The monsoon blockbuster

Dr Akhilesh Gupta in a talk with Vibha Sharma gives an insight into the complexities of the most-awaited weather phenomenon

Dr Akhilesh Gupta

The northwest India is eagerly awaiting for its share of 2006 monsoon rains, which the India Meteorological Department has predicted to be "near normal and on time".

But if you thought the southwest Indian monsoon was some regular yearly ordinary weather phenomenon, think again. The monsoon saga has all the makings of a Bollywood blockbuster. There is action, suspense, drrrr..ama, joy and, to top it all, sacrifice in this multi-starrer movie. The performance of its actors and directors, who by the way remain constant every year, decide whether that particular year’s run will be a super-hit, average or a flop.

The most awaited weather phenomenon in the country even has the ruling party in jitters till the time it actually premiers at the southernmost tip of Kerala every year on June 1. Marked year to year, variations in dates of onset and withdrawal of the monsoon can occur and there have been several occasions in the past when the monsoon arrived over certain parts of the country about a fortnight earlier or later than the normal dates. Similarly, it has also withdrawn earlier or later.

But one thing is sure, that the monsoon has Indian weathermen in jitters and till the time its exact date or time, extent and progress is not 100 per cent sure through various weather models, the information is not shared with the public and kept tightly under wraps.

After all, it is not only the farmers and the common man of the country who await the monsoon eagerly every year. The health of the country’s economy, planners, Sensex, Nifty, practically everyone, depends upon this single weather phenomenon.

Dr Akhilesh Gupta, a weather expert with more than 20 years’ experience in weather forecasting from the National Centre for Medium Range Forecast, gives the readers of The Tribune an insight into the complexities of the Indian monsoon.

Moist winds

First and foremost, monsoon is basically a large-scale weather system, affecting nearly 18,000 km area from east to west and 6,000 km from south to north, covering entire central Africa, almost entire South and South-East Asia, parts of North Australia, North Indian Ocean parts of South Indian Ocean, China Sea, and parts of West Pacific Ocean. The monsoon current does not only affect India and Pakistan, but nearly 25 countries in these regions.

"Monsoon is basically a large-scale sea breeze system caused by temperature difference between land and sea in which moist winds blow from sea to land in the lower levels and dry air moves from land to sea in upper levels, so that a complete circulation is maintained," explains Dr Gupta.

Mascarene High

Interestingly, the Indian monsoon current is formed due to a high-pressure area near a small island in the Southern Hemisphere over the South Indian Ocean called Mascarene.

Mascarene, located at 30 degree south latitude and 70 degree east longitude, is about 4,000 km far from the Indian mainland. Winds from this high-pressure area, termed as Mascarene High by scientists, start blowing towards the northern hemisphere along the east Somalia coast.

The east Somalian coast has a north-south oriented hilly area from where these winds turn eastwards towards Kerala and Karnataka in the Indian peninsular region in the form of a low-level westerly jet.

The winds go towards the Bay of Bengal and from here make another turn towards the North-East and enter the northern plains through an east-west oriented monsoon trough which runs along the Indo-Gangetic plains. "This westward turn is shaped like a spiral," Dr Gupta adds.

So, the monsoon winds cross the Equator, go through the Arabian Sea, turn eastward and reach Peninsular India, make another turn westward and enter eastern part of India, cover central India and northern plains and enter Pakistan.

Pressure gradient

Each time India gets good rain, it is thanks to our neighbour in the North-West and the sacrifice made by the people of Rajasthan and Gujarat who suffer high temperatures to ensure successful monsoon year after year.

"Monsoon is a sea-breeze activity and more the land is heated, more would be the intensity of the current from sea to land. Prior to monsoon, more is the heating over North, Central and Western India, more is the tendency for a good monsoon rains. High temperatures between April and May are very favourable for the monsoon to establish, which is why the pre-monsoon period is extremely critical. The monsoon circulation is established only due to temperature difference between land and sea," says Dr Gupta.

While high temperatures are crucial for timely and strong monsoon, another powerful factor is the Heat Low that decides the temperature and pressure difference between land and the sea. In case of India, it is the Heat Low centered in Jacobabad in Pakistan, a low-pressure area that doesn’t have moisture that controls the sea-breeze flow towards the land.

Whenever temperature difference is more, it gets converted into pressure difference or what we call pressure gradient along the West Coast of India. This pressure gradient culminates into the strong flow of current from the high-pressure area, i.e. sea, into the low-pressure area, i.e. land. Heat Low is a shallow system where heating alone decides its intensity which is in a way related to monsoon strength.

Fortunately, soil characteristics in the western parts of the country, like west Rajasthan and Gujarat are such that they get heated up very fast. There is also little vegetation in these areas. Even Pakistan has large areas under sandy and rocky type soil. Such area extends up to parts of Middle East and North Africa.

Timely rains

But to complete the circle, the monsoon current also has to go back to the Mascarene High. A high-pressure cell is centered over the Tibetan Plateau called the Tibetan High. From the Tibetan High, there is an outflow in the upper levels between 8 and 15 km height towards the Bay of Bengal from where the current crosses the Indian Peninsula and descends down and goes back to the Mascarene High. The strongest easterly wind flow is seen at 15-km level in the form of what we call Tropical Easterly Jet that passes through the peninsular India along 13 degree North before moving to southern Hemisphere.

Dr Gupta says each of the seven components of the monsoon is equally important for the success of the monsoon over the Indian subcontinent. Here, we are not taking into account several external factors like La Nina, El Nino, southern oscillations or Indian Ocean dipole that influence the monsoon.

Monsoon rainfall is also not uniformly distributed. Timely rains with requisite intensity rightly spread over can make the monsoon ideal for optimum agricultural production and adequate water storage for power generation. But rarely are we able to get such ideal conditions.

Though on a gross scale the monsoon may be considered as a seasonal rain producing mechanism there are several small-scale systems embedded in the general monsoon current. The weather producing systems within the monsoon is different in different parts of the country.

In 2006, southwest monsoon set in over Kerala on May 26, about six days earlier than normal date. It reached Mumbai on May 31, 10 days before the normal date.

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