The onset of north-east monsoon is likely to take place by the end of this week. And, this time, the monsoon may not yield bountiful rainfall.

Pointing out that the normal date of onset is October 20 with an allowance of seven days, Deputy Director General of Meteorology Y.E. A. Raj says as a prelude to the onset, the change in wind pattern – from westerlies to easterlies – takes place by mid-October. But, easterlies have not yet set in. Also, the south-west monsoon has not yet withdrawn from southern States.

Dynamical model

However, the dynamical model developed by the Meteorological Department indicates that the arrival of the monsoon will happen on October 30 or 31.

Dr. Raj says rainfall during the monsoon is likely to be “near normal.” According to meteorological standards, normal rainfall is regarded if the deviation from the anticipated rainfall is in the range of minus 19 per cent to plus 19 per cent. He also refers to two factors – La Nina and Southern Oscillation – that may work against bountiful rainfall during the monsoon.

The prevailing La Nina and the positive phase of Southern Oscillation (SO) point to likely deficient rainfall during the monsoon this time. (La Nina, the opposite of El Nino, pertains to the anomalous decline in the temperature of the central tropical Pacific Ocean while the southern oscillation refers to the air pressure difference between Tahiti in the Pacific and Darwin in the northern Australia).

SO index

The SO index should be negative for heavy rain in October and the first half of November. But, the index has remained positive.

Dr. Raj also says that though the parameters do not have a one-to-one relationship with the pattern of rainfall during the monsoon, they are among the factors that influence the behaviour of the monsoon.

“Our monsoon systems – south-west or north-east – are too complex to be dictated by one or two parameters,” he adds.

For Tamil Nadu, the north-east monsoon is the main rainy season, accounting for about 43 cm of the annual rainfall of around 92 cm. Since 2004, the deviation of rainfall during the north-east monsoon has always been on the positive side.

The figures of deviation were one per cent (in 2004); 79 per cent (2005); 15 per cent (2006); 21 per cent (2007) and 31 per cent (2008) and 13 per cent (2009).

Even though the State is witnessing isolated rainfall, the prolonged dry spell is causing concern among farmers and water managers alike. As of now, the Mettur reservoir, the lifeline of farmers in the Cauvery basin of the State, has realised only 81 thousand million cubic feet (tmcft) of water. The shortfall in realisation of Cauvery water from Karnataka is 82 tmc ft.

Tamil Nadu has been pressing Karnataka to make good the shortfall at least substantially, if not fully.

Had the distress-sharing formula of the Central Water Commission been adopted, Tamil Nadu should be given over 35 tmcft more for the period June-September, an expert says.

Mettur storage

On Tuesday, the storage of Mettur was about 27 tmcft, which is expected to last about three more weeks, given the present rate of inflow and discharge.