Explained: Monsoon forecast tweak – stronger El Nino, neutral IOD

The IMD makes two long-range forecasts each year, in April — generally in the third week — and June. The April forecast gives the likelihood of the amount of rainfall for the country as a whole for the four-month monsoon season from June to September. It updates this forecast in June, and gives likely region-wise and month-wise distribution of rainfall. The June forecast is normally in the second week; it has come early this year.

In April, scientists were unsure exactly how the El Nino would shape up. There is a strong correlation between an El Nino — the abnormal rise of sea surface temperatures in the equatorial Pacific off the coast of South America — and a weak monsoon.

The US National Oceanic and Atmospheric Administration (NOAA) had announced the development of El Nino conditions in April. But as D Sivananda Pai of IMD explained, data then was mainly from subsurface temperatures. It was only subsequently that surface temperature data was available. “Subsurface temperature can be misleading, like it happened last year. In April, there were conditions for El Nino, but it dissipated by June and July,” he said.

But this year, El Nino conditions are not just strong, there is also a 90 per cent probability that it would continue throughout the monsoon season, the IMD has said.

A similar oceanic warming feature, the IOD, sometimes neutralises the effect of an El Nino. A temperature gradient between the eastern and western Indian Ocean sea surfaces has contrasting impact on the monsoon. A positive IOD, in which the western Indian Ocean gets warmer, helps the southwest monsoon gather more moisture and push northward towards the Indian mainland.

In April, IMD noted “slight negative” IOD conditions. This has turned neutral now, but is unlikely to turn positive. IMD has said there is a 50 per cent chance that neutral IOD conditions might continue.