The Hindu Business Line : Even chances for a `break monsoon'

INDICATIONS of sustained rain activity in peninsular India and along the southwest coast seem to cloud the weathermen's ability to take a call on a classic `break monsoon' setting in any time soon.

Speaking to Business Line, Dr Akhilesh Gupta, Director, National Centre for Medium Range Weather Forecasting (NCMRWF), said that on a scale of one to five, he would rate chances of a break monsoon occurring at three.

`Break monsoon' is manifest in the interruption of monsoon rainfall by prolonged spells of sparse rainfall during the mid-monsoon months of July and August over the plains of northern India. This is brought about when a prevailing monsoon system moves from the plains in a northerly direction toward the submontane region of the Himalayas, taking the monsoon trough along with it.

A western disturbance, with a strengthening core of westerlies, is threatening to sweep the prevailing upper air cyclonic circulation with the monsoon trough in tow to the Himalayan foothills by Friday. This should normally deprive the plains of significant ascending motion and normal rainfall.

During the break-monsoon period, descending motion takes place from the stratosphere to the lower troposphere over the plains, while ascending motion occurs in the regions to the north and south of the plains.

As the descending motion persists over the plains, the break monsoon intensifies and the weather turns mainly dry. The phenomenon ceases and normal monsoon conditions are re-established when the monsoon trough returns to the plains and intensifies.

This occurs in association with tropical low-pressure systems developing at or near the Bay of Bengal and moving toward the plains.

According to Dr Gupta, most weather models show the possibility of the westerlies strengthening and making their presence felt at a height of 1.5 km to 3 km from Thursday onwards. Another major indication suggestive of a `break monsoon' emerges from the European Centre for Medium-Range Weather Forecasts (ECMWF), which sees an intense cyclone developing over the western Pacific by Monday next.

This system will be active as a low-pressure area as early as Friday. Such a scenario would lead to rain activity being shifted half way across the globe, with the rampaging Pacific system sucking in the monsoon currents. This condition is identified best with a `break monsoon' over India.

The contra-indication to this is provided by the pattern of rainfall activity in the peninsula and along the west coast. During a `break monsoon', rainfall in the peninsula weakens along the west coast, which is matched only by a spurt in downpour over Tamil Nadu and the Northeastern States. But near-term predictions do not suggest this happening, Dr Gupta said.

In its weather forecast for the next five days, the NCMRWF said that the monsoon system in the form of an upper air cyclonic circulation lay over Jharkhand and adjoining Chhattisgarh on Monday. This is likely to move in a northwesterly direction during next 2-3 days, only to be accosted by the westerlies positioned over Punjab and adjoining Himachal Pradesh and heading to the east.

The interaction of the systems may lead to fairly widespread to widespread rains over many parts of northwest India over the next 2-3 days. Isolated heavy to very heavy rains are likely over Uttaranchal and Himachal Pradesh.

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