El Nino threat during the Indian monsoon 2023 is growing big. Warming of Equatorial Pacific Ocean has commenced, albeit more authentically closer to the coast of Peru. Triple Dip La Nina will fade away soon to make ENSO neutral. Atmosphere has been slower to respond and remain La Nina like. Neutral conditions will gradually give way to El Nino, more so during 2nd half of the season.
February 2018
Precisely, El Nino is the warming of the ocean surface or above average Sea Surface Temperatures (SST's) in the central and eastern tropical Pacific Ocean. This warming result shift in the atmospheric circulation (Walker's Circulation). Consequently, rainfall reduces over India, Indonesia and West Pacific ring countries, during monsoon. The low level trade winds which normally blow from east to west along the equator, either weaken or reverse their direction.
Currently, many lead countries like, United States of America, Australia, Japan, India has different threshold for declaring El Nino event. Bureau of Meteorology Australia looks more critically at the trade winds, SOI (Southern Oscillation Index), Sea Surface Temperature in Nino 3 & 3.4 region and numerical model output before declaring the event. BOM observes +/- 0.8 degree as threshold of SST. The United States Climate Prediction Centre and International Research Institute (IRI) closely observes the SST's in the Nino 3.4 region, tropical atmosphere response and mandatorily considers NOOA's Oceanic Nino Index (ONI) for several seasons in a row.
El Nino pattern does not repeat itself verbatim. However, in general, El Nino events tend to develop during the period Apr-Jun and quite often reach their maximum intensity during Oct-Feb.
El Nino projection based on initial conditions of Feb 2023 is finding semblance with Feb 2018. Both are evolving El Nino, albeit 2023 appears to be stronger than 2018. El Nino share starts with 30% in June, reach 50% by July and climbs to >/= 60% during 2nd half of season.
Monsoon 2018 ended with below normal rainfall recording 90.6% of long period average (LPA). It narrowly missed mild drought. As a rule, El Nino is not to be read in isolation. Indian Ocean Dipole also has a weighty role to play with. IOD in 2018 was mostly neutral, to start with cold neutral and changed to warm neutral after July. Strong +VE IOD has the potential to neutralise blues of El Nino, to some extent. Powerful positive IOD in 2019 reversed the ill effects of El Nino and the season ended with above normal rainfall.
Indian Ocean Dipole index value for the week ending 12 Feb 2023 was -0.05 degree. But these are still early days . IOD projections do indicate the index crossing threshold of + 0.4 degree in June 2023 and stay +ve thereafter.
El Nino prediction at this time of the year does suffer accuracy due to 'spring barrier'. More authentic forecast with precision is likely in April. IOD also need to be watch
