Monsoon hindered by land, sea anomalies

The south-west monsoon broke over Kerala on May 31 this year but ran into a weak phase from June 18 to 23.
Despite revival of flow over Arabian Sea around June 24, there has been no further advancement of monsoon beyond the area covered until June 18.
By this time, it had covered parts of Chhattisgarh, remaining parts of Orissa and West Bengal, Sikkim, entire Jharkhand and some parts of Bihar.

MONSOON ORIGIN
The monsoon originates from the southern hemisphere from a high-pressure area called ‘Mascarene High' situated near Mascarene Islands in South Indian Ocean.
Winds from this ‘high' flows towards Somalia coast and then turn towards Indian peninsula to form a low level jet (in the form of strong westerly to southwesterly flow of winds).
It is this mechanism that decides the strength of monsoon flow, says Dr Akhilesh Gupta, leading operational forecaster and Advisor to the Department of Science and Technology, Apparently, the flow from Mascarene High got weakened from June 18 onwards, putting a drag on onward flow over Arabian Sea and hence the overall monsoon.
The flow, however, got revived from June 24. But it could not penetrate east India, much less North-West India, after it ran into a now-familiar rogue north-westerly flow from the opposite direction.

WESTERLY DOMINANCE
Barring monsoon season, North-West India has a dominant northwesterly to westerly winds over the Gangetic Plains.
These are associated with the passage of western disturbances originating in the extra-tropical (beyond tropical) region.
Advent of monsoon forces westerly winds to shift north and allow monsoonal easterly flow to establish over the Gangetic plains. In meteorological parlance, this amounts to the establishment of the ‘axis of monsoon' or ‘monsoon trough.'
The ‘axis' gets fully established with the monsoon progressing over the entire peninsular India. Strong flow over Arabian Sea and Bay of Bengal sets the stage for formation of a monsoon-friendly low-pressure area over the North Bay of Bengal.
The ‘low' moves in a west-northwest direction to bring monsoon into the Gangetic Plain. ‘Low' forming anywhere else in the Bay may not bring in the desired results (a ‘low' reported in the northwest Bay on Friday is likely to fall in this category).
The ‘axis' of monsoon did establish in the initial phase, but from June 18, the dominance of northwesterly to westerly winds became entrenched over Northwest India.

SEVEN COMPONENTS
They have been inhibiting the easterly winds from penetrating beyond East Uttar Pradesh. This is one factor that could adversely impact the process of formation of a ‘low' in the North Bay.
Monsoon has seven main components which help sustain and maintain its energy and strength. These are: (i) the ‘heat low' formed by sustained heating of land over West Rajasthan and adjoining Pakistan (ii) the monsoon trough or ‘axis' over the Gangetic Plain (iii) high pressure cell at about 5-6 km height over Tibet (iv) tropical easterly jet at 13-14 km height from Tibet across Indian peninsula and goes to southern hemisphere (Mascarene Islands) where it descends (v) high pressure cell at sea level over Mascarene Islands (vii) low level jet at 1.5 km height along Somalia coast (east coast of Africa) which flows from southern hemisphere to Arabian Sea across equator and (vii) low level jet at 1.5 km height over the Arabian Sea flowing onward into Bay of Bengal across the Indian peninsula.
All these components are dynamically and physically interconnected and draw strengths from each other through a feedback process. If one component becomes weak (or strong) the other also starts responding to this change, but with a lag of few days.

MUCH BETTER SITUATION
As on June 30, the rainfall over the country as a whole is -16 per cent, with North-West India at -18 per cent; Central India -26 per cent; South Peninsula +5 per cent and East and North-East India -17 per cent.
Forty-three per cent of districts in the country received normal to excess rains whereas 49 per cent have so far had deficient to scanty rains.



Compared to last year, the situation is much better as last year around this time about 72 per cent of districts had deficient to scanty rains.
As per the latest update on the long-range forecast of monsoon rainfall released by India Meteorological Department, the rainfall over the country as a whole for the 2010 season (June to September) is likely to be 102 per cent; and those for July and August, 98 per cent and 101 per cent.
The forecast indicates that as the season progresses, the rainfall performance is expected to improve.
It may be worth mentioning here that the months of June, July, August and September have respectively 18, 33, 29 and 20 per cent share out of the total seasonal rainfall.