Locally adverse factors worsened by a developing El Nino in the Equatorial and East Pacific have combined to weaken the monsoon during the first month of June.

PV Joseph, monsoon researcher of renown and a former Director of India Met Department, analyses the 40 per cent-plus deficit in rainfall till date in an e-mail interview.

The monsoon has run up a deficit of 40 per cent in June even before El Nino has peaked. So do you think the deficiency can be attributed to El Nino alone?

Large monsoon deficiency in June is caused by many factors.

First, delay in monsoon onset over Kerala and slow northward movement of monsoon with long halts is one cause.

This year, the monsoon onset over Kerala was delayed by five days and northward movement by a week to 10 days over the areas covered by it so far.

El Nino is a factor for the deficiency in monsoon rains of June.

Southward intrusion of mid-latitude westerly winds at levels 6 to 14 km in the atmosphere particularly over Northwest India (winter flows as in western disturbances) is a factor causing reduced rainfall potential of monsoon. It has happened during the whole of this June.

If, as models suggest, El Nino has not indeed peaked, what could we expect in July-Aug-September by when it is expected to peak?

Typical El Nino warming of equatorial east Pacific ocean begins in March to May season. The ocean surface temperature anomaly there rises rapidly during our monsoon months June to September and rises at a slower pace thereafter to reach the highest temperature anomalies in the El Nino by the end of the year (December to the following February).

As it develops El Nino is found associated in many cases with deficient monsoon rainfall of India. The strongest El Nino of 100 years in 1997, however, could not cause deficiency in Indian monsoon rainfall.

Do you subscribe to the view that it's not El Nino, but extended summer showers and impaired heating of Northwest India which did the monsoon in?

Air temperature near the surface of the earth over northwest India has a great control on the monsoon rainfall of India as it affects north-south pressure gradient that drives the monsoon winds.

There is high correlation between surface air temperature of the March to May season, particularly May, over Northwest India and the monsoon rainfall of India of June to September.

The heating was displaced mostly to East India during April-May. Is that what is driving the rainfall in the East currently?

Not really. After making an onset over Kerala, the monsoon current moved north over the Bay of Bengal to reach Northeast India.

Thereafter, further northward and westward movement was slow. So it continued to rain in Northeast India. If a low-pressure area or depression forms over North Bay of Bengal, conditions will change drastically and Central and Northwest India will get monsoon rains and rainfall will decrease over Northeast India.

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