As El Niño Plays Villain, Could the Indian Ocean Dipole Save Monsoon 2019?

The first is the poor role played by pre-monsoon rainfall so far this year. Regions like Tamil Nadu, Gujarat, and Maharashtra are reporting over 60% deficiency in pre-monsoon showers. The second twist is the emergence of a weak, positive phase of the dreaded El Niño Southern Oscillations (ENSO), a phenomenon known to suppress the ISMR.

What is ENSO? Here's a quick primer: It is the anomalous warming of the Central and Eastern Pacific Ocean, which reduces the moisture incursion to the Indian subcontinent (necessary for good rainfall). The consensus among the scientific community, based on the output from the weather models, is that the June and July of 2019 is likely (close to 60% probability) to undergo weak ENSO-like conditions, with a Nino 3.4 index of 0.7.

(Generally, a value between 0.5 and 1 is linked to weak ENSO and a value of less than 0.5 signifies neutral conditions.)

So El Niño, in its weak form, is looming large—and it is likely to stick around for a bit (until July 2019), complicating the plot for those who’ve been praying for a robust monsoon. However, the story does not end there. Just like every coin has two sides, so does the ISMR.

Introducing: the Indian Ocean Dipole

The Indian monsoon is also influenced by another dominant system called the ‘Indian Ocean Dipole (IOD)’. As the name suggests, IOD is a much more local phenomena and signifies the temperature gradient between the Western and Eastern parts of the Indian Ocean (IO). Generally speaking, IOD has three phases: positive, neutral, and negative. A positive phase signifies warmer-than-normal Western IO and cooler-than-normal Eastern IO, while the negative phase designates the opposite. Neutral IOD conditions indicate normal temperatures in the Western and Eastern IO. Research has shown that a positive IOD (+IOD) generally bodes well for the ISMR and has the potential to offset the ill-effects of an ENSO season, thereby bringing normal rainfall to India.

The question, therefore is: where does IOD stand this year?

The IOD starts developing in May and attains a peak around September. Given that 2019 may go on record as a weak El Niño year, the development of IOD would be a key factor in deciding the fate of the 2019 monsoon season. As per the data from the Bureau of Australian Meteorology, up until end of April, neutral IOD conditions prevailed in the Indian Ocean. (The IMD also reported neutral IOD conditions at the beginning of May and predicted that it could turn ‘weakly positive’ during May and June.) However, with the passage of Cyclone Fani, there are indications that the Eastern IO could cool significantly (due to the ocean upwelling process), thus potentially setting up a +IOD-like condition in the near future!

The IOD index currently has a value of 0.06 (neutral). All weather models currently suggest that by start of June 2019, the IOD index will move above the threshold value of 0.4 needed to create a +IOD like conditions, which has a good influence on the Indian monsoon.

As per the author’s analysis, the ISMR is likely to hit the Andamans by May 22, and will move into the mainland–Kerala–around June 2, which would mark a near-timely onset. It will then spread its wings further north to Karnataka and Mumbai by June 12. If a +IOD does persist in June, then there is a fairly decent chance of a “normal” ISMR, as it may offset the negative influence of ENSO. This would be a blessing for drought-hit regions, which are still coping from the deficient 2018 monsoon season.

The ‘claim to fame’ for IOD was in 1997, where monsoon was close to normal when a +IOD event coincided with a record-breaking ENSO year, where rainfall was predicted to be below-normal. This year, if the IOD maintains its current trajectory, it could once again be the fortuitous hand of fate (much like the movie scriptwriter’s) that saves the hero at the last minute, answering all our prayers in true cinematic fashion.

(Sridhar Balasubramanian is Associate Professor at Department of Mechanical Engineering & Associate Faculty in IDP Climate Studies, IIT Bombay. Views are his own. Email: [email protected]).

This article is a guest column reflecting the author’s opinions, and does not necessarily represent the official views of The Weather Channel.