Opinion Questioning the monsoon

But within this overall agreement with the forecast,the monsoon gave quite a few anxious moments this year. While its onset,over the Kerala coast,happened around the normal date,the formation of tropical cyclone Phet over the Arabian Sea interfered with its northward progression during the first week of June. After the cyclone dissipated,monsoon winds travelled quickly to cover all of peninsular India by the third week of June.

However,the monsoon went into a prolonged weak spell after that and did not move forward for about two weeks. By July 1 it was almost 17 days behind its normal schedule. It had not even entered Uttar Pradesh,where the normal date for the arrival of the monsoon is June 14. Talk of a second consecutive year of drought,at least in the northern half of India,had already started.

But yet again,the development of a cyclonic circulation and its interaction with western disturbances in northwest India resulted in widespread rainfall in most parts of northern India; and by July 6,it had covered the entire country — 10 days in advance!

These fluctuations are being attributed to the irregular behaviour of “Mascarene High” — a high-pressure area over the Mascarene islands in the southern Indian Ocean,which makes a crucial contribution to the strength of monsoon winds.

More surprising even than the fluctuations was the prolonged spell of rain over the northwestern parts of the country in the second half of the season. When it was time for a retreat,monsoon rains were lashing western UP,Delhi,Uttarakhand,Punjab,

Himachal Pradesh and adjoining areas,resulting in major flooding. Barring the monsoon season,India’s northwest is dominated by westerly winds through most of the year. These winds are associated with the perpetual passage of western disturbances that originate in the extra-tropical region extending beyond Pakistan. In the monsoon months,rain-bearing winds from the Bay of Bengal generally push the westerlies back. This year,they were too weak to do so. This was the cause for the delayed rain in the first half of the season. In the second half,however,an interesting feature developed over this region. A north-south trough of an extra-tropical nature anchored over northwest India,extending up to Pakistan and Afghanistan. This system interacted positively with the tropical low-pressure areas that get formed routinely this time of the year. This extra-tropical trough,which persisted over the area for the last two months,not only resulted in heavy rainfall but was also the reason for extreme weather events like the cloud-burst in Leh and the flooding in Uttarakhand.

The most remarkable feature of this year’s monsoon has,however,been the complete absence of any depression in the Bay of Bengal. Normally,a depression is formed almost every week in the monsoon season. These low-pressure systems over the Bay of Bengal are absolutely crucial for the movement of monsoon winds over the eastern,northern and northwestern parts of India. In the last 120 years,there has been only one instance where not a single depression was formed in the Bay of Bengal. This was the year 2002. Not surprisingly,that was one of the worst drought years in the last two decades.

But this year has brought good rainfall despite the absence of depressions — the only instance of its kind. Scientists are at a loss to explain this unusual event. One of the reasons,they say,could be the fact that this year’s monsoon rains have been engineered mainly from the Arabian Sea side. The strength of the monsoon on the Bay of Bengal side was weak for most of the season. As such,the absence of depressions over this area did not have a large adverse impact on the rainfall,as is expected in normal situations. But it is an interesting subject for further investigation by the researchers and scientists that study the behaviour of monsoons.

amitabh.sinha@expressindia.com