The lesser known north-east monsoon

N. Gopal RAJ

A La Nina, the opposite of an El Nino, is generally beneficial for the south-west monsoon but could it weaken the north-east monsoon?

As the south-west monsoon draws to a close, the direction of the surface winds change and the stage is set for the north-east monsoon. Much of the country is heavily dependent on the south-west monsoon for most of its annual rainfall.

However, many parts of southern India also receive considerable rain from the north-east monsoon. Tamil Nadu, in particular, typically gets nearly half its annual rain during this monsoon. In fact, as much as 60 per cent of the rain that t he State’s coastal regions receive in a year is from this monsoon.

The north-east monsoon’s arrival is marked by a reversal in the direction of low-level winds over north coastal Tamil Nadu, according to Y.E.A. Raj, director of the India Meteorological Department’s regional centre at Chennai.

Instead of coming from a south-westerly direction, the winds start blowing from the north-east. After the winds shift, persistent rainfall over coastal Tamil Nadu is taken to mark the monsoon’s onset.

Between 1901 and 2000, the onset of easterly winds took place between September 23 and November 1, Dr. Raj wrote in a paper published in Mausam, the IMD journal. The normal date for this change in wind direction was October 15.

During the same period of one hundred years, the monsoon’s onset happened between October 4 and November 11, with the normal date for onset being October 20.

This year, the reversal in wind direction took place around October 9, according to Dr. Raj. But the easterly winds were still relatively weak and needed to strengthen for the monsoon onset to occur.

Labelling the two monsoons after the prevailing winds during those seasons gives the misleading impression that rain is brought to India by winds blowing from the south-west from June to September and, subsequently, is transported by winds from the north-east during October to December, pointed out Sulochana Gadgil of the Indian Institute of Science. It would perhaps be better to call the two seasons “summer monsoon” and “post-monsoon.”

As one can readily discern from satellite weather pictures, there is, in fact, no difference in the basic nature of the cloud systems that provide rain during the two seasons, said Dr. Gadgil.

Driven by heat from the sun’s rays, a band of clouds, often hundreds of kilometres long in the east-west direction, forms over the equatorial Indian Ocean and then moves northward over the Indian subcontinent.

In the case of the summer monsoon, the cloud band ultimately settles over the plains north of Mumbai. During the post-monsoon season, the cloud band stays over the country’s southern part.

El Nino effects

A great deal of effort within the country and abroad has gone into understanding the south-west monsoon, which provides nearly 80 per cent of India’s countrywide rainfall. The north-east monsoon has, by contrast, been far less studied. But it has been known for some time that an El Nino has very different effects on the south-west and north-east monsoons.

This abnormal warming of the equatorial waters of the central and eastern Pacific has often been associated with failure of the south-west monsoon.

But the very same phenomenon appears to have just the opposite effect on the north-east monsoon, leading to more bountiful rain.