Why IMD failed to foresee above normal monsoon

As the four-month rainfall season ended with "above normal" monsoon, initial predictions by the India Meteorological Department and the private forecaster Skymet Weather were found to be contrary to the outcome.

While IMD in April had said the country would receive 96 per cent of the Long Period Average, Skymet had predicted 93 per cent of the LPA.

Both had given an error margin of plus or minus 5 per cent.

Ninety-six per cent of LPA falls on the border line of "below normal" and "normal" rainfall. Skymet had cited El Nino as the reason behind a possible below normal rainfall.

However, as the rainfall season ended, IMD recorded a whopping 10 per cent more rainfall. The monsoon rainfall was the highest since 1994, according to the weather department.

Of the four-month season, July recorded 105 per cent of rainfall of the LPA. August recorded 115 per cent of the LPA, a first since 1996 (119 per cent). Similarly in September, the rainfall (152 pc of the LPA) was the second highest after 1917 (165pc of the LPA).

Mohapatra, director-general of IMD, said the department would do a detailed analysis.

He, however, said IMD was able to gauge the trends like El Nino turning neutral and the Indian Ocean Dipole turning positive. He added that IMD did not change the forecast even when June ended with 33 per cent deficiency.

"We will carry out an analysis on this," he said, adding that there has been a significant improvement in seasonal forecast.

Mohapatra said several global models had predicted below normal monsoon because of the El Nino but IMD stuck to its prediction and did not revise it.

The El Nino is associated with the heating of Pacific waters, while a Positive Indian Ocean Dipole is linked to cooling of the Indian Ocean waters. El Nino is a global phenomenonm while the IOD is regional and has an impact mostly on the Bay of Bengal and Arabian Sea.

"In spite of several global models indicating a strong possibility of continuation of a El Nino episode during the monsoon season and possibly a below normal monsoon, IMD had predicted a normal rainfall (96-104 per cent of LPA).

"While issuing the forecasts, based on IMD's models, it was suggested that the El Nino episode will weaken further and a positive Indian Ocean Dipole (IOD) event will emerge in the Indian Ocean. Moreover, IMD also predicted that the monsoon performance would be better in the second half compared to the first half," a statement by the IMD said.

IMD's analysis on weakening of El Nino and development of a positive IOD and the second half monsoon rainfall being above normal were thus proved correct, it said. "However, quantitatively, realised rainfall during the second half was more than what IMD predicted," it added.

Citing reasons behind the above normal rainfall, Mahesh Palawat, Vice-President (Climate Change and Meteorology), Skymet Weather, said El Nino, most of the time, overwhelms IOD. But this time, he added, it was the other way around.

Most global models indicated below normal rainfall, Palawat said.

The Madden-Julian Oscillation (MJO) is the major fluctuation in tropical weather on weekly to monthly timescales. The MJO can be characterised as an eastward moving 'pulse' of cloud and rainfall near the equator that typically recurs every 30 to 60 days.

Palawat said whenever there is MJO, the monsoon activity increases. It is also difficult the predict the MJO activity in April, he added.

"We will factors all these aspects in future," he said.