India’s monsoon weakens

Flooding in Uttar Pradesh in 1970s [Getty/Gallo]

Across much of India, the last few days have seen a significant easing in the monsoon rains. Ratnagiri, on the west coast, some 225 kilometers south of Mumbai, had reported 24 hour rainfall totals of up to 184 millimetres a week or so previously. By the start of this week that had dropped to a ‘mere’ 59 millimetres.

We always need to be cautious when interpreting monsoon rainfall. It is an interesting phenomenon of the monsoon that the rain, on a day-to-day basis, is highly variable. It exhibits ‘active’ and ‘break’ phases – short-lived increases and decreases in rainfall intensity.

Yet late July should signal a weakening of the monsoon rains. For example, average rainfall for Mumbai on the west coast, falls from 700 millimetres in July to 400 millimetres in August. The picture in the north and east of the country is less clear.  Here,  much of the summer rain owes its origin less to the southwesterly winds and more to the development of monsoon depressions which form in the Bay of Bengal before travelling westwards on the easterly upper winds.

So whilst we can expect further heavy monsoon rain in many parts of the country until at least late September, the intensity may well be easing.

So far, many Indian states have seen at least average rainfall for the 2011 monsoon. Many western and central northern states have had above average rainfall. The main area where there has been a rainfall deficit is in the far east across Assam and West Bengal. But that may change in the next month or two.