IMD tracks changes in Indian Ocean for monsoon impact

NEW DELHI: The India Meteorological Department (IMD) on Wednesday predicted a normal monsoon for 2017 but the accuracy of its forecast will depend on the variability of a less discussed climatic phenomenon called the Indian Ocean Dipole (IOD), apart from the keenly observed El Nino.The country's national weather forecaster will hopefully have a better picture of both these factors in June when it releases its updated forecast.

Unlike El Nino, which is an abnormal warming of equatorial Pacific Ocean, the IOD is defined by the difference in the sea surface temperatures between the two equatorial regions of the Indian Ocean -the western end near the Arabian Sea and the eastern one off Indonesia. Positive IOD conditions are likely to be favourable for a normal above normal monsoon.

The IMD in its pre-forecast presentation on Wednesday noted that neutral IOD conditions are, at present, prevailing over the Indian Ocean and more clarity on it is expected next month. "The latest forecast from the Monsoon Mission Climate Forecasting System (MMCFS) indicates that the weak positive IOD conditions are likely to develop during the middle of the monsoon season and persist for some more months.

Positive IOD conditions are likely to be favourable for a normal above normal monsoon," said D S Pai, director of long-range forecast at IMD, while making his presentation on Tuesday .

Pai noted that extreme sea surface temperature conditions over the Pacific, particularly El Nino conditions, and a positive IOD development over equatorial Indian Ocean are known to have strong influence on the Indian summer monsoon. IMD is, therefore, "carefully monitoring the sea surface conditions over the Pacific and Indian oceans".

Although there is no fully established correlation between summer monsoon rainfall in India and the IOD, IMD's records show that IOD was positive in four of the 15 El Nino years (during 1951-2016) and the rainfall was normal (96-104% of the Long Period Average) in three of these four years.

If one looks at only the El Nino phenomena, records show that the country had deficient monsoon rainfall (less than 90% of the LPA) in nine out of 15 El Nino years. In two of these years, the country received normal rainfall of above 100% of the LPA. One of these two years was 1997, which saw one of the strongest El Nino years of the last century .

Incidentally , 1997 was the year of positive IOD when the country had normal rainfall of 102% of the LPA -indicating a peculiar interplay of both these climatic phenomena which impact the Indian summer monsoon.