Southwest Monsoon forecast this season, by all the global weather agencies, was riding on the back of La Nina and IOD. Anticipating, the early occurrence of La Nina (Pacific Ocean) and positive Indian Ocean Dipole (Indian Ocean), the seasonal rains were expected to be augmented and have a better finish than a normal monsoon. However, the southwest monsoon lived up to its reputation of throwing a surprise, anytime – anywhere, and pulled off an impressive victory, despite ‘no show’ of these two dependable parameters. ENSO-Neutral conditions have rarely triggered ‘above normal’ monsoon, with success rate of 1:10. In the last 45 years, since 1979, the monsoon rainfall was above normal (106%) in 1990 and excess (112%) in 1994. ENSO-Neutral had also set off drought conditions (<90%) twice during this period, more famous being the severe drought of 1979 (81%).
El Nino/La Nina events are difficult to simulate and predict because there are many mechanisms driving them. This has hindered the capacity to produce accurate predictions and help society prepare to mitigate the potential damage. Global warming or climate change is surely not behind the El Nino/La Nina events. But, the El Nino event does drive the global warming and thence, the climate change.
ENSO: La Nina conditions generally follow strong El Nino events but high uncertainty always remain regarding its strength and duration. Seasonal forecast models seem to have better skills in predicting El Nino but these forecasts lack clarity in La Nina episodes. Bureau of Meteorology (BoM) has forecast neutral ENSO -IOD until February. As per BoM, there is yet no consistent and sustained signal of La Nina weather pattern, which results heavy rainfall in Asia and drought in Americas. However, the statement of non-occurrence of La Nina by BoM stand to a reason. The standards of threshold marker are different and are pegged at +/- 0.8°C, as against Climate Prediction Center, edging it at +/- 0.5°C. Obviously, achieving an anomaly of -0.8°C may be difficult, in the back drop weak and brief La Nina event. Should a La Nina develop in the coming months, it is forecast to be relatively weak and short lived, too. Most models indicate return of ‘neutral’ by March 2025.
The Nino indices in the equatorial Pacific Ocean remain inconsistent. Nino 3.4 index, after having barely touched the threshold of -0.5°C last week, dropped yet again below the mark at -0.4°C. Nino 4 and Nino 3 also showed slight warming. The steadiness of these indices need to be ascertained to bring in uniformity.
IOD: The Indian Ocean Dipole is currently neutral. The weekly index value as on 29 Sep 2024 was -0.39°C, narrowly missing the threshold of -0.4°C. This is the lowest value of the index since 30June2024, when it briefly touched -0.4°C. Most models indicate that the IOD is likely to remain neutral, but within the negative margins. An IOD event is quite unlikely.
MJO: The Madden-Julian Oscillation has propagated eastward to Western Hemisphere. The pulse is likely to move further eastward over the Indian Ocean but possibly with a weaker amplitude. It is expected to reemerge in the Maritime Continent by mid-October.
Monsoon 2024 accomplished stellar performance against all odds. It goes on to prove that it has very strong internal dynamics and yet not fully understood. The largest synoptic feature on the globe is a very powerful heat engine with unparalleled energy equivalent. Decoding monsoon will continue to be a challenge and a puzzle to reckon with.