Announcement of La Nina is always conditional. Necessarily, the temperature anomaly must exceed the threshold value for 5 consecutive episodes of an overlapping 3 month season. Also, the atmospheric features should remain consistent and aligned with the state of the sea surface. El Nino and La Nina events are characterized by a +ve/-ve ONI (Oceanic Nino Index) greater/less than or equal to 0.5°C/-0.5°C. ONI is the three-month running - mean SST departures in the Nino 3.4 region. By historical standards, to be classified as a full-fledged El Nino/ La Nina episode, the threshold must be exceeded for a period of at least 5 consecutive overlapping 3-month seasons. Climate Prediction Center (CPC) does consider El Nino/ La Nina conditions to occur when the monthly Nino 3.4 OISST departure meets or exceeds +/- 0.5°C, along with consistent atmospheric features. These anomalies must also be forecasted to persist for 3 consecutive months. This category could be for an ordinary and simple El Nino/ La Nina. Typical El Nino/ La Nina does not compromise with the duration of ocean-atmosphere coupling and strictly follows the logic of five episodes or more.
La Nina conditions, as envisaged this season, may last for a few months to a few years. Recently, a triple dip in La Nina was observed for three monsoon seasons (2020-2022). The longest-ever La Nina, since 1950, was witnessed from AMJ-1973 to MAM-1976. The shortest episode of La Nina during this period was five months, from JAS-2016 to NDJ-2017. Practically, this episode lasted from August to December 2016. The ONI had dropped to -0.3 in Jan 2017. The La Nina of 2024 could even be shorter than this episode and may last, at best, from October 2024 to Jan 2025. As the neutral conditions are likely to dominate from Jan 2025 itself, the La Nina may get truncated to just three months, between October and December 2024.
ENSO: Equatorial Sea Surface Temperatures are above average in the Western Pacific and slightly below average in the eastern Pacific Ocean. ENSO -Neutral conditions are expected to continue with weak La Nina likely to emerge in October 2024. Then, it is likely to persist through the Northern Hemispheric winters of 2024-25.
The ocean and atmosphere need to align and sync well for typical ENSO conditions. Nino indices have declined significantly during the last week. Nino 3.4, the marker for ONI has dipped to -0.2°C, the lowest value since March 2023. The eastern half of the Nino region has breached the threshold mark of -0.5°C. The Southern Oscillation Index (SOI), representing the state of the atmosphere has a monthly average of 0.9 for August 2024, the highest positive value, since Feb 2023. It may be construed that the ocean-atmosphere coupling is becoming sturdy, enough to pave the way for La Nina, more likely in Oct-Nov 2024.
IOD: The Indian Ocean Dipole is neutral. The IOD index value for the week ending 01Sep2024 was +0.15°C. The IOD is likely to remain within the neutral bounds of +/- 0.4°C, till the fall of the year. In any case, the IOD becomes inconsequential for the post-monsoon rains in the Indian Sub-Continent. As such, La Nina and IOD have a poor correlation in the autumn and winter months of the Northern Hemisphere.
MJO: The Madden-Julian Oscillation remained fairly coherent over the Maritime Continent, since the last week of August 2024. It is likely to remain in Phase 4&5 till mid-September, favouring stormy conditions in the Western Pacific and South China Sea. Courtesy MJO, tropical cyclone Yagi formed over the Philippines Sea and tracked over the Philippines and into the South China Sea. Storm Yagi intensified into a super typhoon, the most powerful storm in the last one decade to strike Hainan, the southernmost province of China. The remnants of the typhoon may travel across Vietnam, Laos, and Myanmar and emerge into the Bay of Bengal around 10-11 Sep 2024.
The absence of La Nina and positive IOD during the southwest monsoon had the least impact on the seasonal activity. These two oceanic parameters were expected to boost the monsoon rainfall. Courtesy, these two drivers, the monsoon was forecast to remain bountiful. The mystery of the monsoon and the potential of its internal dynamics is still far from understood.
