Geoscience Reference
In-Depth Information
Figure 11.30
Rainfall (mm) produced in three days over a 50,000-km
2
area of central India northeast of Nagpur by a severe, westward-
moving monsoon depression, during September 1926.
Source
: Dhar and Nandargi (1993). Copyright © John Wiley & Sons Ltd. Reproduced with permission.
to occur on the windward coasts and mountains of India,
Burma and Malaya. Without such disturbances, the
distribution of monsoon rains would be controlled to a
much larger degree by orography.
A key part of the southwest monsoonal flow occurs
in the form of a 15 to 45 m s
-1
jet stream at a level of
only 1000 to 1500 m. This jet, strongest during active
periods of the Indian monsoon, flows northwestward
from Madagascar (Figure 11.31) and crosses the equator
from the south over East Africa, where its core is often
marked by a streak of cloud (similar to that shown in
Plate 14) and where it may bring excessive local rainfall.
The jet is displaced northward and strengthens from
February to July; by May it has become constricted
against the Abyssinian Highlands, it accelerates still
more and is deflected eastward across the Arabian
Sea towards the west coast of the Indian peninsula.
This low-level jet, unique in the trade wind belt, flows
offshore from the Horn of Africa, bringing in cool
waters and contributing to a temperature inversion that
is also produced by dry upper air originating over Arabia
or East Africa and by subsidence due to the convergent
upper easterlies. The flow from the southwest over the
Indian Ocean is relatively dry near the equator and near
shore, apart from a shallow, moist layer near the base.
Downwind towards India, however, there is a strong
temperature and moisture interaction between the ocean
surface and the low-level jet flow. Hence, deep convec-
tion builds up and convective instability is released,
especially as the airflow slows down and converges near
the west coast of India and as it is forced up over the
Western Ghats. A portion of this southwest monsoon
airflow is deflected by the Western Ghats to form 100-
km diameter offshore vortices lasting two to three days
