Nino conditions in the equatorial Pacific this season resembled largely with ‘Canonical El Nino’. Similar conditions were observed last in 2014, a drought year. Canonical ENSO is a naturally occurring phenomenon that involves fluctuating ocean temperatures in the tropical Pacific. During such an episode, eastern Pacific Ocean gets warmed more than the central and western parts of the ocean. Sometimes, when El Nino is full blown, the west and central Pacific may observe even negative SST anomaly.
El Nino, La Nina, +VE IOD, -VE IOD are crucial oceanic variables. However, these frequently occurring phenomenon have no correlation and any of the combination is feasible. The only strong correlation is invariably seen between -VE Southern Oscillation Index (SOI) and El Nino. That is how, ENSO is considered as ocean-atmosphere coupling with El Nino representing the ocean and SOI constituting atmospheric arm.
ENSO: During the last 4 weeks, above average SST’s have strengthened near the Date Line. Earlier, Nino 3.4, the principal indicator for assessing, monitoring and predicting El Nino, showed signs of plateauing with index hovering around 1.5°C and 1.6°C for nearly 8 weeks. However, this index seems still not done and dusted, as the temperature anomaly has risen significantly to 1.8°C, as on 06thNov 2023.
The SOI is one measure of the large scale fluctuation in the air pressure occurring between the western and eastern tropical Pacific. The negative phase of the SOI represents below normal air pressure at Tahiti (Australia) and above normal air pressure at Darwin ( Australia). The strongly negative margin seen earlier seem to be shrinking. The monthly average of SOI has dropped to -0.5 in October from the seasonal high of -1.3 in September 2023. SOI is more vulnerable to fluctuations and need to be observed for longer durations.
IOD : Like Nino 3.4, Dipole Index is also fluctuating. The IOD index was +1.55°C for the week ending 05Nov 2023, a significant rise from its earlier mark of +1.36°C on 29Oct 2023. The IOD index has been above the threshold value of +0.4°C for the last 12 weeks. Accordingly, 2023 qualifies as positive IOD event year. IOD event typically starts around May and peak between August and October. It rapidly decays when the monsoon arrives in the Southern Hemisphere around the end of ‘spring’ (SH). During positive IOD, westerly winds weaken along the equatorial Indian Ocean, allowing warm waters to shift towards Africa. Such temperature distribution is good for rains over Southern Peninsula and leads to poor rainfall in Australia.
MJO: The Madden-Julian Oscillation is nearly indiscernible. The weak pulse is likely to exit the Indian Ocean and go around the Maritime Continent with a compact amplitude. The transient feature will sail across West Pacific, which otherwise has finished its peak cyclone season, normally observed between May and October. No storm is likely to form over the Indian seas, till the 3rd week of November.
The tropical Pacific atmosphere and oceanic anomalies are consistent with an ongoing El Nino event. As the monsoon trough prepares to shift in the Southern Hemisphere, the pressure gradient between Tahiti and Darwin weakens. The fractured pattern disturbs the SOI distribution. Invariably, it turns neutral during late winters and early spring of Northern Hemisphere. Weakening of atmospheric arm degrades the Oceanic Nino Index (ONI) also and gradually reverts to ‘neutral’ conditions.
