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Monsoon 2021: Cool Pacific Ocean and warm Arabian Sea can together open sluice gate of Monsoon

May 11, 2021 7:00 PM |

Monsoon 2021

A large majority of model forecasts predict Sea Surface Temperatures (SSTs) to return to near normal in the Central Pacific though a La Nina advisory remains in effect by now. During the last 4 weeks, SSTs continue to be below average across the Equatorial Pacific Ocean. However, the margin of cooling is gradually dropping and may turn zero-zero in another 3- 4 weeks to coincide with the arrival of monsoon over the Indian Sub Continent.


La Nina projection continues to wield a spike halfway through the monsoon season and the consistent pattern over the last few months rules out any meaningful warming of the Central and East Pacific. The Nino index 3.4 is hosted in the Central Pacific and is the principal measure for monitoring, assessing, and predicting ENSO. This also remains the base value for the 9 months long projection for historical perspective. A cool Pacific means the SSTs do not breach the 0-degree Celsius mark to cross over to the warm segment. Also, the ENSO configuration is shared between 'Neutral' and 'La Nina' markers. El Nino at best may have token presence limited to less than 15%.

Nino Indices

Indian Ocean Dipole (IOD) is defined by the difference in SSTs between the eastern and western tropical Indian Ocean. A warm equatorial Arabian Sea as compared to the right flank over Indonesian waters of the Indian Ocean will spell out a +ve IOD. Even neutral IOD with a score between zero-zero and +0.4 degrees celsius is considered good for the monsoon rains. Weekly SSTs in the Indian Ocean are warmer than average and the warm anomalies have increased in strength over the past fortnight. IOD is neutral now and the latest weekly value of the index to 09th May was +0.34 degrees celsius, a marginal increase from its previous fortnight value of +0.32 degrees celsius. Fluctuation in the forecast projection may be large as the model forecast is not reliable at this time of the year. Accuracy improves as the monsoon season advances and can generally be trusted after the southwest monsoon establishes over the Indian subcontinent.


Marginally cool Pacific negating El Nino and the warm Arabian Sea with borderline +ve IOD can lead to a promising season. Despite various forecasts and numerical model estimates, the monsoon could still remain mysterious. After all it has to prove its reputation.

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