This branch flows up between Ceylon and Sumatra. A branch of this powerful current strikes the western coast of Burma where the windward slopes of the Arakan and Tenasserim mountains receive heavy rainfall. Akyab records a total of over 425 cm of rainfall from June to September.
Here also, the rain-shadow effect is more pronounced to the leeward. Mandalay situated in the middle Irrawady Valley receives only about 45 cm of rainfall.
Another current of the Bay of Bengal branch, which is more southerly, moves towards Assam where Mausinram (near Cherrapunji), situated on the southern slope of the Khasi hills, has the unique distinction of recording the highest annual average precipitation (965 cm) in the world.
This is because of its peculiar geographical location. Mausinram is flanked on all sides by the Garo, Khasi and Jayantia hills except a gap through which the rain bearing winds enter and are forced to rise, thus yielding the heaviest rainfall.
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More than 2250 cm of rain have been recorded here in one year and 102 cm in one day (June 14, 1876). Shillong, hardly 40 km away on the top of Khasi Hills, receives only about 140 cm of rainfall from June to September.
A current of the Bay of Bengal branch recurves westward and advances up the Gangetic Plain towards the Punjab. It may be mentioned that the westward movement of the monsoon current takes place around the eastern end of a trough of low pressure developed over northern India.
The movement of winds is, of course, parallel to the Himalayan ranges. The rainfall occurring in the Gangetic Plain is partly controlled by the relief, and partly by the cyclonic storms or monsoon depressions which follow the track of low relief and low pressure along the southern fringe of the plains.
It is to be noted that in this region the monsoon current blows from a southeasterly direction. The rainfall decreases from east to west and from north to south.
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The main reason why the amount of rainfall decreases westwards is the increasing distance from the source of moisture. The southward decrease in rainfall is due to the increasing distance from the Himalayas which cause the forced ascent of rain-bearing air currents.
In the Himalayan foothills the zone of maximum precipitation is located at about 1500 meters beyond which there is a gradual decrease in the amount of rainfall. The east-west decrease in rainfall is borne out by a comparison of the amount of rainfall recorded during the southwest monsoon period at Calcutta (115 cm) and Multan (12.5 cm).
In the extreme northwest the rainfall is not only meager, but also highly variable. In fact, the strength of Ganges Valley current of the monsoon determines the arrival or failure of rainfall. Occasional interruptions in the development of pressure systems over this region cause a pronounced change in the wind direction.
On occasions, an anticyclonic condition develops in the Punjab region so that northwest winds start blowing in U.P. and the surrounding regions. These winds are dry and sometimes produce acute drought conditions in the affected regions.
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Remember that the destination of the Arabian Sea branch as well as the Bay of Bengal branch of monsoon is the low pressure centre formed over the northwestern region of the subcontinent. However, the direction of winds in Peninsular India is predominantly southwesterly.
In the northern plains, on the other hand, the air movement is generally from southeast to northwest. The southern margin 6f northern plain is the most frequented track of monsoon depressions that form over the head of the Bay of Bengal.
These cyclonic storms move from east to west along the foot of peninsular hills giving copious rainfall on their southern sides.
In the absence of these storms originating over the Bay of Bengal, the southern side of the plains would be deprived of monsoon rains. These rains are of immense significance to the rice cultivation in these areas.