) of the low-level (1 km)
Somali jet stream over the Indian
Ocean.
Source
: After Findlater (1971), repro-
duced by permission of the Controller of
Her Majesty's Stationery Office.
and capable of bringing 100 mm of rain in twenty-four
hours along the western coastal belt of the peninsula. At
Mangalore (13°N), there are on average twenty-five
rain-days per month in June, twenty-eight in July and
twenty-five in August. The monthly rainfall averages
are 980, 1060 and 580 mm, respectively, accounting for
75 per cent of the annual total. In the lee of the Ghats,
amounts are much reduced and there are semi-arid areas
receiving less than 64 cm per year.
In southern India, excluding the southeast, there is a
marked tendency for less rainfall when the equatorial
trough is furthest north. Figure 11.20 shows a maximum
at Minicoy in June, with a secondary peak in October as
the equatorial trough and its associated disturbances
withdraw southwards. This double peak occurs in much
of interior peninsular India south of about 20°N and in
western Sri Lanka, although autumn is the wettest period.
There is a variable pulse alternating between active
and break periods in the May to September summer
monsoon flow (see Figure 11.16) which, particularly at
times of its strongest expression (e.g. 1971), produces
periodic rainfall (Figure 11.32). During active periods
the convective monsoon trough is located in a southerly
position, giving heavy rain over north and central India
and the west coast (see Figure 11.16). Consequently,
there is a strong upper-level outflow to the south, which
strengthens both the easterly jet north of the equator and
the westerly jet to the south over the Indian Ocean. The
other upper-air outflow to the north fuels the weaker
westerly jet there. Convective activity moves east from
the Indian Ocean to the cooler eastern Pacific with an
irregular periodicity (on average forty to fifty days for
the most marked waves), finding maximum expression
at the 850-mb level and clearly being connected with
the Walker circulation. After the passage of an active
convective wave there is a more stable break in the
summer monsoon when the ITCZ shifts to the south.
The easterly jet now weakens and subsiding air is forced
to rise by the Himalayas along a break trough located
above the foothills (see Figure 11.16), which replaces
the monsoon trough during break periods. This circu-
lation brings rain to the foothills of the Himalayas and
the Brahmaputra valley at a time of generally low
rainfall elsewhere. The shift of the ITCZ to the south
of the subcontinent is associated with a similar move-
ment and strengthening of the westerly jet to the north,
weakening the Tibetan anticyclone or displacing it
northeastward. The lack of rain over much of the sub-
continent during break periods may be due in part to
the eastward extension across India of the subtropical
high-pressure cell centred over Arabia at this time.
It is important to realize that the monsoon rains
are highly variable from year to year, emphasizing the
role played by disturbances in generating rainfall within
the generally moist southwesterly airflow. Droughts
occur with some regularity in the Indian subcontinent:
between 1890 and 1975 there were nine years of
extreme drought (Figure 11.33) and at least five others
of significant drought. These droughts are brought
about by a combination of a late burst of the summer
monsoon and an increase in the number and length of
the break periods. Breaks are most common in August
to September, lasting on average for five days, but they
may occur at any time during the summer and can last
up to three weeks.
The strong surface heat source over the Tibetan
Plateau, which is most effective during the day, gives rise
to a 50 to 85 per cent frequency of deep cumulonimbus
