Role of mountains in the development of the Indian summer monsoon circulation and the associated weather

Abstract

Some special features of the Indian summer monsoon mentioned by Hahn and Manabe (l 975) in their model simulated with mountains are discussed with reference to the model for surface layers given by Banerji (1930 a and b) while takin g into acco unt the effect of moun tain ranges on the a ir flow and configura tion of isobars on the basis of thermo-dynamical and hydrodynarnical consideration and the experience of the author for fifty
years about the facts of weather.

It is observed that the lower monsoon circulation and the upper tropospheric circulation developed indepe nden t.
Iy, The barriers to the west of the east coa st of Africa are respon sible for the deflection of the south-east trades north to northeastward acro ss the equator under the influence of the heat low (location influenced by barriers) over south Pakistanan d the monsoou trough circulation in the levels up to about 600 mb which is set up due to the influence of the moun tains on the Indian sub-continent.

The westerly jet is not a part of the monsoon circulation and its shift from the south to the north of Tibet need not
necessarily coincide with the onset of the monsoon. There would not appear any cause and effect relation  between the development and strength of eas terly jet and the onset and performance of the monsoon in terms of rainfall.
The low level jet is embedded in the monsoon current and is noticed even to the east of Madagascar and in the
western Indian Ocean and the adjoining parts of eastern coastal area s of Africa (Findlater 1969 a and b) and would not appear to be developing due to upwelling of water off the Somalia coast which has led to its being named Somalia jet (Bunker 1965).

The seasonal rainfall can be understood on the basis influence of mountains, although proximit of the tracks
followed by the depressions while moving west to northwestwards from the north Bay of Bengal, increase, the amount of rainfall. Small amounts of rainfall over the northwest of the sub-contonent would appear to be due to absence of barriers over thecoast extending From Karachi in Pakistan to veraval in G ujratt (India)and the presence of low level inversion due to spreading of relatively warmer drier air between about  1 and 3 km from the Baluchistan plateau side over the lower cool moist monsoon air. There is no ITCZ over the monsoon trough area upto about 600mb once the monsoon is established. ' Breaks' in monsoon rain would appear to be associated with the high pressure anomalies over the monsoon trough area and the adjoi ning area to th: south which prevent the southern hemispheric air from being drawn to the west coast of India.